Bringing together two sets of constraints

Focusing on the learners:

Distinguishing–eliciting–connecting. How will you:

• build connections between the macroscopic and the sub-microscopic
• develop a model of photons that is based on a wide range of phenomena
• avoid confusing them with the detritus of history

Teacher Tip: These are all related to findings about children's ideas from research. The teaching activities will provide some suggestions. So will colleagues, near and far.

Focusing on the physics:

Representing–noticing–recording. How will you:

• represent photons
• model frequency-dependent phenomena
• connect existing models of energy with shifting energy in chunks
• separate power in pathways from energy in stores

Teacher Tip: Connecting what is experienced with what is written and drawn is essential to making sense of the connections between the theoretical world of physics and the lived-in world of the children. Don't forget to exemplify this action.

A fascinating and challenging step forward

In episode 01, Radiating from source to absorber, a description was developed of light and sound as originating in vibrations that travel from a source as electromagnetic and mechanical waves. This picture of light and sound waves will be a familiar one to students, not just from studies of science in school but also through common usage of these terms in everyday life.

In this episode, we shall be asking the students to take a fascinating and challenging step forward in their learning. This involves coming to accept that light does not shift energy in a steady stream but in tiny, discrete units, or chunks.

Shifting energy in discrete amounts

A common approach to introducing the idea that light shifts energy in discrete chunks or photons is to state that light not only has wave properties but also can be thought of as being like a stream of particles. Such a description of a wave/particle model for light can be unsettling for students, and we think you could be more helpful by being more careful, unlike this teacher:

Igor: So are you saying that light is not waves – that we have been taught the wrong stuff?

Teacher: No… certainly not! What I'm saying is that there are certain things that are best described in terms of light as waves and others which are best described in terms of light as particles.

Igor: But how can light be both a wave and a particle?

One of the problems here is that of describing light as a stream of particles, with the word particles suggesting something real and substantial. This, not surprisingly, prompts questions from alert students:

Harriet: How can you have light particles when light travels through empty space from the Sun? There is nothing there! Are there particles of light passing through space?

These non-productive images and thoughts might be avoided simply by not using the shorthand term light as particles. This episode introduces the ideas that in light shifts energy in discrete amounts. It adds nothing to this new idea to describe these chunks of energy as particles. You might also seek to avoid the equally unhelpful phrase wave-particle duality.

Continuous streams and discrete chunks of energy

In introducing the idea that light shifts energy in discrete chunks, it is important to emphasise to students the key differences from a purely wave-based model:

In a wave model of light, energy is shifted in a continuous stream along the heating as radiation pathway. Imagine a continuous flow of the orange energy liquid introduced in the physics narrative Shifting energy between stores, gradually filling up an energy store as it is absorbed. With the photon model, the image is one of a stream of orange energy liquid droplets filling an energy store chunk by chunk. Furthermore, different kinds of light (infrared, visible, ultraviolet) have energy chunks, or photons, of different value. For example, ultraviolet photons can be thought of as larger droplets than infrared photons. In other words, ultraviolet light shifts energy in bigger chunks.

The same droplet-like model applies to emptying stores – no longer a steady outward flow of liquid, but a drip-by-drip emptying.

Teacher Tip: Now we're emptying and filling stores not by pouring orange liquid but spoonful by spoonful.

Why don't lorry drivers get sunburned arms?

It is tempting to argue that whether the energy of a light beam arrives in a steady continuous stream, or in a steady line of photons, it all adds up to the same thing (in the same way that a series of tiny dots very close together looks very much like a continuous line). This is not the case for many phenomena, which can only be explained in terms of the existence of light photons of different sizes. The following example is a useful one for teaching.

Thinking about the teaching

Lorry drivers spend long hours sat in their cabs, often with full exposure to the Sun. But it is well known that lorry drivers don't get sunburned arms. Why should that be?

The first point to bear in mind is that sunburn is caused by ultraviolet light being absorbed by the skin. As it happens, ultraviolet light is also absorbed by glass, so in the case of the lorry driver, the ultraviolet light arriving from the Sun will be stopped by the lorry window. Hence no ultraviolet light lands on the skin of the driver and no sunburn results. But, again, why should that be?

It is clear that although ultraviolet light is absorbed by the glass of the window, other visible and infrared frequencies are transmitted through the glass. Why don't these radiations cause sunburn? The answer to this question lies with the light-as-photons model.

Ultraviolet light produces sunburn simply because the individual photons of energy shift sufficient energy to trigger the process when they are absorbed by the skin. In the case of infrared or visible light, it doesn't matter for how long the radiation falls on the skin, there will be no sunburn, simply because the individual photons don't shift enough energy.

