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Light travels in straight lines

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

A question to start

You can hear a friend talking round a corner, but you cannot see them. Why can sound get round the corner when light cannot?

The idea

Light travels in straight lines. It does not bend round corners on its own. So if something is in the way, the light cannot reach your eyes.

You can see the straight lines when light shines through mist, smoke or dust: the beams of a torch or of sunlight through trees look like straight rods.

holes in a straight line

Worked examples

1. Three cards. Make a hole in the middle of three cards and stand them in a row. You can only see a torch through all three when the holes are in a perfectly straight line. Move one card and the light is blocked.

2. Sunbeams. On a misty morning in a forest, you can see straight beams of sunlight between the trees.

3. Round a corner. Sound spreads out round corners, but light keeps going straight. That is why you hear your friend but cannot see them.

Try it

  1. How does light travel?
  2. Why can you see a torch beam in a dusty or misty room?
  3. You move the middle card a little. What happens to the light?
  4. Why can't you see round a corner?
Answers
  1. In straight lines.
  2. The light bounces off the dust or mist, showing the straight path of the beam.
  3. It is blocked; you cannot see the torch any more.
  4. Light travels in straight lines, so it cannot bend round the corner to your eyes.

Common mistake

"You can see the light beam in clear air." You only see a beam when something in the air, like dust or mist, bounces some of its light to your eye. Mars's dusty air is good at showing beams.

Where it leads