Light

What is light?

Close your eyes. Everything goes dark. Open them. The room comes back: the colours, the shapes, your friend's face. What changed? Light.

Light is a form of energy that lets us see. Without light there is no seeing at all. In a room with no windows and no lamp, you cannot see your own hand, no matter how close you hold it.

Light comes from the sun, from fire, from lamps and torches and screens. It travels out from these sources, bounces off the things around us, and enters our eyes. That is how seeing works, and in this book you will find out exactly how each step happens.

You will also learn why shadows have the shape they do, why a mirror shows you backwards, and why a spoon in a glass of water looks bent.

Try this: Sit in a room and list everything that is giving off light right now.

Sources of light

A source of light is anything that makes its own light.

Natural sources are made by nature:

• The sun, by far the brightest
• The stars, which are suns very far away
• Fire and lightning
• Some animals, such as fireflies and certain deep-sea fish

Artificial sources are made by people:

• Electric bulbs and LED lamps
• Torches and phone screens
• Candles and kerosene lamps
• Television and computer screens

Some things look bright but are not sources. The moon does not make its own light. It reflects sunlight, like a giant mirror in the sky. A white shirt in the sun is bright, but it is only bouncing light back at you.

Try this: Make two lists: five sources of light, and five things that are bright but are not sources.

The sun, our biggest source

Almost all the light on Earth comes from the sun. It is a huge ball of burning gas, 150 million kilometres away, and yet its light reaches us in about eight minutes.

Sunlight gives us daytime. It makes plants grow. It warms the land and the sea. Even the light from a lamp at night came from the sun in a way, because the electricity was made from fuel or water or wind, and all of those got their energy from the sun.

Safety: Never look straight at the sun, not even for a second, and never through binoculars or a telescope. Its light is so strong that it can damage the back of your eye for ever.

Try this: Stand outside at midday and again at five o'clock. Which time feels brighter? Which time feels warmer?

Light at night in Ghana

When the sun sets, we need other sources. In towns, electric bulbs light the streets and homes. In many villages, and when the lights go off, people use other things.

A kerosene lamp burns oil through a wick. The flame is the source of light. The glass around it stops the wind blowing it out and spreads the light.

A candle burns wax. A torch uses a battery to light a small bulb. A solar lamp stores sunlight in a battery during the day and gives it back at night.

Each source is also a source of heat, except the LED torch, which stays cool. That is one reason LED lights are so useful: they do not waste energy as heat.

Try this: Ask an older person what they used for light at night when they were your age.

Light travels in straight lines

Light does not bend round corners. It moves in straight lines from its source. Scientists call these lines rays.

You can see this when sunlight comes through a gap in the clouds or a crack in a roof. The beam is straight. Dust in the air shows the path, like a ruler drawn in the sky.

Because light goes straight, you cannot see round a corner. If Kofi hides behind a wall, the light bouncing off him hits the wall and never reaches your eyes.

This one simple rule explains shadows, mirrors, periscopes and the pinhole camera. Remember it: light travels in straight lines.

Experiment: the three cards

You can prove that light travels in straight lines with three pieces of card.

You need: three pieces of card the same size, a pencil, some modelling clay or three lumps of dough to stand the cards up, and a torch or a candle.

Steps:

1. Make a hole in the middle of each card with the pencil, in exactly the same place.
2. Stand the cards in a row, about 10 cm apart, with the holes in a straight line.
3. Put the torch at one end. Look through the holes from the other end. You see the light.
4. Now move the middle card a little to one side. Look again.

What happens: The light disappears. The holes are no longer in a straight line, so the light cannot get through.

Why: If light could bend, it would curl round and find the hole. It cannot. It goes straight, hits the card, and stops.

Try this: Replace the torch with the window on a sunny day. Does it still work?

Transparent, translucent, opaque

What happens when light hits something depends on what the thing is made of.

