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# What is Light

What we call light is actually only a relatively small part of the electromagnetic spectrum. Visible light consists of only a relatively narrow band between short wave infra-red and ultraviolet radiation, with wavelengths between 760nm (Red) and 380nm (Violet). The image below shows the visible spectrum with its familiar rainbow of colours at different wavelengths.

A useful way to remember the order of colours with increasing frequency in the visible spectrum is to think of Mr. ROY G. BIV (Red, Orange, Yellow, Green, Blue, Indigo, Violet).

Figure 1 - The visible spectrum of light.

### The Speed of Light

According to Einstein's Theory of Relativity, nothing in the universe can accelerate beyond the velocity of light. It also tells us that light always travels at the same speed, no matter what the relative motion of the observer or the source. Thus, light emitted from a moving object does not travel with the speed of light plus the original speed of the object, it simply travels at the speed of light. An observer moving towards the object would also experience the light passing them at the speed of light, not the speed of light plus their own velocity. This may seem strange as it would appear to contradict some of the laws of motion, however it has been confirmed in many experiments.

Having just said that light always travels at the same speed, it is important to qualify that by saying that it always travels at the same speed in the same medium. Quite the opposite of sound, the denser the medium through which light travels, the slower its speed. When light passes from one medium to another, it will actually change its speed, a principle that causes the lens in a telescope or magnifying glass to work.

Medium Speed of Light
Vacuum (Space) 299 792 458 m/s
Air (1 Atm) 299 702 547 m/s
Ice (0Ã‚Â°C) 228 849 204 m/s
Water (20Ã‚Â°C) 225 407 863 m/s
Glass (Clear float) 199 861 638 m/s
Table 1 - The speed of light in various media.

### Wave or Particle

Light, quite unlike sound, displays dual characteristics such that it can be simultaneously thought of as an energy wave or a series of tiny energy particles called photons. This effect is covered in more detail in the Electromagnetic Spectrum topic.

Lighting - the Electronic Textbook
http://www.saud.ku.edu/book/contents.htm
Lighting Design
Light: Behaviour