Gemma Fallon, of Ardgowan School, asks :-

How are mirrors made?

John Campbell, a condensed matter physicist with interests in archaeology, responded.

Light is reflected off the surface of any material so if the surface is flat enough the surface acts as a mirror.

Probably the first mirrors were pools of water with dark bottoms. For water only about one fiftieth of the incident light is reflected so it is not a great mirror and the quality is degraded by vibrations and dust on the surface. With the discovery of metals, ancient peoples such as the Romans learned how to make much more efficient mirrors by polishing smooth and flat a metal such as speculum which is an alloy of copper and tin. (Speculum is the Latin word for a mirror.) Such mirrors were manufactured by experts so were expensive and only available to rich people.

Glass has the advantage that it can be manufactured very flat and cheap but only about one twenty-fifth of the incident light is reflected. Good mirrors were mass-produced by chemically depositing shiny silver metal on glass. Because silver is soft and tarnishes in air it was placed on the back of the glass surface and painted over to protect it. Most mirrors you see today were made this way. These mirrors have a problem of some reflection off of the front surface. Aluminium metal doesn't tarnish as readily as silver so it can be plated onto the front of a glass surface by vapourising the aluminium in a vacuum. Most large telescope mirrors are made this way but the aluminium does degrade and so must be recoated about once a year.

If you look at an oil slick on water you will notice that coloured light is reflected of it. Cunning materials scientists have learned how to put many films of very carefully controlled thickness and composition onto plastic or glass so that the surface reflects visible light but transmits infrared light (heat). You will normally see such mirrors at your dentists or behind the bulb in an overhead projector.