Light is pure energy. It travels from place to place at an absolutely fantastic speed - 300,000,000 metres per second or 186,000 miles per second! There is nothing that can travel faster than light: nothing can even approach this speed. Even the fastest spacecraft we use today travel at only about 30 Km per second. For everyday purposes on Earth we therefore say that light travels instantaneously from one place to another.
Another property of light is that, for all general purposes, it travels from one point to another in a straight line. This is a very important property that we will use extensively in our discussions later.
Of course, we all know that light has colour. Indeed, white light is made up of a wide range of different lights, specified by quantities called frequency and wavelength. In simple terms we think of white light as being made up of the colours of the rainbow. Precisely, we can express the relationship between frequency, wavelength, and speed of light as:
frequency x wavelength = speed of light
Because the speed of light is always constant, as the wavelength gets bigger, the frequency gets smaller and vice versa. At one end of the spectrum, red light has long wavelength and, at the other end, violet light has short wavelength.
We shall discuss further the importance of the different colours of light to the use of lighthouses but for the moment we shall consider only white light.