5. Speed Of Light

Light is one of the most fundamental aspects of the universe. Light gives us the information about universe as a whole such as the ages of the stars, life cycles of stars and the temperature of celestial objects which makes it possible because of its speed. The speed of light is held to be the absolute upper speed limit of anything in the universe and it is the most important property of light. The speed of light in absolute vacuum is an universal physical constant important in many areas of physics and its exact value is 299,792,458 meters per second. While traveling through another medium such as air, water or glass, the density of medium will slow down the light's speed. Therefore, light can also tell us the relative densities of the medium.

The light year is a unit of length used to express astronomical distances. A light-year is the distance that light travels in vacuum in one Julian year which means 365.25 days. Because it includes the word 'year', the term 'light-year' may be misunderstood as a unit of time. But, it actually not. The light-year is a unit of distance which is a product of the Julian Year (365.25days) and the speed of light (299,792,458m/s) and it measures about 9.46 x 10¹² km in length. Sunlight takes about 8 minutes 17 seconds to travel the distance from the surface of the Sun to the Earth.