Coding classes in Monmouthshire

Light passing through any circular opening spreads a little, which blurs every point into a tiny disc. The smallest angle a lens can separate, its angular resolution at the diffraction limit, is about 1.22 times the wavelength divided by the opening's width. Green light has a wavelength of about 550 billionths of a metre. The learner codes the formula, converts the angle from radians into arcseconds, the astronomer's unit where 3,600 make a degree, and multiplies by distance to find the smallest detail that could be seen. The distances and telescope sizes are our own choices; the record does not say what the club's telescope was.

Doubling the width of the lens halves the finest angle, so a 10 centimetre telescope could in principle pick out detail about twenty times finer than the pupil of an eye. Real eyes do worse than their pupil suggests, typically about a minute of arc, because of the retina behind them. But the telescope has a limit too: the air. Warm and cool pockets bend light slightly and set a shimmer that astronomers call seeing. On a good night that is about one arcsecond, and along the ground on a sunny day it is usually worse. One arcsecond at 10 kilometres is about 5 centimetres.

So which limit wins? The learner finds the crossover by setting the diffraction limit equal to one arcsecond: an opening of about 14 centimetres. For any telescope smaller than that, the lens is the bottleneck; for anything bigger, the air is. A classic slip also appears: forgetting to convert arcseconds to radians. Treat one arcsecond as one radian and the program claims the smallest visible detail at 10 kilometres is 10 kilometres wide, an answer so wrong it is easy to catch, if someone checks.

Spotting the missing limit in a confident answer is a skill a Monmouthshire teenager builds by coding the sums. The longer argument is in why teenagers should still learn to code in 2026.