Chapter 5 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

S2.6.5 Chapter 5

Big and small shadows

Why is my shadow big near a lamp and small far away?

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

  • Say which side of a thing its shadow falls on
  • Explain how a lamp makes a shadow bigger or smaller
  • Say why this hardly shows in sunlight

Hold your hand in front of a lamp. A dark shape falls on the wall behind it. That is a shadow. A shadow is a place where a thing blocks the light.

The light is on one side of your hand. The shadow is on the other side. A shadow always falls away from the light.

A person stands on the ground. The Sun, labelled Sun, is low in the sky on the left. The shadow of the person lies on the ground to the right, labelled shadow. Labels: Sun, shadow.Sunshadow
The Sun is on the left. The shadow falls on the right.

A shadow has the outline of the thing. The outline is the line round its edge. A shadow has no colour. It has no small details. A red cup and a blue cup make the same dark shadow.

Draw it first

Draw the shadow of a card

  1. A torch on the left, labelled torch, and a wall on the right, labelled wall. A card, labelled card, stands between them. Two lines run from the torch past the top and the bottom of the card to the wall. The part of the wall between the lines is black and is labelled shadow.torchwall

    1 Draw a torch on one side and a wall on the other.

  2. A torch on the left, labelled torch, and a wall on the right, labelled wall. A card, labelled card, stands between them. Two lines run from the torch past the top and the bottom of the card to the wall. The part of the wall between the lines is black and is labelled shadow.torchwallcard

    2 Draw a card between them.

  3. A torch on the left, labelled torch, and a wall on the right, labelled wall. A card, labelled card, stands between them. Two lines run from the torch past the top and the bottom of the card to the wall. The part of the wall between the lines is black and is labelled shadow.torchwallcard

    3 Draw lines from the torch past the edges of the card.

  4. A torch on the left, labelled torch, and a wall on the right, labelled wall. A card, labelled card, stands between them. Two lines run from the torch past the top and the bottom of the card to the wall. The part of the wall between the lines is black and is labelled shadow.torchwallcardshadow

    4 Light cannot pass the card. Fill in the wall between the lines. That is the shadow.

Now ask a question. What if the card moves? The wall stays where it is. How far the card is from the torch is its distance.

A torch on the left, labelled torch, shines on a wall on the right. A card, labelled card, is halfway between the torch and the wall. Two lines run from the torch past the top and the bottom of the card. They make a dark patch on the wall, labelled shadow. The shadow is 2 times as tall as the card.torchcardshadow
The card is halfway to the wall. The shadow is 2 times as tall as the card.

See it three ways

As a picture

A torch on the left, labelled torch, shines on a wall on the right. A card, labelled card, stands close to the torch, 1 step away. The wall is 4 steps away. Two lines run from the torch past the top and the bottom of the card. They make a tall dark patch on the wall, labelled shadow. The shadow is 4 times as tall as the card.torchcardshadow

In words

Move the card nearer the torch. Light spreads out like a fan from the torch. Behind the card, it has more room to spread. So the shadow grows. Move the card away, and the shadow shrinks.

In symbols

Torch to wall: 4 steps. Card 2 steps from the torch: shadow 2 times as tall. Card 1 step from the torch: shadow 4 times as tall.

The Sun is a light too. But the Sun is very, very far away. Its light comes to us almost side by side. It does not spread like a fan. Hold a book in the sun, near a wall. Move it nearer the wall, or farther away. Its shadow stays about the same size.

Two pictures, one above the other. At the top, a lamp sends three rays to a wall, and the rays spread out like a fan. At the bottom, the Sun sends three rays to a wall, and the rays run side by side. Labels: lamp, rays spread out, wall, Sun, rays side by side.lamprays spread outwallSunrays side by side
A lamp is near, so its rays spread out. The Sun is far, so its rays run side by side.

Sunlight makes shadows change in a different way. A shadow changes when the Sun climbs or sinks in the sky. A later chapter shows how.

Try it

Make a shadow bigger

You need: a torch, or the torch on a phone, a plain wall in a dim room, your hand, or a book.

Be safe: Do not shine the torch at anyone's eyes.

  1. Dim the room. Put the torch on a table, shining at the wall.
  2. Hold your hand close to the wall. Look at its shadow.
  3. Move your hand slowly toward the torch. Watch the shadow.
  4. Stop halfway. Look. Then stop near the torch. Look again.

What you will see: The shadow grows as your hand gets nearer the torch. Close to the wall, it is about the size of your hand. It is dark, with no colour and no small details. Near the torch, the edge may look soft.

If you cannot try it, this is what happens.
Three black bars stand side by side. They show how tall the shadow of a hand is. The bar labelled near wall is the shortest. The bar labelled halfway is twice as tall. The bar labelled near torch is four times as tall.near wallhalfwaynear torch

A wrong idea many people have

A shadow is always the same size as the thing.

Why it feels right: In sunlight, a wall shadow is about the size of the thing.

The test that breaks it: Hold your hand near a torch. Look at the wall. The shadow is far bigger than your hand. Move your hand toward the wall, and watch it shrink.

What is true: A lamp can make a shadow much bigger than the thing. The nearer the lamp, the bigger the shadow.

Key words

shadow
a place where a thing blocks the light.
outline
the line round the edge of a shape.
distance
how far one thing is from another.

Back to your guess

The answer is B: A thing near the lamp makes a bigger shadow.

Light spreads out from a lamp, like a fan. Behind a thing near the lamp, light has far to spread. So the shadow grows.

