If you’ve ever been involved with weather monitoring you’ll know that a spherical lens can focus sunlight to a point. The Campbell-Stokes sunlight recorder counts the hours of sunlight per day by burning a trail across a calibrated sheet of paper.

When teaching vision I use a large spherical flask full of dyed water and a number of different lenses to simulate the eye, including short- and long-sightedness.

Until very recently this flask was stored on a windowsill in direct sunlight. Can you guess why we moved it?