John Stringer and Dinah Starkey explore religious festivals and the science of illumination
We see shadows when light rays are stopped; and because light rays travel in straight lines, the shadows resemble the shape of the object. If the light source is bigger than the object, some of the light rays can get round behind the object, and then we get two shadows - a dark central umbra and a lighter penumbra.
Nearly 1,000 years ago, the Arab scholar Ibn al-Haytham Alhazen explained how the Sun's image could be produced in a darkened room because rays of light passing through a tiny hole formed an image on the wall opposite.
Smooth, reflective surfaces are attractive. Cars gleam with polish, and food may be shiny with glaze. During winter festivals, baubles and decorations are shiny and reflective. All surfaces reflect - but mirror surfaces reflect absolutely.
A sheet of glass reflects five per cent of the light falling on it - 95 per cent passes through. But when the glass is silvered, virtually all of the light is reflected. Mirrors reflect images - pictures of what is in front of them. The image is formed because the surface of the mirror is so smooth. Still water reflects an image too - Narcissus fell in love with his own reflected image in water in the Greek legend.
The earliest known mirrors date back 7,000 years and are made of polished obsidian, a type of stone. Later, mirrors were made from polished metal. The first glass mirrors were made in Venice in the year 1300 and, in 1835, the modern silvering process made it possible to make mirrors cheaply.
The colour of light
"In a very dark chamber, at a round hole, about one third part of an inch broad, made in the shutter of a window, I placed a glass prism, whereby the beam of the sun's light, which came in at that hole, might be refracted upwards towards the opposite wall of the chamber, and there form a coloured image of the sunI " These words begin Isaac Newton's description of his experiments with optics, which he performed as a young man at Cambridge University in 1669.
What Newton went on to discover was that a triangular block of transparent material - glass, silica, or nowadays, plastic - can "bend" a ray of light, splitting the white light into its constituent colours. The ray of each colour bends at a definite angle - red bends least, and is followed in turn by "orange, yellow, green, blue, indigo, deep violet". These colours are continuous and infinite in number, and Newton may have described only seven "rainbow colours" because seven was regarded as a mystical number.
The rainbow had been first explained by a German scientist, Theodoric, in the 14th century as the bending, reflection and dispersion of the Sun's rays by rain or mist.
All this is counter-intuitive - how can white be made of colours? - and it also contradicted the belief of Newton's times, that white symbolised purity and innocence.
But colour, Newton explained, was mysterious. "What we call "coloured" is merely a disposition to reflect this or that sort of rays more copiously than the rest."