# 02Photons shift energy

• ## 01 Modelling photons filling storesRa02TAnugget01 Activity

### 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.

• ## 02 Photons and filtersRa02TAnugget02 Activity

### Linking activity in the laboratory with a photon description

What the activity is for

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).

• ## 03 Counting photons one by oneRa02TAnugget03 Activity

### A photon counter; the Geiger–Müller tube

What the activity is for

This activity is all about hearing photons arrive, one by one. Draw out the random nature of their arrival, and the fact that the distance between the source and the detector seems to affect that rate.

What to prepare

• a Geiger–Müller tube, connected to a counter with an audible output
• a gamma source
• some absorbers (thin sheets of lead and aluminium are suitable)

Safety note: This activity uses a radioactive source, and these should not be handled by students under 16 years of age.

What happens during this activity

Switch on the Geiger–Müller tube, and connect the counter, introducing it as a photon counter. Then introduce the gamma source as a photon source. Draw out some expectations about the detected count rate – what you hear – as the source is moved closer to or farther away from the detector. You might support the development of these expectations by asking whether the source looks more like a beam, more like a point source or somewhere in between. In any case you will find that the count rate drops with increasing separation between the detector and the source. This corresponds very nicely with the drop in brightness as you move farther and farther away from a light source. You might choose to have a light source to hand, precisely to draw out this connection.

If you have such a light source, you could start the next part of the experiment by placing an absorber in front of it (layers of tracing paper or greaseproof paper are probably about right). Draw out from the class what they'd expect to hear from your photon counter if you now place appropriate absorbers in front of the photon source. You'll need to be careful here to draw out the analogies in such a way that makes it obvious that we are dealing with two kinds of photons: those corresponding to optical frequencies and those corresponding to much higher frequencies, that we cannot see, but that our special photon counter can see.

You can extend the experiment further by comparing thicknesses of lead with thicknesses of photocopy paper, and thicknesses of aluminium with thicknesses of tracing or greaseproof paper. You'll need to practise to make this convincing. Do make the connection with the action of an optical filter, by linking the action of the additional sheets to the absorption of a fraction of the photons. Each sheet of absorber removes a certain proportion of the photons from the beam.

• ## 04 Photons shift energyRa02TAnugget04 Activity

### Photons shifting energy: photochromic paper

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.

• ## 05 Diagnostic questions about photonsRa02TAnugget05 Activity

### Diagnostic questions exploring the understanding of lighting as a stream of photons

What the activity is for

The diagnostic questions can be used for two main purposes:

• to encourage students to talk and think through their understandings of the light as photons model
• to provide the teacher with formative assessment information about the students' understandings of light as photons.

What to prepare

• printed copies of simple questions on photons

Support sheet

What happens during this activity

It would be a good idea to get the students to work in pairs on these questions, encouraging them to talk through their ideas with each other. Collect responses from all of the pairs and discuss as a whole class.

Alternatively, the questions might be set for homework prior to the lesson, so that you have time to read through the responses.

Question

Two light sources give out identical beams of red light. The supply to one of the sources is turned up to increase the brightness of the beam. The source with the brighter beam gives out:
1. the same number of photons/second of higher frequency.
2. more photons/second of the same frequency.
3. more photons/second of higher frequency.
4. more photons/second of lower frequency.

B. Identical beams means same frequency. A brighter beam means more photons/second.

Question

Ultraviolet light causes sunburn of the skin whereas infrared light does not. This is because:
1. the infrared light needs more time to build up sufficient energy to cause sunburn.
2. the infrared photons are bigger than the ultraviolet photons.
3. the ultraviolet photons are bigger than the red photons.
4. the intensity of the infrared light needs to be increased.

C.

Question

When even a very weak beam of blue light is shone on sodium metal, an effect takes place such that electrons are released immediately from the surface of the sodium. This effect must be due to:
1. many relatively low-energy photons arriving each second.
2. a few relatively low-energy photons arriving each second.
3. many relatively high-energy photons arriving each second.
4. a few relatively high-energy photons arriving each second.