Images, Resonances, Echoes, take 9

Photons are finally here. Who knew such a slow arrival was possible? Maybe COVID tunnelling is a phenomenon after all or at least COVID-induced lassitude. Introspection suggests the latter, unreliable as it is.

Topics around light are typically taught by a mixture of assertions about light and waves and use of ray diagrams to make predictions. I suggest that a radical overhaul would improve matters.

- By providing a unified predictive account of the phenomena associated with light.

- By connecting thinking about light to a well-founded way of developing the idea of energy.

- By exposing younger children to quantum re-imaginings of the lived-in world.

Perhaps the last has the most cultural value right now: we desperately need the imagination to see how our world might be, rather than trying to get it back to what it was. So creativity based on wrestling with empirical evidence – sound like physics to me.

Images - that cat, entanglement, non- locality

Non-locality and entanglement are as real as it gets in the world we have.

(Wikipedia’s image of this reality.)

- Schroedinger’s cat

- Wigner’s Friend

- Bell’s theorem.

There is well-evidenced unavoidable weirdness, unlike the lived-in world. ’Spooky action at a distance’ is for real.

A photon is about the simplest accessible quantum object – it enables seeing and you can characterise it using just frequency. (I think Qbits and spin don’t make a good entry point for younger children, because it’s a long haul to see what they can do for you. Others disagree, and are developing schemes to teach about quantum physics for older children.)

To take the idea of photons seriously is to engage with Physics as seeking unification, rather than ad-hoc rules of thumb. If experiment compels rules on us, the aim is to have only a few, so they should be of wide scope.

More reading

Ball, P. Beyond Weird. 2018. The Bodley Head.ödinger%27s_cat and onward links

Resonances - exploiting the idea of a photon

See something, and it’s either bright or dim and of a particular colour. Accounting for this is easy with photons.

Photons and the box of seeing

But photons are not little bullets of light that travel from source to detector. You know where they’re emitted, and where they’re detected, but between those two events, you’re guessing.

You can vary what’s between source and detector and figure out how that affects the brightness.

Vary what's between source and detector

Several steps along in the sequence( for the full sequence, see the further reading), predicting where is bright and dim requires that photons don’t restrict themselves to what we might imagine as obvious paths. So the model evolves to sum contributions from different explored paths.

multiple paths

Using this single approach, but varying the paths allows you to develop a unified account of refraction, reflection, diffraction, interference and propagation.

A triplet of paths for the mirror

Two pairs of triplets for interference

Prefer the physical? You can do all of these on a benchtop with a pizza-cutter or technical Lego ‘photon wheel’ to perform the geometrical experiments and a large piece of paper. (Special thanks to my class of guinea pigs who tried all this out).

And you’d be showing and measuring the phenomena you’re predicting.

This sequence is not a suggestion to replace physics with slick software simulation, instead exploiting the affordances of software to represent, suggesting that new didactical paths are workable.

More reading

Feynman, Richard P. (1985). QED: The Strange Theory of Light and Matter. Princeton University Press. (A special mention to the first year sixth-former who put this on my desk in 1990, and asked why my lessons were not more like this.)

Lawrence, I., Whitehouse, M. (eds) (2000) Advancing Physics AS CD. Institute of Physics Publishing, Bristol. (chapters 6&7)

What it would be like to think through this sequence as a teacher:

What a supporting children’s text might look like:


Images, Resonances, Echoes 05, for trip times being the basis of journeys.

Images, Resonances, Echoes 04 for resonance as lock and photons are available as keys.

Images, Resonances, Echoes 06, for a good way of teaching using the idea of energy.

Echoes - you can’t beat string

The software activities of engineering a lens and engineering a focussing mirror can both be done of the beach top, without any need to involve the photon at all.



You just need five identical lengths of string with coloured bands on, showing the phase, as preparation for a superposition argument.

To model a lens, start with the bands lined up at the ‘source’. Demand that they’re also lined up at the ‘detector’. The shape of a converging lens emerges because of the need to stack up the bands for the most direct paths beaten source and detector.

To model a focussing mirror, start with an array of evenly spread strings, with the bands aligned. Demand that they’re also lined up at the ‘detector’, after a reflection. The shape of the mirror emerges because the paths far away from the centre line must traverse a shorter distance parallel to their original direction to make up for the greater distance they must cover after reflection to reach the centre line.

Both arguments rely on distance setting trip time: then the differences in trip time between paths set the phase differences.

This pair initially published as activities on the Advancing Physics AS, chapter 6, and before that, nursed into life in what was then my physics prep room.

More reading

Lawrence, I., Whitehouse, M. (eds) (2000) Advancing Physics AS CD. Institute of Physics Publishing, Bristol. (chapter 6)