How far can light travel?
Teaching Guidance for 11-14
Reasons for not seeing glowing objects?
Wrong Track: The light runs out before it gets to us.
Wrong Track: Light's only got quite a long range: then it stops.
Right Lines: Light keeps going until it gets absorbed. But there might not be enough light entering your eye for you to see something – although your dog might still be able to!
Light doesn't run out or get used up
Thinking about the learning
On the one hand pupils are quite prepared to accept that light can travel 150 million kilometres from the Sun to the Earth and yet at the same time believe that light from their torch beam gets used up in a matter of a few metres.
The light from the torch becomes progressively more spread out and may also be scattered and absorbed by particles in the air (more in the physics narrative). The light is not 'lost', it just becomes more spread out, scattered and absorbed.
This applies to the very bright sunlight, or to the somewhat dimmer torch. Light just does not 'get lost'.
Thinking about the teaching
Although the spreading and scattering of light are not referred to in all curricula, they are certainly necessary to offer any kind of plausible explanation for the relatively short range of a torch beam. One teaching colleague regales his pupils with the mantra:
Teacher: Light doesn't run out, it spreads out!
Furthermore, the only reason that you can actually see the torch beam cutting through the night air is because light is being scattered to your eye. These are ideas that pupils are ready to accept and we would advise that you use them. See the teaching activity that depends upon the scattering of the intense laser beam by chalk particles in the air.