Light, Sound and Waves

Seeing with light - an activity

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

What the Activity is for

Seeing beams of light.

This is part of a suggested sequence:

A repertoire to develop the idea of seeing

Pupils will very often agree that to be able to see, light must enter the eye. However, it is possible to set up situations where this apparently firmly-fixed belief can be productively challenged, and thereby strengthened. This is the aim for this demonstration.

What to Prepare

  • a source of fine dust – board duster or talc
  • a low power laser that provides an intense, fine beam of light

Safety note: Take care to ensure that light from the laser cannot pass directly into anyone's eyes. A class 2 laser should be used, although a laser pointer can be used if under the careful control of the teacher. You should be aware, however, that some laser pointers are incorrectly assigned a relatively low power rating and are potentially more dangerous than they might appear.

Even if a laser beam does enter the eye, the blink-aversion response is sufficiently fast to avoid any damage to the retina except with the most powerful lasers.

What Happens During this Activity

Set up the laser so that, when switched on, the beam travels across the front of the room, landing on a white screen so that the red spot is clearly visible.

Start by describing the set-up to the pupils, showing the laser and pointing to the screen across the room at which the beam is directed. For the sake of this demonstration, all that the pupils need to know is that the laser provides an intense, or very powerful, beam of light.

Explain that you are going to put the room lights out in a moment and ask the pupils to predict what they will see when the lights are out (with a good black-out) and the laser is switched on. This is where the pupils' ideas about seeing are challenged.

Challenging predictions

Former opinion: You'll see the beam cutting through the darkness.

Latter opinion: You'll just see a spot on the screen where the beam hits.

Experience has shown that the former opinion is very common among pupils in the lower secondary school age range. Make sure to bring it out into the open, so that it gets effectively challenged. It's not enough to simply show the process and assume the learning will be automatic.

Now switch on the laser (with appropriate theatrical build-up). Nothing seems to happen. A red spot appears on the screen at the side of the room, but the laser beam itself can't be seen.

The big question is: Why can't we see the laser beam? (Because none of the laser light is entering our eyes). This can lead to an interesting discussion of situations where it is possible to see beams of light cutting through the air. Pupils may well refer to laser shows at pop concerts, the projection lights in cinemas, car headlamps on a foggy night. In each of these cases there is something which will scatter the light into our eyes.

An interesting follow-up question is: How come we can see the red spot on the screen? (Here laser light is being reflected in all directions from the wall and some travels to our eyes).

Use chalk dust from the board rubber (if you still use one) or a fine powder such as talc, or alternatively a fine water spray to scatter the beam. The room light is off and the effect is stunning as the beam becomes visible as light is scattered away from it.

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