[{"data":1,"prerenderedAt":854},["ShallowReactive",2],{"layer:light:extend":3},{"layer":4,"contentHash":836,"dependencyHashes":837,"approval":848,"releaseId":853},{"schemaVersion":5,"conceptId":6,"locale":7,"depth":8,"revision":5,"title":9,"subtitle":10,"summary":11,"objectives":12,"estimatedMinutes":18,"plate":19,"blocks":40,"sourceIds":831,"reviewStatus":832,"authoring":833},1,"light","en","extend","Waves, particles and the light you cannot see","Beyond visible light: wave versus particle, a real chocolate-bar experiment, and looking into the past with light-years.","Step past visible light into the wider spectrum, meet the wave-versus-particle debate (light is genuinely both), measure light's speed with a microwave and a chocolate bar, see how bending stretches every day, and use light-years to look into the past.",[13,14,15,16,17],"Place visible light within the wider electromagnetic spectrum by wavelength.","Give at least one piece of evidence each for light behaving as a wave and as a particle.","Carry out (or understand) a real measurement of the speed of light using a microwave oven.","Explain why atmospheric refraction adds a few minutes of daylight to every day.","Use light-years to say how far into the past you are looking at a given astronomical object.",44,{"title":20,"rows":21},"Lesson plate",[22,25,28,31,34,37],{"label":23,"value":24},"Depth","Extend",{"label":26,"value":27},"Reading time","≈ 44 minutes",{"label":29,"value":30},"Prior knowledge","Deepen: refraction, dispersion, scattering",{"label":32,"value":33},"Chapters","9",{"label":35,"value":36},"Labs","Sorting games, ray boxes, matching, prism bench",{"label":38,"value":39},"Goes beyond syllabus","Yes — quantum ideas simplified honestly",[41,45,51,57,60,97,109,153,158,199,204,208,219,224,227,238,280,285,288,301,313,341,345,359,364,367,372,383,405,410,415,426,431,434,463,474,478,483,488,491,509,520,531,535,539,544,547,579,583,594,598,613,618,621,631,636,661,666,671,702,706,731,800,811,817,822],{"id":42,"type":43,"markdown":44},"intro-extend","prose","Everything so far has treated light as a ray — a straight arrow that reflects and refracts. That model is enormously useful and completely wrong about what light actually *is*.\n\nThis layer opens the two big questions physicists spent three centuries fighting over: is light a wave, or a stream of particles? (Answer: astonishingly, both, depending on the question you ask.) It also steps past the narrow band of wavelengths your eyes can catch, tries a genuine speed-of-light project with a chocolate bar, and leaves you with open questions nobody has fully answered.",{"id":46,"type":47,"variant":48,"title":49,"markdown":50},"how-to-read-e","callout","observation","What to expect in Extend","Some of this is genuinely graduate-level physics, simplified as honestly as possible rather than dumbed down. It is fine — expected, even — to finish a chapter with a question rather than a tidy answer. That is the actual state of some of this material for professional physicists too.",{"id":52,"type":53,"title":54,"eyebrow":55,"navLabel":56},"e-ch1","chapter","Beyond what your eyes can see","Chapter 01","1 Beyond visible",{"id":58,"type":43,"markdown":59},"ems-setup","Visible light — roughly 400 to 700 nanometres — is only a sliver of a vastly wider family called the **electromagnetic spectrum**. Every member of that family is the same underlying phenomenon (an oscillating electric and magnetic field) travelling at the same speed, c, differing only in wavelength — and therefore in frequency and in energy.",{"id":61,"type":62,"caption":63,"columns":64,"rows":68},"table-ems","table","The electromagnetic spectrum, longest wavelength to shortest",[65,66,67],"Band","Roughly","A familiar use",[69,73,77,81,85,89,93],[70,71,72],"Radio","metres to kilometres","AM\u002FFM broadcast, mobile phone signals",[74,75,76],"Microwave","millimetres to centimetres","Microwave ovens, radar, satellite links, Wi-Fi",[78,79,80],"Infrared","700 nm to about 1 mm","TV remotes, thermal cameras, the warmth you feel from a fire",[82,83,84],"Visible light","400–700 nanometres","Everything in this topic so far",[86,87,88],"Ultraviolet","10–400 nanometres","Sunburn, sterilising water, some security ink under a UV lamp",[90,91,92],"X-rays","0.01–10 nanometres","Medical and dental imaging, airport baggage scanners",[94,95,96],"Gamma rays","under 0.01 nanometres","Emitted by radioactive decay and in cancer radiotherapy",{"id":98,"type":99,"title":100,"problem":101,"steps":102,"help":107},"we-fm-radio","worked_example","One wavelength across the whole spectrum: an FM radio station","All India Radio's FM stations broadcast around 100 MHz. Using wavelength = c ÷ frequency, how long is one wave of an FM radio signal, and how does that compare with visible light?",[103,104,105,106],"Wavelength = c ÷ frequency = 299,792,458 ÷ 100,000,000.","That gives about **3.0 metres** — roughly the length of a small car.","Visible light's wavelength (400–700 **nanometres**, or 0.0000004–0.0000007 metres) is about **4,285,714 times shorter** than an FM radio wave.","Same speed, same underlying phenomenon, wavelengths differing by a factor of millions — that single ratio is most of what the electromagnetic spectrum is.",{"simplerExplanation":108},"A radio wave is metres long; a light wave is a tiny fraction of a millionth of a metre long. Both travel at the same speed.",{"id":110,"type":62,"caption":111,"columns":112,"rows":116},"table-ems-numbers","One representative wavelength from each band, with its frequency and photon energy, all computed from the same two formulas (f = c ÷ λ and E = hc ÷ λ)",[65,113,114,115],"Wavelength","Frequency","Photon energy",[117,121,125,129,134,139,143,148],[70,118,119,120],"1 m","3.00 × 10⁸ Hz","0.0000012 eV",[74,122,123,124],"1 cm","3.00 × 10¹⁰ Hz","0.00012 eV",[78,126,127,128],"2,000 nm","1.50 × 10¹⁴ Hz","0.62 eV",[130,131,132,133],"Visible (red)","700 nm","4.28 × 10¹⁴ Hz","1.77 eV",[135,136,137,138],"Visible (violet)","400 nm","7.49 × 10¹⁴ Hz","3.10 eV",[86,140,141,142],"200 nm","1.50 × 10¹⁵ Hz","6.2 eV",[144,145,146,147],"X-ray","0.1 nm","3.00 × 10¹⁸ Hz","12.4 keV",[149,150,151,152],"Gamma ray","0.001 nm","3.00 × 10²⁰ Hz","1,240 keV",{"id":154,"type":47,"variant":155,"title":156,"markdown":157},"aha-ems-pattern","aha","One pattern rules the whole table","Read across any row of the table above and two things always move together: shorter wavelength always means both higher frequency (f = c ÷ λ, so smaller λ forces bigger f) and higher photon energy (E = hc ÷ λ, the same relationship). Radio waves are gentle giants: long, slow-oscillating, and each individual photon carries almost no energy at all. Gamma rays are the opposite extreme