Light: how you can see anything at all

Close your eyes. The room is still there — the table, the window, the person next to you — but you cannot see any of it.

Open them. Everything comes back at once.

Nothing about the room changed. What changed is that light is once again getting into your eyes. Light is the messenger that carries the world to you, and it is doing it right now, millions of times over, from every direction, faster than anything else in the universe.

This lesson is about what light is, where it comes from, how it travels, and why it makes shadows, mirror images and rainbows.

There is one rule behind all of seeing:

You can only see something if light from it enters your eye.

Not light near it. Not light around it. Light from it, arriving at your eye and landing on the back of it.

That sounds obvious until you ask the next question: where did that light come from? Only two answers are possible.

1. The thing made the light itself — a flame, a bulb, the Sun, a firefly.
2. The thing bounced light that came from somewhere else — this page, your hand, a wall, a cricket ball, the Moon.

Almost everything you look at in a day is in group 2. The world is mostly dark objects catching and throwing back light from a few bright ones.

Hold up whatever you are reading this on and ask: is it luminous or not?

If it is a printed page, it is non-luminous. Light from a bulb or a window is falling on it, bouncing off the white paper in all directions, and a tiny fraction of that bounce happens to travel into your eye. Take the page into a cupboard and shut the door and the page vanishes, even though it is still white and still right in front of your nose.

If it is a screen, it is luminous. Millions of tiny red, green and blue lamps are making light and firing it at you. That is why a phone still works in a dark room, and why it is tiring to read in the dark: your eye is staring at a small bright thing in a black surround.

Light sources split into natural and artificial.

Natural sources were here before us: the Sun above all, then the stars, lightning, forest fire, glowing lava, and living things that make their own light — fireflies, some fungi, and a great many deep-sea creatures.

Artificial sources are ones we built. For most of human history that meant fire: a burning stick, then oil in a clay diya, then a wax candle, then a kerosene lantern. All of these work the same way — something burns, the soot in the flame gets white-hot, and hot things glow.

Then, in the last 150 years, we learned to make light with electricity instead, and the light in your home stopped being a fire.

Here is the second big idea, and nearly everything else in this lesson falls out of it:

Light travels in straight lines.

You have seen the evidence without noticing it. A beam of sunlight slipping through a gap in a curtain into a dusty room is a straight bar of light, not a curve. The beam from a car's headlamp on a foggy night is straight. The cone of light from a cinema projector to the screen is straight. Sunbeams breaking through a gap in monsoon clouds come down in straight spokes.

In each case you are not really seeing the light itself — you are seeing dust or fog or water droplets in the beam, each speck bouncing a little of it towards your eye. The light going past you is invisible. A laser beam in clean air is invisible from the side; add a puff of dust or a drop of milk in water and the straight line springs into view.

Put something in a beam of light and one of three things happens.

Almost all the light goes through, and you can see clearly through it: the material is transparent. Window glass, clean water, air, cling film, a clear plastic bottle.

Some light goes through but scrambled, so you get brightness without a picture: translucent. Tracing paper, frosted bathroom glass, a thin cotton kurta, oiled paper, a lampshade, mist.

No light goes through at all: opaque. Cardboard, wood, metal, a brick wall, your hand, this book, the Moon.

These are not fixed labels stuck to materials for ever. They depend on thickness. Water is transparent in a glass and opaque at the bottom of the ocean. Paper is translucent as one sheet and opaque as a ream.

A shadow is not a thing. It is an absence — the patch of a surface that light could not get to, because something opaque was in the way and light will not bend around it.

That is why a shadow needs exactly three ingredients, and fails if any one is missing:

1. a source of light,
2. an opaque object to block it,
3. a screen — a wall, the floor, the ground, a sheet — for the shadow to fall on.

Take away the screen and the shadow has nowhere to be. Take away the object and there is nothing to block the light. Switch off the source and everything is dark, which is not the same as a shadow.

Stand outside on a sunny day and your shadow tells the time.

