Activity 1.2 - How does light travel?

• Understanding that light is present in the space around a light source
• Understanding that the direction light travels to an observer may be modelled as a straight line ray

The incandescent bulb is placed in the centre of a darkened classroom and switched on. The students gather close to the bulb and, if they can see light from the bulb, should be asked to raise their hands. They then line up around the walls of the classroom with a large space between each of them and the process repeated. The teacher should then ask whether they would be able to see light from the bulb if they stood in the gaps that were deliberately left between each of them. Finally, the students should face the wall of the classroom and asked to raise their hands if they can still see light from the bulb. The bulb should be turned off for a moment, and then turned back on, and the students asked if they wish to reconsider whether they can see light from the bulb when not facing it. Using the acetate and overhead projector, the bulb is represented by a dot in the centre and the relative positions of the students and their direction of view marked on the acetate with arrows for the three cases. This forms roughly-concentric rings of different diameters. The students should then discuss how they think the light reached them and the teacher should guide this discussion towards conclusions that involve light ‘spreading out’ from the bulb. This can be directly compared to sound waves. They should then discuss how they think light reached their eyes when facing the wall and consider the direction that light appears to travel from the bulb to each observer. The teacher can guide the discussion towards conclusions involving straight line paths or ‘rays’ from the bulb.

The aim of this exercise is to establish in the students’ minds that light is a wave, that light exists in the space around them, that light reflects from the wall, and that light can be modelled using rays. These concepts are reinforced in Worksheet 1.2.

As an additional component, the students could be asked to consider where they would need to stand to see the light from the bulb after a box (with a small hole in the side) is placed over it. They can then test their ideas by performing a similar ‘hands-up’ experiment to that at the beginning of the activity, and can map the positions from which they can see the light from the bulb. This can be used to show the validity of the ‘ray’ model in predicting where the students need to stand to see the light exiting the box.

• If you changed the size of the hole in the box, would this change where in the room you could see the bulb?
• What happens to the light that doesn’t come out of the hole in the box?