in every column at once.",{"id":159,"type":160,"component":161,"componentVersion":5,"config":162,"objective":193,"textAlternative":194,"help":195},"lab-ems-sort","interactive","sort-game",{"prompt":163,"bins":164,"items":171,"seconds":192},"Order matters here: sort each electromagnetic band by roughly how long its wavelength is.",[165,168],{"id":166,"label":167},"long","Longer wavelength than visible light",{"id":169,"label":170},"short","Shorter wavelength than visible light",[172,176,180,183,186,189],{"id":173,"label":174,"bin":166,"why":175},"radio","Radio waves","Radio wavelengths run from metres to kilometres — far longer than any visible wavelength.",{"id":177,"label":178,"bin":166,"why":179},"microwave","Microwaves","Millimetres to centimetres — still much longer than visible light.",{"id":181,"label":78,"bin":166,"why":182},"infrared","Just longer than red light, from about 700 nanometres to a millimetre.",{"id":184,"label":86,"bin":169,"why":185},"ultraviolet","Just shorter than violet light, from about 10 to 400 nanometres.",{"id":187,"label":90,"bin":169,"why":188},"xray","Far shorter still: about 0.01 to 10 nanometres.",{"id":190,"label":94,"bin":169,"why":191},"gamma","The shortest of all, under 0.01 nanometres — small enough to pass through many materials.",0,"Sort six electromagnetic bands into longer or shorter wavelength than visible light.","Six electromagnetic bands — radio, microwave, infrared, ultraviolet, X-ray and gamma — sorted by whether their wavelength is longer or shorter than visible light's 400–700 nanometre range. Radio, microwave and infrared sit on the long side; ultraviolet, X-ray and gamma sit on the short side, with visible light as the narrow dividing band in between.",{"hints":196},[197,198],"Infrared literally means \"below red\" — just past the red end, on the long side.","Ultraviolet means \"beyond violet\" — just past the violet end, on the short side.",{"id":200,"type":47,"variant":201,"title":202,"markdown":203},"misc-radio-not-sound","misconception","\"Radio waves are a kind of sound\"","Radio waves are electromagnetic radiation, exactly the same family as light, just with a much longer wavelength — they travel through the vacuum of space at the speed of light and carry no vibrating air at all. A radio *receiver* converts the electromagnetic signal into an electric current, which a speaker then converts into sound — three separate physical processes chained together, easily mistaken for one.",{"id":205,"type":47,"variant":155,"title":206,"markdown":207},"aha-why-visible","Why is \"visible\" light visible, of all possible wavelengths?","It is not a coincidence. Using Wien's displacement law (peak wavelength = a constant ÷ surface temperature), the Sun's surface, at about 5,778 K, radiates most strongly at around **502 nanometres** — squarely inside what humans call the visible band, close to green. Eyes almost certainly evolved to be most sensitive exactly where the available light was brightest. A creature evolving under a cooler red star, or a hotter blue-white one, would very plausibly call a *different* slice of the electromagnetic spectrum \"visible\".",{"id":209,"type":99,"title":210,"problem":211,"steps":212,"help":217},"we-xray-energy","Why X-rays need thick lead aprons and gamma rays need even more shielding","Using the rule E (in eV) = 1240 ÷ wavelength (in nanometres), estimate the energy of a single medical X-ray photon of wavelength 0.1 nm, and compare it with a single green light photon (about 550 nm).",[213,214,215,216],"X-ray energy = 1240 ÷ 0.1 = **12,400 eV**, or about 12.4 keV (kilo-electron-volts).","Green light energy = 1240 ÷ 550 ≈ **2.25 eV**.","Ratio: 12,400 ÷ 2.25 ≈ **5,511 times more energetic** for a single X-ray photon than a single green light photon.","That enormous jump in energy per photon is exactly why X-rays and gamma rays can damage living cells and need careful shielding, while visible light, however bright, cannot: a photon's ability to break chemical bonds depends on its individual energy, set by its frequency, not on how many photons arrive at once.",{"simplerExplanation":218},"Shorter wavelength means a far more energetic photon. X-ray photons carry thousands of times more energy each than visible-light photons, which is why they need shielding and light does not.",{"id":220,"type":53,"title":221,"eyebrow":222,"navLabel":223},"e-ch2","Is light a wave? The case for yes","Chapter 02","2 Light as a wave",{"id":225,"type":43,"markdown":226},"wave-case","A wave's calling card is **diffraction and interference**: waves bend slightly around obstacles and through narrow gaps, and two overlapping waves can reinforce or cancel each other. In 1801, Thomas Young shone light through two narrow, closely spaced slits and saw exactly this — a pattern of bright and dark bands where the light from the two slits alternately reinforced and cancelled. Particles fired through two slits, like tiny bullets, could never do that; only waves interfere.",{"id":228,"type":99,"title":229,"problem":230,"steps":231,"help":236},"we-cd-diffraction","A rainbow you can hold: diffraction from a CD","A CD's data track is a spiral of tiny pits spaced about 1.6 micrometres (1.6 × 10⁻⁶ m) apart — closely spaced enough to diffract light noticeably, acting like a reflective diffraction grating. At what angle does green light (550 nm) diffract from a CD's surface, and why does tilting a CD in the light show a rainbow?",[232,233,234,235],"For a diffraction grating, sin(angle) = wavelength ÷ spacing = 550 × 10⁻⁹ ÷ 1.6 × 10⁻⁶ = 0.344.","angle = arcsin(0.344) ≈ **20.1°**.","Red light (700 nm) diffracts at about 25.9°, and violet (400 nm) at about 14.5° — every colour bends by a different amount because diffraction, like refraction, depends on wavelength.","Tilt a CD under a lamp and you are sweeping through a whole range of angles at once, catching a different colour brightly reflected at each angle — a rainbow made entirely by diffraction, with no prism or raindrop involved at all.",{"anotherExample":237},"A DVD packs its data track twice as densely (about 0.74 micrometres), which bends light through even larger angles — try comparing a CD and a DVD under the same lamp.",{"id":239,"type":160,"component":161,"componentVersion":5,"config":240,"objective":274,"textAlternative":275,"help":276},"lab-wave-particle-sort",{"prompt":241,"bins":242,"items":249,"seconds":192},"Is this observation best explained by light behaving as a wave, or as a particle (photon)?",[243,246],{"id":244,"label":245},"wave","Best explained by waves",{"id":247,"label":248},"particle","Best explained by particles",[250,254,258,262,266,270],{"id":251,"label":252,"bin":244,"why":253},"young","Young's