Chapter 4 of 8

All 8 chapters
  1. 1Reflection, readunderstood
  2. 2Refraction, readunderstood
  3. 3Lenses and the eye, readunderstood
  4. 4The colours of light, readunderstood
  5. 5Why things have colour, readunderstood
  6. 6Sound as a wave, readunderstood
  7. 7The speed of sound, readunderstood
  8. 8Pitch, loudness and hearing, readunderstood

S3.9.4 Chapter 4

The colours of light

Where do the colours of a rainbow come from?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Split light into colours with a mirror in a dish of water
  • Name the colours of the spectrum in order
  • Explain how raindrops make a rainbow

After a shower, Lucía sees a bow of colours in the sky. The Sun is behind her and the rain is in front. Where do the colours come from?

Sunlight looks white. But white light is a mix of many colours. You can pull the mix apart. A prism is a block of clear glass with flat sides, shaped like a wedge.

When light goes into a prism and out again, it bends twice. Each colour bends by a different amount. Red bends the least. Violet bends the most.

Draw it first

Draw white light going through a prism

  1. In the first picture, a ray labelled white light travels towards a triangle labelled prism. In the second picture, the ray goes through a prism and spreads into a fan that lands on a screen as seven bands: red, orange, yellow, green, blue, indigo and violet. Labels: white light, prism, screen, red, orange, yellow, green, blue, indigo, violet.white lightprism

    1 Draw a ray of white light going towards a prism.

  2. In the first picture, a ray labelled white light travels towards a triangle labelled prism. In the second picture, the ray goes through a prism and spreads into a fan that lands on a screen as seven bands: red, orange, yellow, green, blue, indigo and violet. Labels: white light, prism, screen, red, orange, yellow, green, blue, indigo, violet.redorangeyellowgreenblueindigovioletwhite lightprismscreen

    2 The prism spreads the light out. Draw seven bands on a screen. Red is where the light bends least. Violet is where it bends most.

The band of colours is a spectrum. The colours run in this order: red, orange, yellow, green, blue, indigo, violet.

In truth, the band changes smoothly from red to violet, with no sharp edges. The seven names are a custom. People agreed on seven names to help them talk about it.

The picture is drawn with a wider fan than real glass makes, so that the names fit. In real life, the bands are much closer together.

See it three ways

As a picture

A table with two columns, order and colour. 1 red. 2 orange. 3 yellow. 4 green. 5 blue. 6 indigo. 7 violet. Labels: order, colour, red, orange, yellow, green, blue, indigo, violet.ordercolour1red2orange3yellow4green5blue6indigo7violet

In words

The colours always come in the same order. Red bends the least, so it is at one end. Violet bends the most, so it is at the other.

In symbols

red, orange, yellow, green, blue, indigo, violet.

A rainbow is a spectrum made by raindrops. Each raindrop is a tiny ball of water. Sunlight goes into a drop and bends. It reflects off the back of the drop. Then it bends again as it comes out.

A raindrop drawn as a circle. A ray of sunlight comes in from the left, labelled sunlight. It bends going in, reflects off the back of the drop, and bends again coming out. It splits into two rays that leave the drop at different angles, one for red and one for violet. Labels: sunlight, raindrop, red, violet.sunlightraindropredviolet
The light bends in, bounces off the back, and bends out. Each colour leaves at its own angle. The colours are drawn farther apart than they really are.

Each colour leaves the drop at a slightly different angle. So you see red light from drops in one place in the sky. You see violet light from drops in another place. All the drops together make the bow. Red is on the outside of the bow.

A person, labelled you, stands with the Sun behind, labelled Sun, on the left. Rain, labelled rain, falls in front on the right. A ray of sunlight goes over the person into the rain. Light comes back from the rain to the person. Labels: Sun, you, rain.Sunyourain
The Sun is behind you and the rain is in front. The light goes into the drops and back to your eyes.

This is true everywhere on Earth. You need the Sun behind you and rain in front of you. A rainbow is a sign that the Sun is shining while it rains.

Try it

Make a spectrum with a mirror in water

You need: a shallow dish or bowl of water, a small flat mirror, a plain white wall or a sheet of white paper, a sunny window, or a sunny spot outside next to a shaded wall.

Be safe: Never look at the Sun or at the mirror. Look only at the wall. Never point the light at anyone's face. A mirror can break, so keep it away from the edge of the table.

  1. Put the dish near a window where sunlight reaches it. Hold the paper or stand near a plain wall.
  2. Lean the mirror in the water, so part of the mirror is under water.
  3. Turn the mirror a little at a time until sunlight bounces up onto the wall.
  4. Look at the wall only. You should see a band of colours. Find the red end and the violet end.

What you will see: The colours are in the same order as with a prism. Red is at one end and violet is at the other. The water in front of the mirror is a wedge. It bends each colour by a different amount.

If you cannot try it, this is what happens.
Light comes down into a dish of water, hits a mirror leaning in it, and bounces up as a fan of seven colours that lands on a wall. Labels: sunlight, water, mirror, wall, red, orange, yellow, green, blue, indigo, violet.redorangeyellowgreenblueindigovioletsunlightmirrorwaterwall

A wrong idea many people have

A prism, or the rain, adds colour to light.

Why it feels right: The colours come out of the prism, so it seems the prism made them.

The test that breaks it: Look at the glass, the water or a raindrop. It has no colour. The colours on the wall were already hidden in the sunlight.

What is true: White light is a mix of colours. The prism, the water and the raindrop only spread the colours apart.

Key words

prism
a block of clear glass with flat sides. It spreads white light into colours.
spectrum
the band of colours from red to violet that white light spreads into.

Back to your guess

The answer is B: Sunlight is a mix of colours, and drops of water spread them apart.

