### Photons: filling stores drop by drop

**What the activity is for**

In this activity you can introduce students to the idea that energy can be shifted in small chunks. This is particularly powerful if they have already met energy modelled as an orange fluid.

**What to prepare**

- some yellow food colouring
- a collection of measuring spoons (Careful research will find some in a range of colours corresponding to the spectrum.)
- a selection of beakers to hold the fluid
- some means of marking the beakers to show the store they represent

**What happens during this activity**

This is a demonstration activity, requiring a showman's approach. You might start by revisiting the idea of energy modelled as an orange fluid, by pouring fluid from store to store. In this it's the filling and emptying that's important, not where the stores are. It's a model of the energy description, and we're about to refine that by adding the constraint that there are smallest quantities of orange fluid that can be added or subtracted in the quantum description.

Do take care not to conflate the physical description(no-one knows anything about the photons, or their path

, between emission and detection) with the energetic description.

One fruitful example to pursue is being warmed by the Sun on a summer's day. Discussion here will lead to the idea that a nuclear store is being emptied and a thermal store being filled. The pathway connecting the two – that is emptying the nuclear store and filling the thermal store – is a heating by radiation pathway. It's the pathway that we're going to focus on here.

Now is the time to produce the spoons. The red spoon should be of small volume, the green spoon of medium volume, and the blue or purple spoon of large volume. If you are fortunate enough to find teaspoon, dessert spoon, and tablespoon measures, then you will find that the ratio of volumes is, most conveniently, the same as the ratio of the number of joules added to, or subtracted from, the store by a photon of the corresponding frequency. Present the students with a choice: would you like to be warmed by red light, green light, or blue light? Choose your spoonful and your thermal store can be warmed drop by drop. The drop size varies with the frequency – so with the colour.

Now choose a spoon. Take a spoonful from the nuclear store, talking through the emptying of the store by the heating by radiation pathway, one photon at a time. Add the spoonful to the thermal store, talking through the filling of the store by the heating by radiation pathway, one photon at a time. Take care not to suggest

Repeat the process with the other coloured spoons. Emphasise the point that what is changing is the energy shifted by each photon: its not continuous pouring any more – instead the filling or emptying is one quantum of energy at a time.

You might deliberately move the spoons along different tracks on their way from one store to another – after all, it's only the emitting and absorbing that we're modelling here. Anything to avoid the Wrong Track

idea that photons are little bullets that travel in straight lines from one store to another

.

Now ask how you can fill the store very quickly. Draw out the two possibilities: small spoonfuls, arriving at a great rate or larger spoonfuls, arriving at a lower rate. Act out the two possibilities. You might like to explicitly introduce the idea of compensation here – that a high activity (many photons per second) can compensate for a small amount of energy shifted by each photon. This is true for the power: but not for all effects. Red photons cannot produce sunburn: a reminder that we do need the photon model. Keep in students' minds that there is more to the interaction between light and matter than the power in the beam.