Photons and filters

What the Activity is for

Linking activity in the laboratory with a photon description.

In this activity you get to use the idea that filters work by absorbing different frequencies of photons with a different chance (that is, different levels of probability). This is quite a complicated idea, and should not be underestimated. In particular, you might like to revisit the idea of perceptual and spectral colour introduced in the SPT: Light topic. Re-describing the activity of the filter using photons gets closer to the idea of a mechanism by which the filter might work. Therefore it is a gentle and good introduction to the idea that photons can act as an explanation for the interaction between light and matter, so not following the rather common practice of just introducing photons through the photoelectric effect.

What to Prepare

• a set of filters
• a source of white light
• if possible, some sources of coloured light (coloured light emitting diodes (LEDs) may be suitable)
• possibly a webcam, connected to a large computer screen
• perhaps a prism

What Happens During this Activity

The intention here is to get the students to discuss how the action of filters can be described in terms of their propensity to pass photons. A simple statement about a filter's action is that it passes light of one or a small range of frequencies. But this is only equivalent to saying that it passes light of one or a small range of colours. It therefore feels a bit unsatisfactory as an explanation. By re-describing what is happening in terms of photons, you can introduce a bit more of an idea of mechanism, and begin to introduce the tools to explain what is going on. However, the interaction of light and matter is fantastically complicated and you should emphasise that the models you are introducing here are just an improvement on the simple statements that we started off with. The models are not the final and most complete models now available: those require a much deeper understanding of atoms, molecules and photons. But what we're talking about here is along the right lines.

Perhaps start with a red LED. You might try passing the beam through a prism, to see if you can split the beam into constituent colours. (You hope not to; if you do, choose another LED. Practise, practise, practise!) After this you can describe the beam in terms of photons. You might introduce the term monochromatic. At least you'd want to make the point that the photons are all of one kind: all of one frequency. Contrast this with a different-coloured LED, where the photons are all of the same kind again but of a different frequency. Contrast the first situation also with a white light, where a range of photons are present. The idea is to establish that the mixture of photons present determines the colour of the light that is seen. You might even revisit colour mixing from the SPT: Light topic.

After this introduction, the time has come to introduce the filters. For the selection of filters and light sources that you have, you'll want to work out a sequence where the action of the filter can be persuasively described as removing a fraction of the photons of one or a range of frequencies from the beam. It might be helpful to have a range of filters, and also to have a number of filters of the same colour. Using these combinations you will then be able to make several points about more photons being removed from the beam, by stacking together filters of the same colours, and also points about photons with different ranges of frequencies being removed as you stack together filters of many different colours. In both cases you want to make the point that filters work by removing photons from a beam. These photons are absorbed: they cease to be. You might therefore expect the filters to get warm – they will. This is a real problem if you have very intense light sources – that is, sources emitting many photons in each second. For visible light each photon shifts only around one attojoule (1 × 10^-18 joule). So even if the filter were very well insulated, and none of the energy shifted to the filter as the photons are absorbed was in turn shifted to the surroundings (through the heating by particles pathway), it would take many photons to significantly increase the energy in the thermal store of the filter (and so the temperature of the filter).