Photons shift energy

What the Activity is for

This is a rather fine review activity, and should be undertaken towards the end of an introduction to the idea of photons.

What to Prepare

• a large sheet of photochromic paper
• a powerful light source

Safety note: Do not shine the light straight into students' eyes.

What Happens During this Activity

Introduce the photochromic paper by placing your hand on it, and so adding energy to its thermal store using the heating by particles pathway. It's important to get an energy description in early. The focus here is on energy and not on temperature. So we're using the changing colours of the photochromic paper to show that energy is shifted to the thermal store of the photochromic paper. Of course, if we are investigating a thermal store, change in temperature is precisely the clue that we look for in order to determine whether the store is being filled or emptied. But we suggest steering the conversation towards energy descriptions.

Now now push a rubber back and forth across the photochromic film and again see energy shifted to the thermal store. This time the energy is shifted through the mechanical working pathway.

Finally, use the heating by radiation pathway.

Shine the powerful beam at the photochromic film. Watch the colours change (only deduce the increase in temperature as an intermediate step if necessary) and draw out that energy has been shifted to the thermal store – again! Tell the story about the photons arriving at the photochromic film. Perhaps you have a powerful infrared source. If so, it may be worth trying to again fill this thermal store but this time with photons that are invisible. It's still the heating by radiation pathway, doing remote working. It's a kind of magic. By doing something over here (emptying a chemical store) I can warm something up over there. What happens over here (the emission of photons from the filament) affects what happens over there (the absorption of photons into the photochromic film). Do make links back to do like me later and remote working, introduced earlier in this topic.