The Story of Light

Let There Be Light

Imagine waking up in a room with no windows, no lamps and no light at all. You would see nothing — not your hands, not the walls, not even the room itself. Without light, the world simply vanishes into darkness.

Light is so much a part of every day that we hardly think about it. But light is one of the most wonderful things in nature. It lets us see, it brings warmth, it paints the world in dazzling colours, and it gives plants the energy to grow. This book tells the story of light: where it comes from, what it does, and the surprising secrets it hides.

Where Does Light Come From?

Anything that makes its own light is called a light source. The biggest and most important light source of all is the Sun. The Sun is a giant ball of hot, glowing gas, and the light it sends across space gives us daytime, warmth and life itself.

There are many other light sources too. A burning candle makes light. A campfire glows. A flash of lightning lights up the whole sky for a moment. And humans have invented light sources of our own, like light bulbs, torches and the screens of phones and televisions.

Some living things can even make their own light! Fireflies flash in the dark to send signals to each other, and deep in the ocean, where no sunlight reaches, strange glowing fish create their own gentle light. Making light from inside a living body is called bioluminescence.

How Do We See?

Here is a question that may surprise you. Most of the things around you — your chair, your book, your friend's face — do not make any light at all. So how do we see them?

The answer is that light bounces. Light pours out of a source like the Sun or a lamp, travels through the air, hits an object, and then bounces off it. Some of that bounced light travels into your eyes. Your eyes catch the light and send a message to your brain, which turns it into the picture you see.

This is why you cannot see anything in a pitch-black room. There is no light to bounce off the objects and reach your eyes. Switch on a lamp, and suddenly the whole room appears, because now light is bouncing everywhere.

A mirror is special because it is very smooth and shiny. Instead of scattering light in all directions, it bounces light back in a neat, even way. That is why a mirror shows you a clear reflection of yourself.

The Colours Hidden in Light

Sunlight looks plain and white. But hidden inside that white light is one of nature's best secrets: white light is actually made of all the colours mixed together.

You can see this for yourself in a rainbow. After it rains, the sky is full of tiny floating raindrops. When sunlight shines through them, each raindrop splits the white light into its separate colours. They always appear in the same order: red, orange, yellow, green, blue, indigo and violet.

So why is grass green and why is a strawberry red? It is all about which colours an object keeps and which it bounces back. A leaf soaks up most of the colours in light but bounces back the green, so we see it as green. A red strawberry bounces back red light and absorbs the rest. The colour of everything you see depends on which colours of light it sends back to your eyes.

Shadows: Where Light Cannot Go

Light has one important rule: it travels in straight lines. It cannot bend around corners on its own. This simple fact is what creates shadows.

When something solid gets in the way of light, the light cannot pass through it. So a dark patch appears on the other side, in the exact shape of the object. That dark patch is a shadow.

You can play with shadows easily. Stand outside on a sunny day and you will see a shadow of yourself on the ground. Notice how your shadow changes during the day. In the early morning and evening, when the Sun is low in the sky, shadows are very long. At midday, when the Sun is high overhead, shadows are short. By watching shadows move, people long ago even invented the first clocks, called sundials.

Materials that block light completely, like wood or your own body, are called opaque. Some materials, like frosted glass, let some light through and are called translucent. And materials you can see straight through, like clear glass or clean water, are called transparent.

Light, Heat and Life

Light does much more than help us see. It also carries energy — and that energy is what keeps our whole world alive.

The Sun's light warms the land, the seas and the air, giving us comfortable temperatures to live in. Without it, Earth would be a frozen, lifeless ball.

Even more importantly, plants capture sunlight and use it to make their own food in a process called photosynthesis. Plants are like tiny living factories that turn light into food. Then animals eat the plants, and other animals eat those animals. So nearly all the energy in every living thing began as sunlight. When you eat your lunch, you are really eating energy that came from the Sun.

The Fastest Thing in the Universe

Light has one last astonishing secret: nothing in the entire universe travels faster than light.

Light moves so quickly that it crosses an ordinary room before you could even blink. It could zoom all the way around the Earth more than seven times in a single second. The sunlight warming your face right now actually left the Sun about eight minutes ago and raced across space to reach you.

This means that when we look up at the stars at night, we are looking at very old light. Some stars are so far away that their light has been travelling for thousands of years to reach our eyes. When you gaze at the night sky, you are looking back in time.

What We Learned

Light comes from sources like the Sun, flames and bulbs. We see the world because light bounces off objects and into our eyes. White light is secretly made of all the colours of the rainbow, and the colour of everything depends on which light it sends back to us. Light travels in straight lines, which is why shadows form, and it carries the energy that keeps every living thing alive. Light truly is one of the great wonders of nature.

To see how light from the Sun shapes our days, explore Weather and the Sky. Or look up and discover the distant stars in Understanding Our Universe.