How does a computer store a photo using only 0s and 1s?

A computer stores a photo as a grid of numbers. It chops the picture into tiny squares called pixels, like a mosaic, and records the colour of each one as three numbers: how much red, how much green and how much blue light to mix. Each number runs from 0 to 255, which is exactly what one byte of eight 0s and 1s can hold. Three bytes per pixel, 24 bits in all, and the whole photo becomes a long string of binary.

Why red, green and blue? Because of your eyes. Screens mix coloured light, and our eyes have three kinds of colour-sensitive cells, so three well-chosen lights added together in different amounts can fool them into seeing a huge range of colours. With 256 levels for each of the three, there are 16,777,216 possible combinations.

It started much more modestly. In 1957, the engineer Russell Kirsch and his team built one of the first image scanners and fed it a photo of his three-month-old son. The result was a grid of 176 by 176 pixels, 30,976 in all, and each pixel was a single bit: pure black or pure white, no greys, let alone colours. It was one of the first digital photographs, and in 2003 Life magazine named it one of the 100 photographs that changed the world.

Every picture on your phone is the great-grandchild of that baby portrait: the same grid, just with far more pixels and three bytes of colour in each.