double-slit bright-and-dark banding pattern","Interference — reinforcement and cancellation between two overlapping waves — has no equivalent for a stream of independent particles.",{"id":255,"label":256,"bin":244,"why":257},"cddiff","A CD's rainbow-coloured diffraction pattern","Diffraction, light bending round the edges of closely spaced obstacles, is a defining wave behaviour.",{"id":259,"label":260,"bin":247,"why":261},"photoelectric","Shining light on a metal knocks electrons out only above a certain minimum frequency, however dim the light","Each photon delivers a fixed lump of energy set by its frequency; a photon below the threshold energy can never knock an electron out, no matter how many arrive (the photoelectric effect).",{"id":263,"label":264,"bin":247,"why":265},"solarpanel","A solar cell's electrical output jumps immediately when light switches on, with no build-up delay","Each absorbed photon transfers its energy in one instant, discrete event, not a gradual build-up as a wave model alone would suggest.",{"id":267,"label":268,"bin":244,"why":269},"rainbow2","A rainbow's continuous, unbroken smear of colour","Continuous refraction and dispersion by wavelength is naturally described by a continuous wave picture.",{"id":271,"label":272,"bin":247,"why":273},"ccdpixel","A camera sensor's faintest possible signal arrives as single, discrete clicks rather than a smooth trickle","At very low light levels, sensitive detectors register individual photons arriving one at a time — direct evidence for light's particle side.","Sort six real observations into wave evidence and particle (photon) evidence for the nature of light.","Six genuine physical observations — Young's interference bands, a CD's diffraction rainbow, the photoelectric effect's frequency threshold, a solar cell's instant response, a rainbow's continuous colour smear, and single-photon clicks in a sensitive detector — sorted into whichever picture of light, wave or particle, explains them most naturally. The deliberate lesson of the sort: **both bins fill up**. No single picture explains every card.",{"hints":277},[278,279],"Anything about bending, spreading or overlapping patterns points to waves.","Anything about a sharp threshold or discrete, individual events points to particles.",{"id":281,"type":53,"title":282,"eyebrow":283,"navLabel":284},"e-ch3","Is light a particle? The case for yes, too","Chapter 03","3 Light as a particle",{"id":286,"type":43,"markdown":287},"particle-case","In 1905, Albert Einstein explained the **photoelectric effect** — light knocking electrons out of a metal — by proposing that light itself arrives in discrete packets of energy, later named **photons**. A photon's energy depends only on its frequency: E = h × f, where h is a tiny fixed number called the **Planck constant**. Dimmer light means fewer photons, not weaker ones; a very dim beam of high-frequency violet light can still knock an electron out, while an enormously bright beam of low-frequency red light, below the threshold frequency, cannot knock out a single one.",{"id":289,"type":290,"items":291},"f-photon","formulas",[292,295,298],{"expression":293,"caption":294},"E = h × f","A photon's energy, where h ≈ 6.626 × 10⁻³⁴ joule-seconds (the Planck constant) and f is the light's frequency.",{"expression":296,"caption":297},"Green photon ≈ 2.25 eV","About 3.6 × 10⁻¹⁹ joules — a vanishingly small amount of energy for a single photon.",{"expression":299,"caption":300},"Red photon ≈ 1.77 eV, violet ≈ 3.10 eV","Higher frequency (shorter wavelength) always means more energetic photons.",{"id":302,"type":99,"title":303,"problem":304,"steps":305,"help":311},"we-photon-count","How many photons is a laser pointer, really?","A small green laser pointer outputs about 1 watt of power (a strong one; most are far less). Using E = hc ÷ λ for one green photon (550 nm) and power = energy per second, roughly how many photons leave it every second?",[306,307,308,309,310],"Energy of one green photon: E = hc ÷ λ = (6.626 × 10⁻³⁴ × 2.998 × 10⁸) ÷ (550 × 10⁻⁹) ≈ 3.61 × 10⁻¹⁹ joules.","1 watt means 1 joule of energy leaves the laser every second.","Number of photons per second = total energy per second ÷ energy per photon = 1 ÷ 3.61 × 10⁻¹⁹.","That works out to about **2.77e+18 photons every second** — nearly three billion billion, every second, from a device you can hold in one hand.","This is exactly why light, at everyday brightness levels, feels perfectly smooth and continuous rather than a hail of separate particles: the individual photons are so numerous and so closely spaced in time that no human sense could ever detect the graininess directly.",{"simplerExplanation":312},"Divide the laser's total energy output each second by the tiny energy of one photon, and you get an almost incomprehensibly large number of photons per second.",{"id":314,"type":315,"title":316,"items":317},"timeline-duality","timeline","Three centuries of arguing about what light is",[318,322,325,329,333,337],{"time":319,"title":320,"text":321},"1670s","Newton: particles","Proposed light is a stream of tiny particles (\"corpuscles\"), explaining reflection and straight-line travel well.",{"time":319,"title":323,"text":324},"Huygens: waves","Proposed light is a wave in an invisible medium, explaining refraction (and, later, diffraction) at least as well.",{"time":326,"title":327,"text":328},"1801","Young: wave evidence","The double-slit interference experiment gave strong, direct evidence for light's wave nature, and the particle theory fell out of favour for a century.",{"time":330,"title":331,"text":332},"1860s","Maxwell: electromagnetic waves","Showed mathematically that light is an oscillating electric and magnetic field, unifying light, radio waves and all the rest of the electromagnetic spectrum.",{"time":334,"title":335,"text":336},"1905","Einstein: the photon","Explained the photoelectric effect only by treating light as discrete energy packets, reviving the particle picture — and winning the 1921 Nobel Prize substantially for this, not for relativity.",{"time":338,"title":339,"text":340},"1920s onward","Quantum mechanics: both, properly","A full theory (quantum electrodynamics) shows light is neither a simple wave nor a simple particle in the everyday sense, but a genuinely quantum object that shows either face depending on how you look at it.",{"id":342,"type":47,"variant":201,"title":343,"markdown":344},"misc-either-or","\"Light must be either a wave or a particle — pick one\"","This is the single most natural, and most misleading, way to think about the debate. Light is not secretly one of the two, waiting to be correctly identified. It is a quantum object whose behaviour genuinely matches the wave