Sunlight looks white, but it is a mix of colours. Water drops bend each colour by a different amount, so the colours spread apart. The drops add no colour.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. White light goes through a prism. Which colour bends the least?

    Tap your answer.

    1. Not this one. Violet bends the most. Red is at the other end, bent the least. Go to S3.9.4. Try another.

    2. Not this one. Green is in the middle. The ends are red, bent the least, and violet, bent the most. Go to S3.9.4. Try another.

    3. Yes, that is right. Red bends the least and violet bends the most. The other colours are in between.

    See the answer

    Red.

    Red bends the least and violet bends the most. The other colours are in between.

    • If you chose A: Violet bends the most. Red is at the other end, bent the least. Go to S3.9.4.
    • If you chose B: Green is in the middle. The ends are red, bent the least, and violet, bent the most. Go to S3.9.4.

  2. Which list gives the colours of the spectrum in the right order, starting from red?

    Tap your answer.

    1. Yes, that is right. The colours always come in this order. Red is bent least and violet is bent most.

    2. Not this one. Orange comes before yellow, and indigo comes before violet. The order is the same every time. Go to S3.9.4. Try another.

    3. Not this one. Green comes before blue. Check the order in the table. Go to S3.9.4. Try another.

    See the answer

    Red, orange, yellow, green, blue, indigo, violet.

    The colours always come in this order. Red is bent least and violet is bent most.

    • If you chose B: Orange comes before yellow, and indigo comes before violet. The order is the same every time. Go to S3.9.4.
    • If you chose C: Green comes before blue. Check the order in the table. Go to S3.9.4.

  3. Lucía wants to see a rainbow after a shower. How should she stand?

    Tap your answer.

    1. Not this one. Never look at the Sun. The light must come from behind you and bounce back from the rain in front. Go to S3.9.4. Try another.

    2. Yes, that is right. Sunlight goes over her head into the drops. The drops send coloured light back to her eyes. She never has to look at the Sun.

    3. Not this one. The bow forms opposite the Sun. The rain must be in front of you and the Sun behind you. Go to S3.9.4. Try another.

    See the answer

    With the Sun behind her, and the rain in front of her.

    Sunlight goes over her head into the drops. The drops send coloured light back to her eyes. She never has to look at the Sun.

    • If you chose A: Never look at the Sun. The light must come from behind you and bounce back from the rain in front. Go to S3.9.4.
    • If you chose C: The bow forms opposite the Sun. The rain must be in front of you and the Sun behind you. Go to S3.9.4.

  4. How does a raindrop make colours?

    A raindrop drawn as a circle. A ray of sunlight comes in from the left, labelled sunlight, and goes no farther than the edge of the drop. Labels: sunlight, raindrop.sunlightraindrop
    Tap your answer.

    1. Not this one. A drop has no colour to add. The colours were already in the sunlight. Go to S3.9.4. Try another.

    2. Not this one. A mirror would not spread the colours apart. It is the bending that splits them, and the drop bends light twice. Go to S3.9.2. Try another.

    3. Yes, that is right. The two bends spread the colours apart. The reflection sends the light back towards the person looking.

    See the answer

    Light bends going in, reflects off the back, and bends again coming out. Each colour bends by a different amount.

    The two bends spread the colours apart. The reflection sends the light back towards the person looking.

    • If you chose A: A drop has no colour to add. The colours were already in the sunlight. Go to S3.9.4.
    • If you chose B: A mirror would not spread the colours apart. It is the bending that splits them, and the drop bends light twice. Go to S3.9.2.

  5. Mei leans a mirror in a dish of water. Sunlight makes a band of colours on the wall. Why does the water matter?

    Tap your answer.

    1. Yes, that is right. The water makes a wedge like a prism. Each colour bends by a different amount, so the white light spreads into a spectrum on the wall.

    2. Not this one. Water is clear. The colours were already in the sunlight, and the water only spreads them apart. Go to S3.9.4. Try another.

    3. Not this one. Brighter light would still be white. The bending is what separates the colours. Go to S3.9.4. Try another.

    See the answer

    The water in front of the mirror is a wedge. It bends each colour by a different amount.

    The water makes a wedge like a prism. Each colour bends by a different amount, so the white light spreads into a spectrum on the wall.

    • If you chose B: Water is clear. The colours were already in the sunlight, and the water only spreads them apart. Go to S3.9.4.
    • If you chose C: Brighter light would still be white. The bending is what separates the colours. Go to S3.9.4.

  6. From an earlier chapter

    Review: which glasses help a long sighted eye?

    Tap your answer.

    1. Not this one. That helps short sight. A long sighted eye needs a lens that brings rays together. Go to S3.9.3. Try another.

    2. Yes, that is right. A long sighted eye does not bend the light enough. A converging lens brings the rays together a little before they enter the eye.

    3. Not this one. Flat glass bends light in, and back out by the same amount. The rays leave going the same way. A long sighted eye needs a lens that brings rays together. Go to S3.9.3. Try another.

    See the answer

    A converging lens, thicker in the middle.

    A long sighted eye does not bend the light enough. A converging lens brings the rays together a little before they enter the eye.

    • If you chose A: That helps short sight. A long sighted eye needs a lens that brings rays together. Go to S3.9.3.
    • If you chose C: Flat glass bends light in, and back out by the same amount. The rays leave going the same way. A long sighted eye needs a lens that brings rays together. Go to S3.9.3.

Keep it

Copy this into your exercise book.

White light is a mix of colours. A prism spreads it into a spectrum: red, orange, yellow, green, blue, indigo, violet. Red bends the least. A rainbow is made by raindrops, with the Sun behind you.

Draw: A ray of white light going through a prism and spreading into bands. Write red at one end and violet at the other.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

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