Light is what we see. It can be thought of either as a particle, (the photon), or as a wave. The photon we can easily think of as a small dot travelling through space at the speed of light. Each photon has a particular colour or energy. But how do we think of light as a wave? For this we need to know that light is also an "Electro-magnetic field" -- a combination of electric-field …showed first 75 words of 826 total…

…showed last 75 words of 826 total…Although either is an acceptable description of light, for our purposes in introductory astronomy the wave description will be more useful. The speed of light in a vacuum is commonly given the symbol c. It is a universal constant that has the value c = 3 x 1010 cm/second The speed of light in a medium is generally less than this. Normally the term "speed of light", without further qualification, refers to the speed in a vacuum.