Energy and Thermal Physics

Energy shifted in many small chunks

Physics Narrative for 14-16 Supporting Physics Teaching

Some effects are frequency dependent

Devices for lighting a room switch power to a particular pathway: the heating by radiation pathway. If the source emits at least the frequencies that our built-in detectors respond to, you'll see the environment in colour. However bright or dim the beam, the colour doesn't change. This suggests that there is something intrinsic to whatever it is that forms the physical basis of the beam that is not connected to the amplitude.

The SPT: Light topic established the connection between seeing a particular spectral colour and the frequency of vibration of the beam. This is correct, but a useful and necessary refinement has been added since the turn of the last century.

Increasing the brightness of the beam increases the power in the lighting pathway. For some processes, this description is not good enough: these depend on something about the beam other than there simply being enough power in the pathway. Some effects of the beam are frequency dependent. This should not really be a surprise – even seeing colour is clearly a frequency-dependent effect.

Some effects cannot be produced just by making the light brighter

No matter how bright a beam of red light shone onto a white wall, the wall will never appear violet. There are a whole range of phenomena like this, for which a rather simple modification to the SPT: Light topic model of lighting provides the most plausible, intelligible and fruitful explanation. That is not to say that the deeper consequences of the theory are easy to understand, but it is easy to get started.

Lighting is granular – a patter of very small drops, each shifting a very small amount (a quantum) of energy. Determining the power in a pathway is then a matter of finding the rate of arrival of the drops and the energy shifted by each drop.

The photon model of lighting: granular shifting of energy

This granular delivery of energy during lighting is the photon model of light. This was invented by Planck and Einstein in about 1905, but physicists and philosophers are still arguing about how to interpret it. You'll meet some of these issues in episode 04.

If our eyes had evolved to be about ten times more sensitive, then we'd have got started a lot earlier, because we'd see the individual flashes of light arriving at low beam intensities.

What we can say with confidence, based on reliable empirical evidence, is that power in a lighting pathway depends on energy being shifted from sources in small chunks and also being absorbed in these chunks. These chunks are of a particular size, so are called quanta of energy. The quantum of lighting, and indeed all electromagnetic radiating, is the photon. The evidence is not direct, because we cannot detect single photons, but there are phenomena that can only be explained by this model. Now we can build instruments that do detect single photons. You may have heard the clicks that indicate gamma radiation arriving using a Geiger–Muller tube (more in episode 04).

So a beam of light, on the nanoscale, can be pictured as a stream of photons. But, and it turns out to be a big but, anything apart from the emitting and detecting is inferential. There is very good evidence that emission and absorption happen in chunks. However, we simply don't have any evidence for granularity in transit. How could we have? As soon as you detect a photon – it's destroyed, so you cannot spot it in transit.

Lighting is done by variable sized quanta: red patters; blue batters

Lighting in a red beam (spectral red) is done in small chunks: each granule, or quantum, is typically 3 × 10-19 joule.

Lighting in a green beam (spectral green) is done in larger chunks: each quantum (plural quanta) is typically 3.8 × 10-19 joule.

Lighting in a violet beam (again spectral, not perceptual) is typically 4.8 × 10-19 joule.

These are quite small numbers – between 3.0 and 5.0 attojoule (1 attojoule is 1 × 10-19 joule. That's right down at the scale of energies in atomic systems (30–50 attojoule), which is again a clue as to the quantum origins of light.

To get a brighter beam in each case, simply deliver more quanta in each second. A green beam that seems to be green to us (so a perceptual colour, not a spectral colour) may consist of a range of frequencies (see SPT: Light topic), and so a range of photons.

So monochromatic beams of light vary in the effects they can produce as the energy shifted by each photon varies with the colour. The same applies to other radiations, which are called monochromatic by extension, even though there is no colour. Perhaps you can see how this might begin to explain why ultraviolet light causes sunburn, yet bright red light does not, and maybe even why you cannot pick up (i.e. see) radio stations, or indeed most of the electromagnetic spectrum. (Although there are stories in the press of people picking up Radio 4 through fillings in their teeth, these intriguing anecdotes are best left to post-16 studies, where you can explore the mechanisms with your class.)

is exhibited by Photoelectric Effect
can be explained by the Bohr Model
can be described by the relation E=hf
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