Transparent things let almost all the light straight through. You can see clearly through them. Clear glass, clean water, clear plastic, air.

Translucent things let some light through, but scatter it, so you see only a blur. Frosted glass, thin paper, a plastic water sachet, a thin curtain.

Opaque things let no light through at all. Wood, metal, a brick wall, your body, a book.

Only opaque things make a proper dark shadow. Translucent things make a faint, soft shadow. Transparent things make almost no shadow.

Worked example: Why can you see through a window but not through a door? The window is transparent glass; the door is opaque wood.

Try this: Hold five different objects up to a window. Sort them into transparent, translucent and opaque.

How shadows form

A shadow is the dark place where light cannot reach, because something opaque is in the way.

Remember: light travels in straight lines. When rays from the sun hit Ama, they stop. The rays that miss her carry on to the ground. So there is a bright ground all round and a dark Ama-shaped patch behind her. That patch is her shadow.

Three things are needed for a shadow:

1. A source of light
2. An opaque object
3. A surface for the shadow to fall on

The shadow is always on the opposite side of the object from the light.

Try this: Stand in the sun. Where is your shadow? Turn round. Where is it now?

Shadows change with the sun

In the morning the sun is low in the east. Your shadow is long and points west. At midday the sun is high overhead. Your shadow is short and sits almost under your feet. In the evening the sun is low in the west. Your shadow is long again and points east.

The shadow is always on the side away from the sun. A low sun makes a long shadow because the rays come in at a slant. A high sun makes a short shadow because the rays come almost straight down.

Long ago, people used this to tell the time with a sundial: a stick in the ground whose shadow moves round like the hand of a clock.

Worked example: Kwame's shadow at 9 a.m. points to the west. At 3 p.m. it will point to the east, because the sun has crossed to the other side of the sky.

Try this: Push a stick into the ground in the morning. Mark the end of its shadow with a stone every hour. What shape do the stones make?

Experiment: shadow size

Does a shadow always have the same size as the object? Try this in a dark room.

You need: a torch, a cup, a wall.

Steps:

1. Put the cup near the wall. Shine the torch at it. Look at the shadow.
2. Move the cup closer to the torch, away from the wall. Look again.
3. Move the cup right next to the torch.

What happens: The closer the cup is to the torch, the bigger its shadow. The closer the cup is to the wall, the smaller and sharper its shadow.

Why: Light spreads out from the torch in straight lines, like a fan. Near the torch, the cup blocks a big slice of the fan. Near the wall, it blocks only a thin slice.

This is why puppet-show shadows can be made huge: the puppet is held close to the lamp.

Try this: Make a shadow animal with your hands. Then make it twice as big without moving your hands apart.

Shadow puppets

People have told stories with shadows for thousands of years. You can too.

You need: a white sheet or a plain wall, a torch or lamp, and your hands or cut-out shapes on sticks.

Put the light behind you, pointing at the wall. Hold your hands between the light and the wall. Try these:

• Dog: Hold your hand sideways, thumb up for the ear, fingers together for the nose. Open and close your little finger to make it bark.
• Bird: Hook your thumbs together and flap your fingers.
• Rabbit: Make a fist, then raise two fingers for the ears.

Remember what you learned: close to the light, the shadow grows. Far from the light, it shrinks and gets sharp. Move your hands to make the dog run towards the audience.

Try this: Make a two-minute shadow story with a friend. One holds the light, the other makes the shapes.

Reflection: light bounces

When light hits a surface, some of it bounces back. This is called reflection. Every object you can see is reflecting light into your eyes, otherwise you could not see it.

Rough surfaces, like a wall or your skin, scatter the light in all directions. That is why a wall looks the same from any angle.

Smooth, shiny surfaces, like a mirror or still water, bounce the light back neatly, all in the same order it arrived. That is why you see a picture in them: a reflection.

Still water reflects like a mirror. Throw a stone in, the surface gets rough, and the picture breaks up into ripples.