Check yourself

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

  1. From an earlier chapter

    Which of these makes the darkest shadow?

    Tap your answer.

    1. Yes, that is right. Wood is opaque. It blocks all the light, so the shadow behind it is dark.

    2. Not this one. Light goes through clear glass. It is transparent, so it makes almost no shadow. Go to S2.6.4. Try another.

    3. Not this one. Some light goes through thin paper. It is translucent, so the shadow is faint. Go to S2.6.4. Try another.

    See the answer

    A wooden board

    Wood is opaque. It blocks all the light, so the shadow behind it is dark.

    • If you chose B: Light goes through clear glass. It is transparent, so it makes almost no shadow. Go to S2.6.4.
    • If you chose C: Some light goes through thin paper. It is translucent, so the shadow is faint. Go to S2.6.4.

  2. Ana moves a book nearer to a lamp. The wall stays where it is. What happens to the shadow of the book?

    Tap your answer.

    1. Not this one. You may think the shadow shrinks as the light gets closer. Light spreads like a fan, so it grows. Try another.

    2. Not this one. That is nearly true in sunlight. The Sun is so far away. A lamp is close, so the size changes. Try another.

    3. Yes, that is right. Light spreads out like a fan from the lamp. Near the lamp, it has more room to spread behind the book.

    See the answer

    It gets bigger.

    Light spreads out like a fan from the lamp. Near the lamp, it has more room to spread behind the book.

    • If you chose A: You may think the shadow shrinks as the light gets closer. Light spreads like a fan, so it grows.
    • If you chose B: That is nearly true in sunlight. The Sun is so far away. A lamp is close, so the size changes.

  3. A torch is 6 steps from a wall. A card is 2 steps from the torch. The card is 10 cm tall. How tall is the shadow?

    See the answer

    30 cm

    6 steps to the wall, and 2 steps to the card. 6 ÷ 2 = 3. So the shadow is 3 times as tall. 10 cm × 3 = 30 cm.

  4. In the sun, you move a book away from a wall. Why does its shadow hardly change size?

    Tap your answer.

    1. Yes, that is right. The Sun is very far away. Its light does not spread like a fan. So the shadow stays about the same size.

    2. Not this one. The Sun is our best source of light. It is not a thing that only bounces light. Go to S2.6.1. Try another.

    3. Not this one. It is the other way round. The Sun is far, far away. Its light comes almost side by side. Try another.

    See the answer

    The Sun is very far away.

    The Sun is very far away. Its light does not spread like a fan. So the shadow stays about the same size.

    • If you chose B: The Sun is our best source of light. It is not a thing that only bounces light. Go to S2.6.1.
    • If you chose C: It is the other way round. The Sun is far, far away. Its light comes almost side by side.

  5. Is the shadow of a yellow flower yellow? Say why.

    See the answer

    No. A shadow has the outline of the flower, but no colour.

    The flower blocks the light. Little light reaches the shadow. It is dark and shows no colour.

  6. A lamp is on your left, as high as your head. Which side of you does your shadow fall on?

    Tap your answer.

    1. Not this one. That is the side of the lamp. A shadow falls away from the light. Try another.

    2. Yes, that is right. A shadow falls on the side away from the light. The lamp is on your left. So the shadow is on your right.

    3. Not this one. The lamp lights you from the side. So the shadow falls to the side, away from it. Try another.

    See the answer

    Your right side

    A shadow falls on the side away from the light. The lamp is on your left. So the shadow is on your right.

    • If you chose A: That is the side of the lamp. A shadow falls away from the light.
    • If you chose C: The lamp lights you from the side. So the shadow falls to the side, away from it.

  7. Tariq wants a smaller shadow of his hand on the wall. The torch stays still. What should he do?

    A torch on the left, a hand in the beam and a wall on the right. The hand makes a dark shadow on the wall. Labels: torch, hand, wall.torchhandwall
    Tap your answer.

    1. Not this one. That makes the shadow bigger. The light has more room to spread behind it. Try another.

    2. Not this one. The size does change. A lamp makes a thing's shadow bigger as the thing comes near. Try another.

    3. Yes, that is right. A hand near the wall gives the light less room to spread. So the shadow shrinks.

    See the answer

    Move his hand toward the wall

    A hand near the wall gives the light less room to spread. So the shadow shrinks.

    • If you chose A: That makes the shadow bigger. The light has more room to spread behind it.
    • If you chose B: The size does change. A lamp makes a thing's shadow bigger as the thing comes near.

  8. Why is a shadow dark?

    Tap your answer.

    1. Not this one. Weight does not matter. A thin card and a heavy stone both block light. Try another.

    2. Not this one. A reflection sends light back. A shadow is a place the light cannot reach. Try another.

    3. Not this one. The same wall is bright where light reaches it. A shadow is dark because light is missing. Try another.

    4. Yes, that is right. An opaque thing stops the rays. The place behind it gets no light, so it is dark.

    See the answer

    The thing blocks the light.

    An opaque thing stops the rays. The place behind it gets no light, so it is dark.

    • If you chose A: Weight does not matter. A thin card and a heavy stone both block light.
    • If you chose B: A reflection sends light back. A shadow is a place the light cannot reach.
    • If you chose C: The same wall is bright where light reaches it. A shadow is dark because light is missing.

Keep it

Copy this into your exercise book.

A shadow falls on the side away from the light. It has the outline of the thing, but no colour. Move a thing nearer a lamp and its shadow grows. In sunlight this hardly shows.

Draw: A torch, a card and a wall. Two lines from the torch past the card. Colour the wall between the lines.

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.