Waves, particles and the light you cannot see

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.

This 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.

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.

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.

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.

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.

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.

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.

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.

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.