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Shadows

Waves, sound and light · level 2, lesson 3 of 6

A question to start

In the middle of a sunny day your shadow is short. In the evening it stretches out long across the ground. Why does a shadow change?

The idea

A shadow forms when something blocks light. Light travels in straight lines, so behind the object there is a patch the light cannot reach. That dark patch is the shadow, and it has the same outline as the object.

  • Opaque things, like wood or your hand, block light and make dark shadows.
  • Translucent things, like tissue paper, let some light through and make pale shadows.
  • Transparent things, like clear glass, let almost all the light through and make hardly any shadow.
lampobjectshadow on the screen

The closer the object is to the lamp, the bigger its shadow. Move it towards the screen and the shadow shrinks.

Worked examples

1. Shadow puppets. Hold your hand close to a torch and its shadow fills the wall. Move it near the wall and the shadow is small and sharp.

2. The Sun's height. When the Sun is high, at midday, shadows are short. When it is low, in the morning or evening, shadows are long.

3. A sundial uses the shadow of a stick to show the time as the Sun moves across the sky.

Try it

  1. What makes a shadow?
  2. Does clear glass make a dark shadow?
  3. How can you make a shadow puppet bigger?
  4. When are shadows longest?
Answers
  1. Something blocking light.
  2. No. It lets almost all the light through.
  3. Move it closer to the lamp.
  4. In the morning and evening, when the Sun is low.

Common mistake

"A shadow is a reflection." A shadow is a patch where light is missing, because it was blocked. A reflection is light bouncing back.

Where it leads