Infra-red and ultra-violet

These words are used for special light which is invisible to our eyes. Ultra-violet light is just beyond the violet end of the visible spectrum. Infra-red is just beyond the red end.

You may have heard of a 'black light'. This is sometimes used at dances when the lights are low. It makes chemicals in some white dyes shine out very brightly. In fact, this is not a black light at all. It is a bright source of light which our eyes cannot see because it is outside the visible spectrum. This light is called 'ultra-violet'.

The photograph shows a hotplate ring which is glowing 'red hot'. The reason we can see it is because it is giving off visible red light. In fact, nearly all of the energy being given off is in the invisible 'infra-red' part of the spectrum. You cannot see infra-red light but you can feel it.

Laser light

A laser is a device which produces a beam of very bright and powerful light. Some lasers are big enough to fill a room, others are small enough to fit on a pin head. Lasers are used in medicine, industry, communications and the home. Examples of these uses include heat-treatment of skin disorders, accurate distance measurement, optical fibre communications and reading the surface of a music CD.

Powers of 10

The diagram shows some of the notation used for very large and very small numbers. These are often used when describing the waves in the electromagnetic spectrum (see section 'The electromagnetic spectrum of waves'). These prefixes (called prefixes because they are used at the start of the unit names) are given special names.

Examples for measuring length might start at 1 kilometre. If you keep dividing by 1000 then you get the following distances: 1 metre, 1 millimetre, 1 micrometre, 1 nanometre, 1 picometre.

Examples from the electromagnetic 'family' of waves, of sentences which use these prefixes include: "UHF radio broadcasts at a frequency around one gigahertz." and "Ultraviolet light can have a wavelength as short as one nanometre."