Explaining shadows with ray diagrams
What the activity is for
In this activity the teacher talks through the construction of a simple ray diagram for the formation of a shadow, making clear all of the conventions and other thinking behind it.
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 a screen.)
- a table tennis ball (or some other smooth-surfaced ball) hung from a piece of cotton (super-glue the cotton to the ball)
- this interactive resource, enlarged on a screen or interactive white board.
What happens during this activity
The interactive diagram provides a focus for the attention of the whole class. Your job here is to talk through each of the steps of constructing the ray diagram, engaging the pupils in discussion as you proceed.
First, 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.
Constructing a ray diagram
Demonstrate the shadow to the class and pose the key question:
Teacher: How can we draw a diagram to show how the shadow is formed on the screen?
Now turn the class's attention to the screen. Here is a possible commentary to go alongside whole class use of the animation.
Teacher: [Screen 1] We have a light source here which emits light in all directions.
Teacher: [Screens 2 and 3] Putting a shade over the light makes a beam. Some of the wall is lit, and some not.
Teacher: [Screens 4, 5 and 6] Drawing in special paths the light follows helps to predict which bits will be lit, and which not. These chosen paths are called rays. Every ray that you draw must be shown as a straight line and must have an arrow to indicate the direction in which the light is travelling.
Teacher: [Screen 7] So, now we place the ball in front of the projector … choosing rays carefully, to make predictions as we alter the size of the ball and move the ball.
Teacher: How can we use this diagram to show exactly where the shadow will be formed on the screen?
Teacher: OK, so take the ray of light which is just passing over the top of the ball and the ray which is just passing under the ball.
Teacher: OK, let's try moving the light source backwards and forwards. What do you think will happen to the size of the shadow?
Teacher: What about changing the size of the ball?
Teacher: Now let's think about moving the ball up and down.
Teacher: [Screen 8] Here we can see the beams predicted by the ray diagram.
You might want to switch back and forth between screens 7 and 8, altering the predictions on screen 7 and then seeing how they work out on screen 8.