Light, Sound and Waves

In straight lines

Classroom Activity for 5-11 11-14 Supporting Physics Teaching

What the Activity is for

This is a teacher demonstration activity to show that light travels in straight lines.

What to Prepare

  • a bright light source. Use some kind of point source (where the light comes from a relatively small aperture) to produce a well defined shadow on the screen
  • a table tennis ball (or some other smooth-surfaced ball) hung from a piece of cotton (super-glue the cotton to the ball)
  • a screen

What Happens During this Activity

Hang the ball from a metre rule and hold it in the beam of light between the light source and the screen. With a good black-out for the room and a strong point source of light, a striking shadow will be produced on the screen.

Draw the pupils' attention to the sharpness of the shadow, how clean the line is between light and no light. Also, make the point that the shape of the shadow, although bigger, is exactly the same as the shape of the ball. This leads to the idea that the light must be travelling in straight lines to create such a shadow on the screen. The notion of light travelling in straight lines may seem obvious to many of the pupils. You might wish to explore an imaginary scenario which involves the light from the source bending around the ball. What would the shadow on the screen look like then?

Ask the pupils to predict what will happen to the shape and size of the shadow on the screen, if the ball is moved closer to, and further from, the screen.

In developing these ideas, you are likely to find yourself sketching out ray diagrams for the pupils.

is formalised by Law of Reflection
can be exhibited by Progressive Wave
has the special case Total Internal Reflection
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