description in some experiments (diffraction, interference) and the particle description in others (the photoelectric effect, single-photon detection), with no contradiction, because it is neither a wave nor a particle in the classical, everyday sense of either word — those are simply the two closest analogies human intuition, built for a world of balls and ripples, can offer.",{"id":346,"type":160,"component":347,"componentVersion":5,"config":348,"objective":353,"textAlternative":354,"help":355},"lab-dispersion-wave","prism-lab",{"modes":349,"rounds":352},[350,351],"prism","scattering",6,"Revisit dispersion and scattering, now knowing both are wavelength-dependent wave phenomena.","The same prism and scattering bench from earlier layers, worth one more look with the wave-particle debate in mind: dispersion (different wavelengths refracting by different amounts) and scattering (different wavelengths deflected by different amounts by small particles) are both explained through wavelength — a wave property. Nothing about the photoelectric effect or photon counting shows up here at all, which is itself an illustration of how differently the two faces of light present themselves.",{"hints":356},[357,358],"Every phenomenon in this lab depends on wavelength, a wave property.","Contrast this with the photoelectric effect, which depends on frequency crossing a sharp threshold — closer to a particle picture.",{"id":360,"type":53,"title":361,"eyebrow":362,"navLabel":363},"e-ch4","Lasers: light with one colour, one direction, one rhythm","Chapter 04","4 Lasers",{"id":365,"type":43,"markdown":366},"laser-setup","Ordinary light, even from a single small bulb, is a chaotic mix: many different wavelengths, spreading in every direction, with every photon arriving out of step with every other. A **laser** (Light Amplification by Stimulated Emission of Radiation) produces something genuinely different: light of essentially **one wavelength**, travelling in **one tightly focused direction**, with all its waves rising and falling **perfectly in step** — a property called **coherence**.",{"id":368,"type":47,"variant":369,"title":370,"markdown":371},"def-coherence","definition","Coherent light","Light is **coherent** when its waves stay in a fixed, predictable step with each other over a useful distance and time. Sunlight and bulb light are **incoherent** — a chaotic jumble of wavelengths and phases. A laser's coherence is what lets it stay a narrow, undiverging beam over long distances, be focused to an extremely small, intense spot, and produce sharp interference patterns on demand — the property Einstein's 1917 theory of stimulated emission made possible, though the first working laser was not built until 1960.",{"id":373,"type":99,"title":374,"problem":375,"steps":376,"help":381},"we-lidar","A laser rangefinder, timing light instead of sound","A laser rangefinder fires a pulse at a wall 10.5 m away and times how long it takes to return. Light travels about 30.0 cm per **nanosecond** (a billionth of a second). How long does the round trip take, and why does this device need extremely fast electronics?",[377,378,379,380],"Round-trip distance = 2 × 10.5 m = 21 m = 2,100 cm.","Light covers about 30.0 cm every nanosecond, so time = 2,100 ÷ 30.0.","That gives about **70.0 nanoseconds** — 70 billionths of a second.","A device that measures distance this way must be able to time intervals far shorter than a millionth of a second, which is exactly why laser rangefinders and LIDAR (Light Detection and Ranging, used in self-driving cars and by ISRO-class satellites mapping terrain) rely on extremely fast modern electronics that did not exist before the late 20th century.",{"simplerExplanation":382},"Light covers about 30 cm every nanosecond. A 21 m round trip is 2,100 cm, so it takes about 2,100 ÷ 30 ≈ 70 nanoseconds — astonishingly fast, but measurable with the right electronics.",{"id":384,"type":62,"caption":385,"columns":386,"rows":389},"table-laser-uses","A few everyday and specialised uses of laser light",[387,388],"Use","What the coherence or focus buys",[390,393,396,399,402],[391,392],"Barcode and QR scanners","A tightly focused, single-wavelength spot that reads a printed pattern reliably",[394,395],"Fibre-optic communication","A single, stable wavelength that stays sharp over many kilometres of glass fibre",[397,398],"Laser eye surgery","Extremely precise, focused energy delivered to a tiny, exact spot on the cornea",[400,401],"Laser rangefinders and LIDAR","A short, tightly focused pulse whose travel time gives distance to within centimetres",[403,404],"Laser pointers and light shows","A narrow, undiverging beam visible as a sharp point or line from a distance",{"id":406,"type":47,"variant":407,"title":408,"markdown":409},"nuance-bioluminescence","nuance","Nature had cold, efficient light long before humans did","Fireflies, some fungi and many deep-sea creatures make light through **bioluminescence** — a chemical reaction (between a molecule called luciferin and an enzyme called luciferase) that converts chemical energy almost directly into light, releasing very little as heat. This makes it strikingly efficient: estimates put a firefly's light production at roughly 90% efficient, compared with an old incandescent bulb's roughly 5%, since a filament bulb wastes most of its energy as heat rather than light. LED lighting, developed only in the last few decades, is the first human technology to begin closing that efficiency gap.",{"id":411,"type":47,"variant":412,"title":413,"markdown":414},"careful-laser","careful","Never point a laser at an eye","Even a small laser pointer concentrates its light far more tightly than an ordinary bulb, and can damage the retina in a fraction of a second if pointed directly into an eye — including someone else's, a pilot's, or your own reflection in a mirror. Treat every laser, however small, as something to point only at a wall or a screen, never towards a face.",{"id":416,"type":160,"component":417,"componentVersion":5,"config":418,"objective":420,"textAlternative":421,"help":422},"lab-laser-ray","light-ray",{"initialAngle":192,"showNormal":419,"showAngles":419},false,"See a perfectly straight, undiverging beam — the geometric idealisation a laser approaches closely in practice.","A single ray travelling dead straight with no spreading, standing in for how closely a real laser beam approaches the idealised ray model used throughout this whole topic. An ordinary torch or bulb, by contrast, sends rays out from its source in every direction at once — a laser is the closest real-world object to \"drawing a single ray\" that physics actually offers.",{"hints":423},[424,425],"A laser stays narrow over long