Early in the morning the Sun is low in the east, so your shadow is long and stretches away to the west. By the middle of the day the Sun is high and your shadow is short and squat and points roughly north (if you are in India, north of the Tropic of Cancer). By evening the Sun is low in the west and your shadow is long again, reaching east.

The shadow swings round and changes length because the Sun moves across the sky, not because you moved. People noticed this a very long time ago and built the first clocks out of it: a stick in the ground, a marked stone, a sundial. Jaipur's Jantar Mantar has a sundial nearly 27 metres tall whose shadow crosses its marked scale fast enough to watch.

When light lands on a surface it can be absorbed, pass through, or bounce. Bouncing is called reflection, and every non-luminous thing you have ever seen is doing it.

What makes a mirror special is not that it reflects more light — white paper reflects nearly as much — but that it reflects it tidily. A mirror is flat and smooth down to a scale far smaller than you can see, so a neat bundle of rays arriving together leaves together, still in formation. The pattern survives the bounce, so you get a picture.

Paper looks white for the same reason it is useless as a mirror: its surface is a tangle of fibres pointing every which way, so each ray bounces off at its own angle and the picture is scrambled into an even glow. Lots of light, no image.

Stand a straight straw in a glass of water and look from the side. The straw appears bent at the water line, and the bit under the water looks fatter and shifted sideways. Lift it out: perfectly straight. Put it back: bent again.

Nothing is happening to the straw. Something is happening to the light on its way out.

Light travels a little slower through water than through air, and when it crosses the boundary at a slant it changes direction — it refracts. Your brain does not know that. It traces the rays back in straight lines and places the underwater part of the straw somewhere it is not.

The same bending makes a coin at the bottom of a bucket look nearer the surface than it is, makes a swimming pool look shallower than it is, and is the whole reason spectacles, magnifying glasses, cameras and your own eyes work at all.

White light is not plain. It is a mixture, and you can take it apart.

Send a narrow beam of sunlight through a triangular block of glass — a prism — and it comes out as a fan of colours: red, orange, yellow, green, blue, indigo, violet, blending smoothly into one another. That fan is called a spectrum, and the splitting is called dispersion.

The prism does it by bending each colour by a slightly different amount. Violet is bent most, red least, so they arrive at different places on the wall.

A rainbow is the same event, done by millions of raindrops instead of one piece of glass. Each drop takes in sunlight, splits it, bounces it off its inside and sends it back out — and if you are standing in the right place, the reds of some drops and the violets of others all arrive at your eye together as an arc.

Light is the fastest thing there is. In empty space it travels 299,792,458 metres every second, which everyone rounds to 3 × 10⁸ m/s, or about 300,000 kilometres per second.

That number is hard to feel, so try these:

• In one second, light could go round the Earth's equator nearly 7.5 times.
• In the time it takes you to say "one", light could travel from Delhi to Chennai and back about 130 times.
• Light is roughly 874,000 times faster than sound. That is why you see the lightning and then wait for the thunder.

And yet space is so big that even this speed takes time. Sunlight takes about 8 minutes 20 seconds to reach us. So you never see the Sun as it is; you see it as it was when you were eight minutes younger. If it went out this instant, you would enjoy eight more minutes of a perfectly ordinary afternoon.

Once you start noticing light, India is full of it.

Diyas at Diwali. Rows of tiny oil flames, each one a luminous source, each one throwing flickering shadows because it is small and close. A flame is the oldest lamp there is: hot soot glowing white.

Rangoli. Coloured powder laid on the ground is entirely non-luminous and looks its best in bright, low morning sunlight, when the colours reflect strongly and the grains throw tiny shadows that make the pattern look textured.

Mirror work. The tiny mirrors sewn into Kutchi and Rajasthani embroidery, and the sheesh mahal mirrored halls in old palaces, are built entirely on reflection: hundreds of small flat mirrors, each throwing back a lamp flame, so that one candle becomes a room full of stars.

Photography and cinema. A camera is a light-tight box with a lens; a cinema projector is a very bright lamp throwing a straight cone at a screen. India makes more films than any other country, and every one of them is an argument about where to put the light.