Chapter 2 of 7

All 7 chapters
  1. 1Where light comes from, readunderstood
  2. 2Light goes in straight lines, readunderstood
  3. 3How we see, readunderstood
  4. 4Opaque, translucent, transparent, readunderstood
  5. 5Big and small shadows, readunderstood
  6. 6A shadow stick through the day, readunderstood
  7. 7Mirrors and shiny things, readunderstood

S2.6.2 Chapter 2

Light goes in straight lines

Can light go round a corner?

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

  • Show that light goes in a straight line
  • Use three cards or a tube to test it
  • Explain why a shadow has a sharp edge

A lamp is on in the next room. You cannot see it through the wall. Can light go round a corner?

Light goes in straight lines through air. A straight line has no bends. We draw one thin line of light as a ray. A ray has an arrow. The arrow shows which way the light goes.

A lamp on the left with three straight arrows going out from it to the right, spreading a little apart. Labels: Lamp, three rays.Lampthree rays
Each ray is a straight line. Its arrow shows the way the light goes.

Light can bend where it goes from air into water or glass. That is why a spoon in a cup of water looks bent. But in air alone, light goes straight.

Draw it first

Draw light going through three holes

  1. An eye on the left and a candle on the right. Three cards stand between them, each with a hole, and the holes are in a line. Arrows go from the candle through the holes to the eye. Labels: Eye, Candle.EyeCandle

    1 Draw an eye on the left and a candle on the right.

  2. An eye on the left and a candle on the right. Three cards stand between them, each with a hole, and the holes are in a line. Arrows go from the candle through the holes to the eye. Labels: Eye, Candle.EyeCandle

    2 Draw three cards between them. Each card has a hole. Put the holes in a line.

  3. An eye on the left and a candle on the right. Three cards stand between them, each with a hole, and the holes are in a line. Arrows go from the candle through the holes to the eye. Labels: Eye, Candle.EyeCandle

    3 Draw arrows from the candle, through the holes, to the eye.

Try it

Three cards and a flame

You need: three stiff cards, such as card from a box, a pencil, a ruler, a small lamp or a candle, clay or tape, to make the cards stand up.

Be safe: If you use a candle, a grown up must be nearby. Never use the Sun for this test.

  1. Use the ruler to find the same height on each card. Make a hole there with the pencil.
  2. Stand the cards in a row. Leave a hand of space between them.
  3. Put the lamp or candle at one end. Keep it level with the holes.
  4. Look through all three holes from the other end. Can you see the flame or the lamp?
  5. Slide the middle card a little to one side. Look again.

What you will see: You see the light only when the holes are in line. Move one card and the light is blocked.

If you cannot try it, this is what happens.
Two pictures, each with an eye, three cards and a candle. In the first, the holes are in a line and the arrows reach the eye. In the second, the middle hole is moved up and the arrows stop at the middle card. Labels: holes in a line, middle hole moved up, Eye, Candle.holes in a lineEyeCandlemiddle hole moved upEyeCandle

Now think about a card in front of a torch. A shadow is a dark place where something blocks the light. The card blocks the rays. The rays that miss the card go straight on. So the edge of the shadow is sharp. A small light makes the sharpest edge. A big light makes it softer.

See it three ways

As a picture

A torch on the left, a card, and a wall on the right. Two rays go from the torch past the top and the bottom of the card to the wall. The dark place between them on the wall is the shadow. Labels: Torch, Card, Shadow, Wall.TorchCardShadowWall

In words

Light goes straight. The card stops the rays in its path. The rays that miss it go on. The shadow has a sharp edge.

Two pictures of a paper tube. In the first, the tube is straight. A lamp is at one end, an eye at the other, and an arrow goes straight through the tube to the eye. In the second, the tube is bent. The arrow from the lamp hits the wall of the tube at the bend and does not reach the eye. Labels: straight tube, bent tube, Eye, Lamp.straight tubeEyeLampbent tube
Light goes straight along a straight tube. A bent tube stops it.

Think of a lamp behind a wall. The rays go straight. Some hit the wall and stop. Some pass over it and go on.

A lamp on the left, a wall in the middle and an eye on the right, behind the wall. One arrow goes from the lamp and stops at the wall. A second arrow goes up and over the wall and straight on, far above the eye. No arrow reaches the eye. Labels: Lamp, Wall, Eye.LampWallEye
The light goes straight. It does not turn down to the eye.

A wrong idea many people have

Light can bend round a corner.

Why it feels right: A lamp lights a whole room. It seems to reach round corners.

The test that breaks it: Roll paper into a tube. Look at a lamp through it. Then bend the tube. The lamp is gone from view. Never use the Sun for this.

What is true: In air, light goes in straight lines. It reaches round a corner only by bouncing off things.

Key words

straight line
a line with no bends.
ray
one thin line of light, drawn with an arrow.
shadow
a dark place where something blocks the light.

Back to your guess

The answer is B: No. In air, light goes in straight lines.

In air, light goes in straight lines. It does not go round a corner. It can bounce off things. It also bends where it goes into water or glass.

Check yourself

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

  1. You look at a lamp through a bent paper tube. What do you see?

    Tap your answer.