distances because its light waves stay in step (coherent).","An ordinary bulb's rays spread out because its light is incoherent and multi-wavelength.",{"id":427,"type":53,"title":428,"eyebrow":429,"navLabel":430},"e-ch5","Project: measure the speed of light with a chocolate bar","Chapter 05","5 Chocolate project",{"id":432,"type":43,"markdown":433},"chocolate-setup","Here is a real experiment you can try, using a microwave oven, a chocolate bar (or cheese, or marshmallows) and a ruler. It measures the speed of light using nothing more advanced than arithmetic — no lasers, no astronomy, no spinning wheels.",{"id":435,"type":436,"title":437,"items":438},"steps-chocolate","steps","The chocolate-bar experiment, step by step",[439,443,447,451,455,459],{"title":440,"tag":441,"text":442},"Remove the turntable","step 1","A microwave oven's turntable spreads the heating evenly, which hides the pattern this experiment needs. Take it out, with an adult's help, or prop the dish so it cannot spin.",{"title":444,"tag":445,"text":446},"Lay chocolate flat on a microwave-safe dish","step 2","A thin, even layer works best — break up a bar and lay the pieces edge to edge.",{"title":448,"tag":449,"text":450},"Heat in short bursts","step 3","Run the microwave for 10–20 seconds at a time, checking after each burst, until you see a few clearly melted spots rather than the whole bar melting evenly.",{"title":452,"tag":453,"text":454},"Measure the gap between melted spots","step 4","The melted spots mark the **hot points** of the standing microwave pattern inside the oven, spaced by exactly half a wavelength.",{"title":456,"tag":457,"text":458},"Find the oven's frequency","step 5","Almost every microwave oven has its operating frequency printed on a label inside the door or on the back plate — typically 2.45 GHz (2.45 × 10⁹ hertz).",{"title":460,"tag":461,"text":462},"Calculate","step 6","Wavelength = 2 × (gap between melted spots). Speed = frequency × wavelength.",{"id":464,"type":99,"title":465,"problem":466,"steps":467,"help":472},"we-chocolate","Working the chocolate-bar numbers","A student measures a gap of exactly 6 cm between two melted spots, and their oven's label reads 2.45 GHz. What speed do they calculate, and how close is it to the accepted value?",[468,469,470,471],"Wavelength = 2 × 6 cm = 12 cm = 0.12 m.","Speed = frequency × wavelength = 2.45 × 10⁹ Hz × 0.12 m.","That gives about **2.94 × 10⁸ m\u002Fs**.","Compared with the true value of 2.998 × 10⁸ m\u002Fs, that is within about 1.9% — remarkably close, from a kitchen appliance and a ruler.",{"simplerExplanation":473},"Twice the gap between melted spots is one full microwave wavelength. Multiply that by the oven's printed frequency and you get a speed close to light's true speed.",{"id":475,"type":47,"variant":412,"title":476,"markdown":477},"careful-chocolate","Only with an adult, and only briefly","This experiment needs an adult present: never run a microwave oven with metal inside, never run it empty for long periods, and only heat food in the short bursts the steps describe. The chocolate is a measuring tool here, not a snack afterwards — it has been in an oven with its turntable disabled and may have hot spots.",{"id":479,"type":47,"variant":480,"title":481,"markdown":482},"example-green-flash","example","A rarer relative: the green flash","Occasionally, right at the instant the Sun's last sliver disappears below a perfectly clear, distant horizon (often over the sea), observers report a brief flash of green light. Atmospheric refraction bends different colours by very slightly different amounts, in effect creating a series of overlapping images of the Sun's disc in different colours, stacked vertically; the last visible sliver, exactly when the red and orange images have already dropped from view, can briefly show as green rather than red. It lasts a second or two, needs an exceptionally clear and distant horizon, and is real — not a myth — though far rarer to actually witness than to hear about.",{"id":484,"type":53,"title":485,"eyebrow":486,"navLabel":487},"e-ch6","A puzzle: the sunset that has already happened","Chapter 06","6 Bent sunsets",{"id":489,"type":43,"markdown":490},"atmospheric-setup","Here is a genuine puzzle. The Earth's atmosphere is not uniform: it is denser near the ground and thinner higher up. Light travelling through a gradually changing density bends gradually too, curving very slightly downward as it approaches the ground — which means it curves the *image* of a distant object slightly *upward* as you see it.",{"id":492,"type":493,"prompt":494,"options":495,"explanation":508},"pred-sunset-refraction","prediction","Because of this atmospheric bending, when you watch the Sun's lower edge appear to just touch the horizon at sunset, where is the Sun's true, geometric position?",[496,499,502,505],{"id":497,"label":498},"a","Exactly where you see it — the atmosphere has no effect this close to the horizon",{"id":500,"label":501},"b","Already below the geometric horizon — you are seeing a bent image of a Sun that has, in a strict geometric sense, already set",{"id":503,"label":504},"c","Still well above the horizon; the visible position lags behind the true one",{"id":506,"label":507},"d","The effect only matters for stars, never for something as bright as the Sun","**b.** Atmospheric refraction lifts the apparent position of anything near the horizon by about 34 arcminutes (a little over half a degree) — more than the Sun's own angular width. The Sun you watch \"touching\" the horizon has, geometrically, already dropped below it; you are seeing a refracted image of where it used to be moments ago. Combined with the Sun's own steady crawl across the sky, this bending adds roughly **2.3 extra minutes** of visible daylight at both sunrise and sunset than pure geometry alone would give.",{"id":510,"type":99,"title":511,"problem":512,"steps":513,"help":518},"we-extra-daylight","How much extra daylight does bending give you?","Atmospheric refraction lifts objects near the horizon by about 34 arcminutes (34⁄60 = 0.567°). The Sun crosses the sky at about 360° in 24 hours. How many extra minutes of visible daylight does this bending add, roughly?",[514,515,516,517],"The Sun's apparent speed across the sky: 360° ÷ (24 × 60 minutes) = 360 ÷ 1,440 = 0.25° per minute.","Time for the Sun to cross the refraction-lift angle: 0.567° ÷ 0.25° per minute.","That gives about **2.3 minutes**.","Applied at both sunrise (the Sun becomes visible slightly before it geometrically should) and sunset (it stays visible slightly after), this single bending effect adds a few extra minutes of daylight to every single day, everywhere on Earth, every day of the year.",{"anotherExample":519},"The