How a shadow forms

The idea

Because light travels in a straight line, an opaque object in its path blocks it; the dark patch where light does not reach is the shadow.

Your own shadow

You have seen your own shadow in the Sun or under a light.

What happens to the light when an opaque object is in its way?

• The object blocks it
• The light passes straight through
• The light bends round the object

Follow the rays

Light rays go from a lamp to a screen. Place an opaque object in their way, then take it away.

The dark patch

The rays that meet the object are stopped. They cannot reach the screen. The dark patch that is left is the shadow.

Because light travels in a straight line, an opaque object in its path blocks it; the dark patch where light does not reach is the shadow.

Notes

Because light travels in a straight line, an opaque object in its path blocks it; the dark patch where light does not reach is the shadow.

Check yourself

What is a shadow?

Why does an opaque object form a shadow?

Which rays reach the screen?

The opaque object is taken away. What happens to the dark patch?

• The bright spot of light on the screen. That is where light reaches. The shadow is where it does not.
• The dark patch where light does not reach — correct. Yes! That is the shadow.
• The lamp that gives the light. The lamp is the source of light. The shadow is a dark patch.
• It makes the light brighter behind it. Behind the object there is a dark patch, not a brighter one.
• Light bends round it and meets behind it. Here light travels in a straight line, so it does not meet behind the object.
• It blocks the light, which travels in a straight line — correct. Yes! The light cannot go round it.
• The rays that do not meet the object — correct. Yes! The other rays pass by.
• All of them. The rays that meet the object are blocked.
• Only the rays that hit the object. Those rays are stopped by the object.
• It stays where it was. The shadow needs the object to block the light.
• It disappears, because light reaches that part of the screen — correct. Yes! Nothing blocks the light now.
• It becomes bigger. With no object there is no shadow at all.