The speed of light is the amount of distance the light travels for a unit of time. More over it is 299,792,458 m/s to be exact. Some times we take this value to be 3x10^8 as it is easy for mathematical calculations and this is commonly denoted as c. As in the popular equation E = mc^2 (c here is the speed of light and, m is a mass). This is the maximum speed in which any matter, energy or any other thing can travel.
For short distances this is a speed that the human eye would not be able to pick up. Which means it is almost instantaneous. But for long distances such as sun to earth, this would be very noticeable. Did you know that the time to travel the distance between the earth and the sun is about 8 mins and 17 seconds? This means we would not notice if something happens to sun immediately but we would know that after 8 mins! Amazing right?
This speed can be different to which the light is travelling. For transparent materials such as air, glass or water, the speed of light is less than c. The ratio between the speed of light and the speed of light through another material is called the reflective index of the material. This is the reason to refraction or reflection of light off materials.
Light Years is a unit to measure distance. It is the amount of distance the light travels in an year. This ads up to about 9.4605284 x 10^15m, That is a long distance!
Picture courtesy: http://scienceblogs.com/
Resourses:
Wiki: http://en.wikipedia.org/wiki/Speed_of_light
And my own physics teacher at school! :)
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