same bending is why the low Sun and Moon often look slightly squashed (oval) near the horizon: refraction lifts the bottom edge more than the top edge, since the bottom edge is fractionally closer to the horizon.",{"id":521,"type":160,"component":417,"componentVersion":5,"config":522,"objective":525,"textAlternative":526,"help":527},"lab-refraction-ray",{"initialAngle":523,"showNormal":524,"showAngles":524},5,true,"A simplified single-boundary model of the much more gradual bending that really happens across the whole atmosphere.","A ray box showing one sharp boundary bending a nearly horizontal ray slightly upward — a simplified stand-in for what really happens gradually, over many kilometres, as starlight or sunlight crosses air of continuously increasing density on its way down to the ground. Real atmospheric refraction has no single sharp boundary at all; it is a smooth curve, strongest exactly at the horizon where the path through the densest air is longest.",{"hints":528},[529,530],"Real atmospheric refraction curves gradually rather than bending sharply once.","The effect is strongest for objects right at the horizon and disappears for objects directly overhead.",{"id":532,"type":47,"variant":480,"title":533,"markdown":534},"example-mirage","A cousin of the same bending: the road mirage","On a hot Indian highway at midday, a shimmering \"puddle\" that seems to sit on the road ahead and always retreats as you approach is a **mirage**, and it is the same family of effect as the extra daylight above: light bending because it is crossing air of rapidly changing density. A layer of very hot, thin air right above the scorching road refracts light from the sky so strongly that it curves upward into your eye, and your brain — always assuming light travels in straight lines — interprets that bent ray as reflected light from a wet surface on the ground. There is no water there at all.",{"id":536,"type":47,"variant":201,"title":537,"markdown":538},"misc-mirage-water","\"A mirage is an illusion your eyes make up\"","Your eyes and brain are not inventing anything: real light really is arriving at your eye along a genuinely bent path, refracted exactly as the physics predicts. The *interpretation* — \"that must be reflected light off water\" — is the only part that is mistaken, because your brain's straight-line assumption about light is normally an excellent one that simply fails in this one unusual situation of a very sharp temperature gradient just above the road.",{"id":540,"type":53,"title":541,"eyebrow":542,"navLabel":543},"e-ch7","Light-years and looking into the past","Chapter 07","7 Look into the past",{"id":545,"type":43,"markdown":546},"lookback-setup","Because light takes real time to travel, every single thing you look at, you see slightly in the past — and the farther away it is, the further back in time you are looking. A **light-year** is simply the distance light travels in one year: about 9.46 trillion kilometres.",{"id":548,"type":62,"caption":549,"columns":550,"rows":554},"table-lookback","How far into the past you are looking, for various objects",[551,552,553],"Object","Distance","Light-travel time",[555,559,563,567,571,575],[556,557,558],"The Moon","384,400 km","1.28 seconds",[560,561,562],"The Sun","150 million km (1 AU)","8 minutes 19 seconds",[564,565,566],"Jupiter (at opposition)","about 4.2 AU","about 35 minutes",[568,569,570],"Proxima Centauri (nearest star)","4.25 light-years","about 4 years 3 months",[572,573,574],"Sirius (brightest star in the night sky)","8.6 light-years","about 8 years 7 months",[576,577,578],"The Andromeda Galaxy (naked-eye visible)","2.5 million light-years","about 2.5 million years",{"id":580,"type":47,"variant":155,"title":581,"markdown":582},"aha-andromeda","The most distant thing you can see with just your eyes","On a dark, clear night, away from city lights, the Andromeda Galaxy is faintly visible to the naked eye as a smudge — and the light entering your eye left it about 2.5 million years ago, long before modern humans existed. You are not seeing Andromeda as it is now; nobody alive will ever see that. You are seeing a snapshot mailed across space two and a half million years ago, only just arriving.",{"id":584,"type":99,"title":585,"problem":586,"steps":587,"help":592},"we-proxima-conversation","Why you could never have a normal conversation with Proxima Centauri","Proxima Centauri, the nearest star beyond the Sun, is 4.2465 light-years away. If you sent a radio message there (radio waves travel at the same speed as light) and someone replied the instant they received it, how long would you wait for the reply?",[588,589,590,591],"A radio message takes 4.2465 years to arrive at Proxima Centauri.","Their instant reply then takes another 4.2465 years to get back to Earth.","Total round trip: 4.2465 + 4.2465 = **about 8.5 years**.","Even with a reply sent back the very instant the message arrives, with zero thinking time at all, over eight years pass before you hear back — which is exactly why any real conversation across interstellar distances, with any star at all, is fundamentally impossible in the way a phone call is: the round-trip delay alone rules it out.",{"simplerExplanation":593},"Double the one-way light-travel time to get the round trip: 4.2465 years there, 4.2465 years back, about 8.5 years total.",{"id":595,"type":47,"variant":480,"title":596,"markdown":597},"example-parallax","How anyone actually measured Proxima Centauri's distance","Nobody stretched a tape measure to another star. Astronomers use **parallax**: photograph a nearby star twice, six months apart, when Earth has moved to the opposite side of its orbit around the Sun (a known baseline of about 300 million km), and measure the tiny apparent shift of the nearby star against the much more distant, essentially fixed background stars. A little trigonometry with that shift and that baseline gives the distance directly — the same idea as closing one eye, then the other, and watching a nearby finger appear to jump against a distant wall, scaled up by roughly ten trillion times.",{"id":599,"type":600,"itemId":601,"prompt":602,"check":603,"hints":607,"feedback":610},"prac-lookback","practice","light.extend-lookback","Betelgeuse, a bright red star, is about 640 light-years away. If it exploded as a supernova today, roughly how many years would pass before anyone on Earth could see it happen?",{"kind":604,"answer":605,"tolerance":523,"unit":606},"number",640,"years",[608,609],"Light-travel time in years is simply the distance in light-years.","No extra calculation is needed — light-years already measure time as well as distance.",{"correct":611,"incorrect":612},"Right: a distance