Try this: Look at your reflection in a spoon, a window, a puddle and a phone screen. Which gives the clearest picture? Why?

Mirrors

A mirror is a sheet of glass with a very smooth, shiny layer of metal on the back. It reflects almost all the light that hits it, in perfect order, so you see a clear picture.

A mirror picture is reversed from left to right. Raise your right hand, and the person in the mirror raises the hand on your left. Writing looks backwards. Ambulances sometimes paint their name backwards on the front so that drivers see it the right way round in their mirror.

The picture also appears to be behind the mirror, exactly as far behind as you are in front. Step back one metre, and your reflection steps back one metre too.

Worked example: Kofi holds a page with the word AMA up to a mirror. What does he see? AMA, because A and M look the same reversed. Try KOFI instead and the letters flip.

Try this: Write your name, hold it to a mirror, and copy what you see.

The rule of reflection

Light bounces off a mirror the way a ball bounces off a wall. If it comes in at a slant, it leaves at the same slant on the other side.

Scientists say: the angle in equals the angle out. The ray coming in is the incident ray. The ray going out is the reflected ray. The angles are measured from an imaginary line standing straight up from the mirror, called the normal.

Because of this rule, you can predict exactly where reflected light will go.

Worked example: A torch beam hits a mirror at 30 degrees from the normal. It bounces off at 30 degrees on the other side. If it came in at 60 degrees, it would leave at 60 degrees.

Experiment: In a dark room, shine a torch at a mirror on the floor. Move the torch. Watch the bright spot on the ceiling move the opposite way. The slant in matches the slant out.

Try this: Can you use a mirror to shine a torch beam round a corner onto a wall you cannot see directly?

Experiment: the periscope

A periscope lets you see over a wall or round a corner, using two mirrors and the rule of reflection.

You need: a long cardboard box or two milk cartons, two small mirrors, tape, scissors.

Steps:

1. Cut a window near the top of one side of the box, and another near the bottom of the opposite side.
2. Fix one mirror inside at the top, slanted at 45 degrees, facing the top window.
3. Fix the other mirror inside at the bottom, slanted at 45 degrees, facing the bottom window.
4. Look in the bottom window.

What happens: Light from outside enters the top window, bounces down off the top mirror, bounces out off the bottom mirror and into your eye. You see over the top of the box.

Try this: Use it to look over a wall or round a doorway without being seen.

Refraction: light bends in water

Light travels in straight lines, but when it passes from air into water or glass, it changes direction slightly at the surface. This is called refraction.

Put a spoon in a glass of water. Look from the side. The spoon seems broken or bent at the water line. It is not bent. The light from the underwater part changes direction as it leaves the water, so your brain thinks it came from somewhere else.

A pool looks shallower than it is for the same reason. Never jump into water because it looks shallow.

Refraction is also how lenses work. Glasses, magnifying glasses, cameras and your own eye all use curved glass or jelly to bend light and make a clear picture.

Try this: Put a coin in an empty cup. Step back until the rim just hides the coin. Keep still and have a friend pour in water. The coin appears.

Experiment: the pinhole camera

A pinhole camera makes a picture with nothing but a tiny hole. It proves that light travels in straight lines.

You need: a cardboard tube or a small box, greaseproof paper or thin white paper, tape, a pin.

Steps:

1. Cover one end of the tube with the thin paper. This is the screen.
2. Cover the other end with card and make a pinhole in the middle.
3. In a dim room, point the pinhole at a bright window or a candle.
4. Look at the paper screen.

What happens: A small, faint picture of the window appears, upside down.

Why: Light from the top of the window travels straight through the hole and lands at the bottom of the screen. Light from the bottom lands at the top. The rays cross at the hole.

Try this: Make the hole bigger. The picture gets brighter but blurry. Why?

How we see

Seeing is a chain of events, and every link uses what you have learned.