The picture to have in mind is that of a continuous deluge of photons arriving from the Sun, with a range of different values of energy. The high-energy ultraviolet photons are stopped by the lorry window, while the lower energy photons pass through. Each individual lower energy photon contributes to filling an energy store somewhere, but none of them are able to brown (or pink in more extreme cases) the lorry driver's white arms.

So the process of sunburning involves an energy threshold. In other words, it can only be triggered if chunks of energy of sufficient size are involved.

A photon picture of identical bright and dim bulbs

Wrong Track: The brighter light must be giving out bigger photons of light. That's why it's brighter.

Right Lines: Both bulbs are giving out the same colour light and so the photons from each have the same range and mixture of frequencies. Therefore the energy shifted by the average photon from each is the same. The brighter one is simply giving out more photons per unit time.

Not more photons, just higher energy photons

Thinking about the teaching

The description of bright and dim sources of light of different frequencies in terms of photons is absolutely fundamental to this way of thinking and needs to be emphasised in teaching:

Teacher: So, we turn up the supply to the lamp. What happens? Describe what you can see.

Debbie: It's brighter.

Teacher: Yes, it's brighter. So who can tell me what's happening here in terms of photons?

Tash: They're coming out quicker.

Teacher: Careful! More photons leaving the lamp each second.

Teacher: Now then… one of the students in my other class said that there must be bigger photons coming out from the lamp. What do you make of that?

Roger: It's wrong!

Teacher: Yep… but why?

Karim: Because bigger photons means a different frequency and this is just one lamp.

The key teaching and learning point here is to differentiate clearly between intensity and frequency in terms of the photon model.

Making links between different areas of physics

It's always useful to make links between different areas of learning in physics. Whenever a student says, Oh! It's a bit like what we did before, you have an indication of a deeper form of learning taking place. This is to be encouraged.

A useful link can be made between the power in a lighting pathway and the power in an electrical working pathway. The mathematics takes exactly the same form in each case.

For the lighting pathway:

The rate at which energy is shifted by the pathway depends on the energy shifted by each photon and on the number of photons each second. The product of these two variables gives us the power in the pathway.

For the electrical circuit:

The rate at which energy is shifted by a bulb depends on the energy shifted by each charge (the voltage) and on the quantity of charge each second (the current). The product of these two variables gives us the power in the pathway (the power output for the bulb).

Making links of this kind allows students to begin to see how different topics and ideas fit.

Photons and reflection, refraction and spreading

It is important to recognise that the light-as-photons model applies to the various phenomena discussed in episode 01, including reflection, refraction and the spreading of light from a source. They all involve emission and absorption: it's these two processes that require the photon model.

For example, in the case of the light spreading from a source, we might now picture what is going on in terms of streams of photons spreading out in all directions. The fact of the matter is that the intensity of the light beam will still decrease in magnitude according to an inverse square law with distance. The butter gun analogy still works for photons travelling out through space, but please remember that this is just a picture to help us.

Photons behave as they do: we have to try to describe that in the most helpful way. Find more about this in the Physics Narrative.

Teacher Tip: Photons shift energy in drips, not in a stream. Energy is shifted droplet by droplet.

A very short biography of Planck

The remarkably simple equation, E = h × f, tells us how photon size is related to frequency via Planck's constant. But who was Planck?

Max Planck was born in Kiel, Germany, in 1858. He came from a traditional intellectual family, his father being a professor of law. Planck developed an interest in physics at school and then went on to study physics at Munich University, starting in 1874 at the age of 16. In 1877 he went to Berlin for a year of study with physicists Hermann von Helmholtz and Gustav Kirchhoff, and in February 1879 he defended his dissertation, On the second law of thermodynamics.

In April 1885 the University of Kiel appointed Planck as associate professor of theoretical physics. He was subsequently named as successor to Kirchhoff at the University of Berlin and by 1892 he became a full professor. In 1900 Planck introduced his revolutionary idea, now known as the Planck postulate, that electromagnetic energy can be emitted only in quantised form or in chunks. In other words, the energy can only be a multiple of an elementary unit, E = h × f, where h is Planck's constant. At first he considered that quantisation was only a purely formal assumption… actually I did not think much about it. Nowadays this assumption, incompatible with classical physics, is regarded as the birth of quantum physics and the greatest intellectual accomplishment of Planck's career.

Planck retired from the University of Berlin in 1926, and was succeeded by Erwin Schrodinger. In January 1945, his second son Erwin, to whom he had been particularly close, was sentenced to death by the Nazi Volksgerichtshof because of his participation in the failed attempt to assassinate Hitler in July 1944. Planck died in 1947 at the age of 89.