Speed of Light
Light is the range of electromagnetic frequencies that we can perceive with our eyes. Different colours have different frequencies. The spectrum: red, orange, green, blue, indigo and violet range from 400 to 789 THz (trillion cycles per second) with respective wavelengths of 620mm to 450mm.
Light is produced by many sources but it all travels at the same speed, which is 299,792,458 metres every second in a vacuum. This speed is the maximum that energy can travel and is constant throughout the universe. It is usually denoted as C and can be found in Einstein's famous equation e=mC2 that equates energy to mass.
When we look into the sky we see the stars as they were many millions of years ago because that's how long it has taken the light produced by them to travel to us. The further away we look the further back in history we look. When we look at quasars, which exist at the limit of our ability to observe the universe, we are looking at something that existed in the very early universe. There are no quasars near to us so we conclude that these objects must have existed in the early universe and that they possibly don't exist any more. This is one way that we can gain an insight into how the universe was created and has developed since.