of 640 light-years means the light takes 640 years to arrive, so an explosion today would not be visible from Earth for about 640 years.","A light-year is the distance light travels in one year, so a distance given in light-years directly gives the light-travel time in years.",{"id":614,"type":53,"title":615,"eyebrow":616,"navLabel":617},"e-ch8","Open questions and where light takes people","Chapter 08","8 Open questions",{"id":619,"type":43,"markdown":620},"careers-setup","Studying light seriously opens onto real careers and real unanswered questions — not just tidy textbook problems. First, two puzzles to try before you read the fields and questions below.",{"id":622,"type":99,"title":623,"problem":624,"steps":625,"help":629},"we-puzzle-satellite","Puzzle: the satellite phone call delay","A television signal is bounced up to a geostationary satellite, 35,786 km overhead, and straight back down to a receiving dish. Using distance ÷ speed, how long does that one-way hop take, and why do satellite phone calls have such a noticeable pause?",[626,627,628],"One-way time = altitude ÷ speed of light = 35,786 km ÷ 299,792.458 km\u002Fs ≈ **119 milliseconds**, just over a tenth of a second.","A phone call needs the signal to go up and back down **twice** — once for each direction of the conversation — so a single exchange (your voice up and down, the reply up and down) involves four of these hops in quick succession.","Even a single round trip (signal up, back down, reply up, back down) adds up to roughly **476 milliseconds** of pure travel time, well over a third of a second — long enough for two speakers to accidentally talk over each other, which is the exact, distinctive lag people notice on a satellite call.",{"simplerExplanation":630},"A satellite this far away adds a real, noticeable time delay just from light's travel time — nothing to do with poor equipment.",{"id":632,"type":47,"variant":633,"title":634,"markdown":635},"tryit-puzzle-mirrors","try_it","Puzzle: how many reflections of yourself?","Stand between two parallel mirrors (a bathroom cabinet mirror facing a hallway mirror often works) and count how many images of yourself you can see, getting smaller and dimmer into the distance.\n\nIn theory, perfectly parallel, perfectly reflective mirrors would give **infinitely** many images, each one dimmer than the last because every single reflection loses a small fraction of its light. In practice you will run out of visible images long before infinity, usually after 10–30, once the light lost at each bounce finally makes the image too dim to see. Try counting, then try again after cleaning the mirrors — a cleaner mirror reflects more efficiently and should let you count a few more.",{"id":637,"type":62,"caption":638,"columns":639,"rows":642},"table-careers","A few fields built on understanding light",[640,641],"Field","What it uses",[643,646,649,652,655,658],[644,645],"Optometry and ophthalmology","Lens power, refractive errors, the eye's optics",[647,648],"Astronomy and astrophysics","Telescopes, spectra, look-back time, redshift",[650,651],"Fibre-optic and telecom engineering","Total internal reflection, signal loss, latency",[653,654],"Photography and cinematography","Lenses, exposure, colour, dynamic range",[656,657],"Remote sensing (ISRO and others)","Satellites reading reflected and emitted light across many wavelengths, including infrared, to monitor crops, water and weather",[659,660],"Quantum information science","Individual photons used to carry and process information",{"id":662,"type":47,"variant":663,"title":664,"markdown":665},"question-open-light","question","Open questions for curious learners","Nobody has fully settled these. Pick one and dig in.\n\n- Quantum mechanics describes light with astonishing accuracy, but physicists still argue about what that mathematics actually *means* about reality. What would it take to settle the argument?\n- Could a message ever be sent faster than light by any trick at all? (So far, every serious proposal has failed — why do you think that keeps happening?)\n- Optical computers would process information with light instead of electricity. What would have to be true for that to beat today's electronic computers?\n- If you could design one new use for total internal reflection that does not exist yet, what would it be?\n- Bees and some other animals can see ultraviolet light invisible to us. What might the world look like through their eyes, and how would you even test a guess about that?",{"id":667,"type":53,"title":668,"eyebrow":669,"navLabel":670},"e-ch9","Pulling it together","Chapter 09","9 Pulling it together",{"id":672,"type":673,"title":674,"terms":675},"gloss-extend","glossary","Terms from this lesson",[676,679,683,687,691,695,698],{"term":677,"meaning":678},"Electromagnetic spectrum","The full family of waves that includes radio, microwave, infrared, visible light, ultraviolet, X-rays and gamma rays, all travelling at the speed of light.",{"term":680,"meaning":681,"example":682},"Diffraction","Waves spreading slightly around obstacles or through narrow gaps — evidence for light's wave nature.","A CD's rainbow, or light spreading through a narrow slit.",{"term":684,"meaning":685,"example":686},"Interference","Two overlapping waves reinforcing (brighter) or cancelling (darker) depending on their alignment.","Young's double-slit banding pattern.",{"term":688,"meaning":689,"example":690},"Photon","A discrete packet of light energy, with energy E = h × f.","A green laser emits billions of billions of photons every second.",{"term":692,"meaning":693,"example":694},"Photoelectric effect","Light knocking electrons out of a metal, only above a minimum frequency — evidence for light's particle nature.","Explained by Einstein in 1905.",{"term":696,"meaning":697},"Wave–particle duality","The genuinely quantum fact that light (and matter) shows wave behaviour in some experiments and particle behaviour in others, with no contradiction.",{"term":699,"meaning":700,"example":701},"Light-year","The distance light travels in one year, used to measure vast astronomical distances.","Proxima Centauri is 4.25 light-years away.",{"id":703,"type":704,"prompt":705},"reflect-extend","reflection","Wave–particle duality means light is not really \"a wave\" or \"a particle\" in the everyday sense of either word — those are just the closest analogies from everyday experience. Can you think of another idea in science (or outside science) where the best available words for something do not quite capture what it really is?",{"id":707,"type":160,"component":708,"componentVersion":5,"config":709,"objective":725,"textAlternative":726,"help":727},"lab-extend-match","match-pairs",{"prompt":710,"mode":711,"pairs":712},"Match