Explainer: something about Photons

This piece should be read with The Moody Blues ‘In Search of The Lost Chord’ playing softly in the background.

I’m writing at the time of lockdown and getting as much outdoors action as I can, so its onto the bike and around the country lanes of Dorset as often as possible – exercising the muscles, but also exercising the eye muscles that have been staring at computer screen, TV screen, book, food, wall, the rest of the time.

I got to thinking about the light from all of those leaves on all of those trees coming from the woodlands over a mile away – light headed in my direction, and following me as I ride along the lane! So much light action going on! The photons aren’t following me, of course, I’m just riding through the constant stream of light that’s being broadcast in every direction from every individual leaf.

And if I got off my bike and walked across the fields towards those trees, I would be walking into the light created from the leaf. The cone of light coming into my eye would just widen until my nose was touching the leaf – and still the light wouldn’t become pixelated – there’s SO MUCH light there – so many photons!

Two questions then:

What colour is a photon?

For me, this is the real mind-blast. Ever since Newton whipped out his prism and exposed the rainbow inside the beam of white light we’ve been fed the story that white light can be broken down into separate colours, when – in truth – the white light is just a construct. There is no WHITE LIGHT that isn’t made from a bunch of photons of different wavelengths coming together. That colour mixing is just brain business.

Every photon is an individual wavelength within the electromagnetic spectrum: the visible light spectrum runs from around 390 – 700nm. I make that a range of 310 nanometre widths, so that’s 310 possible colours, if we limit our imagination to one tone per nanometre step – which seems very limiting; you can, after all, have half a nanometer, or a tenth of a nanometre, if you fancy.

It’s reckoned that the red-green and the blue-yellow cones (and don’t start with me on the trichromatic colour theory!) can each recognise 100 colour differences, so that offers up 100×100 variations, which brings us up to 10,000 without blinking. And that’s before we start considering the shading from light to dark – possibly another 1000 variations.

And that’s just for us lazy humans. There are species out there working a lot harder on colour differential than us.

How big is a photon?

A photon is its own thing; it is neither a particle nor a wavelength, so if we ask ‘how big’ we’re obliged to look at things a bit differently. It’s called wave-particle duality . . . no, me neither.

But if we think of it as a wavelength, then we understand its ‘size’ as being the peak-peak of the sine wave. For ‘green’ that would be around 500nm. (500 x 10-9m or 0.0000005m). As for its length – well, that’s piece-of-string territory. A photon from the sun must be – ooh – 93 million miles long. If it’s from the leaf to my eyeball it’s the length of my nose. How cool is that!

So can we see anything smaller than a photon? A quark is 1×10-18m, which is a lot more noughts than a photon. Could we ever see a quark if its so much smaller than the smallest beam of light?
Just wondering.

But here’s the blockbuster thought experiment. Imagine we could find a way to see all the photons in the air in their individual wavelengths. What a weave of colour that would be!

And THAT’S why you need The Moody Blues.

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.And here’s something gorgeous to read during these quiet days . . .

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and a short video from the same exhibition: Liz West, of course.

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