LIGHT

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 …

…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.