1. A source gives out light. The sun shines.
2. The light travels in straight lines and hits an object, say a mango.
3. The mango reflects some of the light. Yellow light bounces off; other colours are absorbed.
4. The reflected light enters your eye through the pupil.
5. The lens refracts the light to make a sharp, upside-down picture on the back of the eye.
6. The back of the eye sends signals along a nerve to the brain.
7. The brain turns the picture the right way up and says: "mango".

If any link breaks, you do not see. No source: darkness. Object hidden: no reflected light. Eyes closed: no light enters.

Try this: Draw the chain as a row of seven boxes with arrows between them.

Inside the eye

Your eye is a small camera made of living parts.

• The cornea is the clear window at the front. It does the first bending of the light.
• The pupil is the black hole in the middle. Light goes in here.
• The iris is the coloured ring. It makes the pupil bigger in dim light and smaller in bright light.
• The lens is a clear, bendy disc behind the pupil. It focuses the light.
• The retina is the layer at the back, covered in light-sensitive cells. The picture lands here.
• The optic nerve carries the signals to the brain.

Try this: Look in a mirror in a bright room. Then cover your eyes for a minute and look again quickly. Watch your pupils shrink.

Light and dark: the pupil

In bright sunshine at Labadi beach, your pupils are tiny dots. In a dark room, they open wide. Why?

The iris works like a curtain. Too much light would dazzle the retina and could hurt it, so the iris closes the pupil down to a pinprick. In dim light, the retina needs all the light it can get, so the iris opens wide.

This is why you cannot see for a moment when you walk from bright sunlight into a dark room. Your pupils are still small. After a minute they open and the room appears.

It is also why you squint when you come out of the dark into the sun. Your pupils are wide open and the light floods in until they shrink.

Worked example: Kofi walks out of a cinema into the afternoon sun and everything looks white. In a few seconds his pupils close and he can see.

Try this: Ask a friend to watch your pupils while you look at a window, then at a dark corner.

Looking after your eyes

Your eyes are the only ones you get. Protect them.

• Never look at the sun, not even during an eclipse, and never through a lens.
• Do not point lasers at anyone's eyes. Even a toy laser can burn the retina.
• Read in good light. Reading in the dark tires your eyes.
• Rest from screens. Every twenty minutes, look at something far away for twenty seconds.
• Keep sharp things away from your face.
• Wash your hands before touching your eyes, to avoid infections like apollo.
• Wear sunglasses or a hat on very bright days.
• Tell an adult if the board looks blurry. Glasses fix it, and the sooner the better.

Many children in Ghana sit at the back of the class and cannot read the board. They think they are bad at school. Really they just need glasses.

Try this: Stand at the back of your classroom. Can you read everything on the board?

Colours in light

White light from the sun is not really white. It is a mixture of all colours. You can see them when sunlight passes through raindrops and splits into a rainbow: red, orange, yellow, green, blue, indigo, violet.

A glass prism does the same. So does a thin film of oil on a puddle, and the back of a CD.

Why do things have colours? A mango reflects yellow light into your eye and absorbs the other colours. A green leaf reflects green. A white shirt reflects every colour. A black shirt absorbs every colour, which is why black clothes get hot in the sun.

Worked example: Why does a red cloth look black in a room lit only by a green bulb? There is no red light to reflect, so it reflects nothing.

Try this: Put a glass of water on a sunny windowsill with a sheet of white paper beside it. Look for a small rainbow.

Light and heat

Most sources of light also give out heat. The sun warms your skin. A candle flame will burn your finger. An old light bulb is too hot to touch after a minute.

Dark surfaces absorb more light, and so they get hotter. Light surfaces reflect more light, and stay cooler. This is why:

• Houses in hot places are often painted white
• A black car gets hotter in the car park than a white one
• Solar water heaters have black pipes, to soak up as much sunlight as possible

Experiment: Put a black cloth and a white cloth side by side in the sun for ten minutes. Touch them. The black one is warmer.