    1. Yes, that is right. Light from the lamp goes straight. It hits the wall of the bend. It cannot reach your eye.

    2. Not this one. Light goes in straight lines. It cannot follow the bend in the tube. Try another.

    3. Not this one. The light would have to go round the bend. It cannot, so the lamp is hidden. Try another.

    See the answer

    You cannot see the lamp

    Light from the lamp goes straight. It hits the wall of the bend. It cannot reach your eye.

    • If you chose B: Light goes in straight lines. It cannot follow the bend in the tube.
    • If you chose C: The light would have to go round the bend. It cannot, so the lamp is hidden.

  2. Three cards have holes in a line. A candle burns behind them. Where must your eye be?

    Tap your answer.

    1. Not this one. Light goes straight from the flame through the holes. It does not go up and over. Try another.

    2. Not this one. Only the light in a straight line gets through all three holes. Try another.

    3. Yes, that is right. The light goes through the holes in a straight line. Your eye must be on that line.

    See the answer

    In line with the holes

    The light goes through the holes in a straight line. Your eye must be on that line.

    • If you chose A: Light goes straight from the flame through the holes. It does not go up and over.
    • If you chose B: Only the light in a straight line gets through all three holes.

  3. Which sentence explains why a shadow has a sharp edge?

    Tap your answer.

    1. Yes, that is right. The rays that miss the card go straight on. The rays that hit it stop. The line between them is sharp.

    2. Not this one. Light goes straight past an edge. It does not curl round it. Try another.

    3. Not this one. The wall does not change the edge. The straight rays do. Try another.

    See the answer

    Rays go straight past the edge of the card.

    The rays that miss the card go straight on. The rays that hit it stop. The line between them is sharp.

    • If you chose B: Light goes straight past an edge. It does not curl round it.
    • If you chose C: The wall does not change the edge. The straight rays do.

  4. From an earlier chapter

    Which of these makes its own light?

    Tap your answer.

    1. Not this one. A mirror only sends back light. It makes none. Go to S2.6.1. Try another.

    2. Yes, that is right. A fire makes its own light. A mirror and the Moon only send light back.

    3. Not this one. The Moon only reflects light from the Sun. Go to S2.6.1. Try another.

    See the answer

    A fire

    A fire makes its own light. A mirror and the Moon only send light back.

    • If you chose A: A mirror only sends back light. It makes none. Go to S2.6.1.
    • If you chose C: The Moon only reflects light from the Sun. Go to S2.6.1.

  5. Draw a torch, a card and a wall. Draw two rays past the card. Show the shadow.

    See the answer

    The torch is on the left and the wall on the right. One ray passes the top of the card. One passes the bottom. The dark place on the wall between them is the shadow.

    Rays go straight. The rays that touch the card stop. The wall behind the card gets no light.

  6. A wall stands between a lamp and Ravi's eye. The lamp is on. Can he see the lamp itself?

    A lamp on the left, a wall in the middle and an eye on the right, behind the wall. Labels: Lamp, Wall, Eye.LampWallEye
    Tap your answer.

    1. Not this one. In air, light goes in straight lines. It does not bend round a wall. Try another.

    2. Not this one. A brighter lamp makes more light. The light still goes straight. Try another.

    3. Not this one. Some light may bounce to him off the ceiling. But no ray goes straight from the lamp to his eye. So he cannot see the lamp. Try another.

    4. Yes, that is right. Rays go straight from the lamp. The wall stops the ones that go toward Ravi.

    See the answer

    No. Light goes straight and the wall blocks it.

    Rays go straight from the lamp. The wall stops the ones that go toward Ravi.

    • If you chose A: In air, light goes in straight lines. It does not bend round a wall.
    • If you chose B: A brighter lamp makes more light. The light still goes straight.
    • If you chose C: Some light may bounce to him off the ceiling. But no ray goes straight from the lamp to his eye. So he cannot see the lamp.

  7. Which is the right way to draw a ray from a lamp?

    Tap your answer.

    1. Not this one. In air, light goes in straight lines. So a ray is a straight line. Try another.

    2. Not this one. The arrow shows the way the light goes. Light goes away from the lamp. Try another.

    3. Not this one. With no arrow, the line does not show the way light goes. Try another.

    4. Yes, that is right. A ray is one thin line of light. It is straight. Its arrow shows the way the light goes.

    See the answer

    A straight line with an arrow that points away from the lamp

    A ray is one thin line of light. It is straight. Its arrow shows the way the light goes.

    • If you chose A: In air, light goes in straight lines. So a ray is a straight line.
    • If you chose B: The arrow shows the way the light goes. Light goes away from the lamp.
    • If you chose C: With no arrow, the line does not show the way light goes.

Keep it

Copy this into your exercise book.

In air, light goes in straight lines. A ray is one line of light, drawn with an arrow. A card blocks rays and makes a shadow. The shadow has a sharp edge.

Draw: Draw a torch, a card and a wall. Add two rays past the card. Shade the shadow.

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

No internet tomorrow? Save the whole book as one file (644 KB). It opens on any phone.