each extend-layer term to its meaning.","connect",[713,715,717,719,721,723],{"a":680,"b":714},"Waves spreading round obstacles or through narrow gaps",{"a":684,"b":716},"Overlapping waves reinforcing or cancelling",{"a":688,"b":718},"A discrete packet of light energy, E = h × f",{"a":692,"b":720},"Light knocking out electrons only above a threshold frequency",{"a":699,"b":722},"The distance light travels in one year",{"a":696,"b":724},"Light shows wave behaviour in some experiments, particle behaviour in others","Match six wave-and-particle terms to their meanings.","A connect-the-pairs game covering this layer's core vocabulary: diffraction, interference, photon, the photoelectric effect, the light-year, and wave–particle duality.",{"hints":728},[729,730],"Diffraction and interference are both wave behaviours.","The photoelectric effect is the strongest evidence for light's particle side.",{"id":732,"type":733,"title":734,"questions":735},"quiz-extend","quiz","Test what you worked out",[736,749,762,774,787],{"itemId":737,"prompt":738,"options":739,"correct":500,"why":748},"light.extend-q-ems","Compared with visible light, radio waves have:",[740,742,744,746],{"id":497,"label":741},"A much shorter wavelength",{"id":500,"label":743},"A much longer wavelength",{"id":503,"label":745},"Exactly the same wavelength",{"id":506,"label":747},"No wavelength at all","Radio waves are metres to kilometres long, far longer than visible light's few hundred nanometres.",{"itemId":750,"prompt":751,"options":752,"correct":500,"why":761},"light.extend-q-young","Young's double-slit experiment provided strong evidence that light behaves as a:",[753,755,757,759],{"id":497,"label":754},"Particle",{"id":500,"label":756},"Wave",{"id":503,"label":758},"Neither — it showed light does not exist",{"id":506,"label":760},"Solid","The bright-and-dark interference banding is a defining wave behaviour with no simple particle explanation.",{"itemId":763,"prompt":764,"options":765,"correct":497,"why":773},"light.extend-q-photon","A photon's energy is directly proportional to its:",[766,767,769,771],{"id":497,"label":114},{"id":500,"label":768},"Colour of the surrounding room",{"id":503,"label":770},"Distance travelled",{"id":506,"label":772},"Speed, which varies by photon","E = h × f: energy rises directly with frequency. All photons in vacuum travel at the same speed, c.",{"itemId":775,"prompt":776,"options":777,"correct":503,"why":786},"light.extend-q-daylight","Atmospheric refraction near the horizon:",[778,780,782,784],{"id":497,"label":779},"Has no effect on how long the Sun appears to be up",{"id":500,"label":781},"Shortens daylight by bending light away from the ground",{"id":503,"label":783},"Slightly lengthens daylight by lifting the Sun's apparent position near the horizon",{"id":506,"label":785},"Only affects the Moon, never the Sun","The atmosphere's bending lifts objects near the horizon, adding a couple of minutes of apparent daylight at both sunrise and sunset.",{"itemId":788,"prompt":789,"options":790,"correct":500,"why":799},"light.extend-q-lightyear","A light-year measures:",[791,793,795,797],{"id":497,"label":792},"A very long time",{"id":500,"label":794},"A distance: how far light travels in one year",{"id":503,"label":796},"A unit of brightness",{"id":506,"label":798},"The lifespan of a star","Despite the name, a light-year is a unit of distance, not time — the distance light covers in one year.",{"id":801,"type":802,"title":803,"points":804},"cheat-extend","summary","Cheat sheet",[805,806,807,808,809,810],"**Electromagnetic spectrum:** radio, microwave, infrared, visible, ultraviolet, X-ray, gamma — all the same phenomenon, differing only in wavelength, all travelling at c.","**Wave evidence:** diffraction (bending round edges) and interference (Young's double slit, a CD's rainbow) — both need overlapping waves, not particles.","**Particle evidence:** the photoelectric effect and single-photon detection. E = h × f: a photon's energy depends only on frequency.","**Wave–particle duality:** light genuinely shows both behaviours; it is a quantum object, not secretly one or the other.","**Atmospheric refraction** lifts objects near the horizon by about 34 arcminutes, adding a couple of minutes of daylight at sunrise and sunset.","**Light-year:** the distance light travels in a year, about 9.46 trillion km. Looking far away always means looking into the past.",{"id":812,"type":813,"conceptId":814,"relation":815,"explanation":816},"conn-extend-electricity","connection","electricity","related_to","Maxwell's 19th-century equations, which first showed light is an electromagnetic wave, are the very same equations behind how alternating current and generators work.",{"id":818,"type":813,"conceptId":819,"relation":820,"explanation":821},"conn-extend-body","human-body-anatomy","applied_in","Detecting single photons in very low light is close to the physical limit of what the eye's retina can do — some experiments suggest a dark-adapted human eye can detect only a handful of photons.",{"id":823,"type":824,"sourceIds":825},"sources-extend","sources",[826,827,828,829,830],"light-nasa-ems","light-nasa-visible","light-britannica-light","light-wikipedia-speed-of-light","light-hyperphysics-rainbow",[826,827,828,829,830],"needs_review",{"generatedBy":834,"notes":835},"claude-code","Draft generated locally; every number computed and asserted in scratchpad\u002Flight\u002Fnumbers.py. Pending owner review.","a64c8d23e3d9e69aaa6eea91393630f239340437ee9b13f9e68f437bb3588324",{"component:sort-game@1":838,"component:prism-lab@1":839,"component:light-ray@1":840,"logic:practice":841,"component:match-pairs@1":842,"source:light-britannica-light":843,"source:light-hyperphysics-rainbow":844,"source:light-nasa-ems":845,"source:light-nasa-visible":846,"source:light-wikipedia-speed-of-light":847},"b164f45a2c8ca08f26c450768ff0231e113e9fe45381eddb34dc6d0548596c38","835c8ba7fd707c60ded46494f4b672aa095199ba9a2628409905335d0b3434a9","d66d44021dc28328ce2ea82e0d6cefc9466b5dc65dbb93c92b05b88161725be6","3d6b0fe1b15255975a32b0fcd94e8019bc959ad45cbf12e136e86149549c6878","2a8ee4ac87460b4e1175a4bb13c96b03d577db06dde95670eb7fcfe4ad787899","0e76ba316118067e955dbaf8cbbabc23d388ad0bf547c518be84bcc95923d376","dce907528f01e833f82d68150b423cc68c60d1c9c88673a96a84eb269bbbd3ce","c7ff6424fcd7299b6942c576c43b211fd02ef110e142a0947dba5a70759c03e6","711ba457ef893111775bd5fd9bfd09632c8ef20e5e7708971e06754f5ef7f4bd","e6620f59bd5a24800f47530a4ed586390c383a730742e4e6bdf5fda99401c43a",{"state":849,"reviewer":850,"selfReview":524,"reviewedAt":851,"method":852},"approved","The library owner","2026-09-20T10:18:37.581Z","owner_bulk","preview-7e1cbbcc4f",1789899598934]