Solar power uses this energy. Solar panels on rooftops in Ghana turn sunlight into electricity, and solar lamps light homes far from the grid.

Try this: Which colour would you paint a water tank to keep the water cool? Which colour to make it warm?

Light for plants

Plants need light to live. In their leaves, they use the energy of light to make their food from water and air. This is called photosynthesis. No light, no food, no plant.

This is why:

• Leaves are flat and wide, to catch as much light as possible
• Plants on a windowsill lean towards the window
• Grass under a stone turns yellow and dies
• Farmers do not plant maize in deep shade

Experiment: Put two bean seedlings in pots. Leave one in the light. Put the other in a cupboard. After a week, compare them. The one in the dark is pale, thin and bending, searching for light.

Every meal you eat started with light. The cassava, the rice, the plantain all grew by catching sunlight. Even the fish ate smaller things that ate plants.

Try this: Find a plant that is leaning. Which way is it leaning, and where is the light coming from?

Light at the speed of light

Light is the fastest thing there is. It travels about 300,000 kilometres every second. In one second it could go round the Earth seven times.

Because it is so fast, light seems to arrive instantly. Switch on a lamp and the room is lit at once. But over huge distances, light takes time.

• Sunlight takes about 8 minutes to reach Earth. If the sun went out, we would not know for eight minutes.
• Light from the nearest star after the sun takes 4 years.
• Some stars you see at night are so far away that their light left them before you were born.

Sound is much slower, about 340 metres per second. That is why you see lightning before you hear thunder. Count the seconds between them and divide by three to get the distance in kilometres.

Try this: Next time there is a storm, count the gap between the flash and the bang.

Experiment: the light maze

Build a maze that only light can solve, using everything you know.

You need: a torch, three small mirrors, a shoebox, modelling clay, a target such as a coin.

Steps:

1. Put the coin in one corner of the shoebox.
2. Cut a small hole in the opposite side for the torch.
3. Stand the mirrors in the box with clay so that the torch beam bounces from mirror to mirror and finally hits the coin.

You will need the rule of reflection: angle in equals angle out. Point a mirror too flat and the beam goes past the next one. Point it too steep and the beam goes back the way it came.

What you will learn: You can steer light anywhere you want with mirrors, but each bounce must obey the rule. This is exactly how a periscope works, and how light travels through the fibre-optic cables that carry the internet.

Try this: Add a fourth mirror and make the beam cross its own path.

Light in everyday life

Once you know how light behaves, you see it at work everywhere.

• Car mirrors use reflection so the driver can see behind.
• Reflective strips on school bags and road signs bounce headlights back to the driver.
• Torches have a curved shiny inside to send all the light forward.
• Glasses use refraction to fix blurry sight.
• Cameras are pinhole cameras with a lens and a sensor.
• Fibre-optic cables carry light signals under the sea, bouncing them along inside a glass thread.
• Traffic lights use red, amber and green because they are easy to tell apart.
• Solar panels turn sunlight into electricity.
• Sundials tell the time from a shadow.

Worked example: Why are road signs easy to read at night when a car approaches, but not when you stand beside them with no light? They do not make light; they only reflect it.

Try this: Walk round your home and school. Find five things that use reflection, shadows or lenses.

Common mistakes

Here are ideas that many people believe but that are not true.

"The moon is a source of light." No. It reflects sunlight. On the side facing away from the sun, the moon is dark.

"We see because light comes out of our eyes." No. Light goes into our eyes. In a totally dark room you see nothing, however hard you stare.

"Shadows are black things that stick to you." No. A shadow is simply a place where light is missing.

"Light bends round corners." No. It travels in straight lines. It only changes direction at a mirror or when it enters water or glass.

"A mirror swaps top and bottom." No. A mirror swaps front and back, which makes writing look reversed left to right.

"Bigger holes make sharper pinhole pictures." No. Bigger holes make brighter but blurrier pictures.

Try this: Explain to a younger child why the moon shines.

Practice: quick questions

Try these without looking back.

1. Name three natural and three artificial sources of light.
2. What do we call a material that lets no light through?
3. Why is your shadow longer at 5 p.m. than at noon?
4. What happens to the angle of a ray when it bounces off a mirror?
5. Why does a straw in water look bent?
6. Which part of the eye lets light in? Which part changes its size?
7. Why is a pinhole picture upside down?
8. Why do black clothes feel hot in the sun?

Answers:

1. Sun, stars, fire; bulb, torch, candle (any correct ones)
2. Opaque
3. The sun is low, so rays come in at a slant
4. It leaves at the same angle it arrived
5. Light refracts as it leaves the water
6. The pupil; the iris
7. Rays cross at the hole
8. Black absorbs light and its energy

Practice: explain it

Write two or three sentences for each. Use the science words: source, straight line, opaque, reflect, refract, retina.

1. Ama cannot see her friend who is hiding behind the classroom door. Explain why.
2. Kofi shines a torch on a mirror and lights up the ceiling. Explain how the light got there.
3. Abena's shadow on the wall grows when she walks towards the lamp. Explain why.
4. Kwame says the pool is shallow because he can see the bottom clearly. Explain why he should not trust his eyes.
5. Explain, step by step, how you see this book.

Checking your answers: Each explanation should name the rule that applies. For question 1, the rule is that light travels in straight lines. For question 2, it is reflection. For question 3, it is that light spreads out from a source. For question 4, refraction. For question 5, the whole chain of seeing.

A science fair project

Here is a project you could show at a school science fair, using what you have learned.

Title: How does the distance from the light change the size of a shadow?

Question: If I move a cup closer to a torch, how much bigger does its shadow get?

Method:

1. Fix a torch 100 cm from a wall.
2. Put a cup 10 cm from the wall. Measure the shadow's height.
3. Move the cup to 20 cm, 40 cm, 60 cm and 80 cm from the wall. Measure each time.
4. Write the results in a table.

Results: Draw a bar chart. The bars will get taller as the cup moves closer to the torch.

Conclusion: Explain using straight lines. Light fans out from the torch, so an object near the source blocks a wider slice of the fan.

Try this: Predict what the shadow height will be at 90 cm before you measure it. Were you right?

What you have learned

You have followed light all the way from the sun to your brain.

• Sources make light. Everything else only reflects it.
• Light travels in straight lines, so it cannot bend round corners.
• Opaque things block light and make shadows. The shadow is on the far side from the source, long when the source is low, big when the object is close.
• Smooth surfaces reflect light in order, which is what a mirror does. The angle in equals the angle out.
• Light refracts when it enters water or glass, which makes spoons look bent and lenses work.
• We see because reflected light enters the pupil, is focused by the lens onto the retina, and the brain reads the picture.
• Light carries energy: it warms dark surfaces, feeds plants, and powers solar panels.

Every experiment in this book used things from your home. Science does not need a laboratory. It needs a question, a torch and a curious mind.

Learn more

• Light facts at Science Kids — short, clear facts about light, speed, colour and reflection
• Light and shadows activity at Science Kids — an interactive activity on how shadows change with the light source
• Light at Ducksters — a readable page on what light is, how it travels and how we see
• Light at DK Find Out — illustrated pages on sources, reflection, refraction and the eye
• Light on Wikipedia — the full story of light for when you want to go deeper

Watch more

Think about it

1. If light could bend round corners, what would shadows look like?
2. The moon is bright at night. Why is it wrong to call it a source of light?
3. Why do you think houses in hot countries are so often painted white?
4. A cat can see in much dimmer light than you can. What might be different about its eyes?
5. Sunlight takes eight minutes to reach us. What does that mean about the sun you see in the sky right now?