The Electromagnetic Spectrum
Light allows us to interact with our world. We can see our surroundings because light bounces off objects into our eyes. We are all familiar with visible light, but this is merely a small portion of the electromagnetic spectrum. In fact, there are many different types of light ranging from short wavelengths, like x-rays, to longer wavelengths, like radio waves. These types of light blend together to make a continuous spectrum of electromagnetic radiation. By filtering out various regions of the electromagnetic spectrum, one can receive different types of information.
Light is composed of tiny wave-particles called photons which all travel at the same speed through space. We call this speed c , and it is approximately equal to 300 million meters per second. Instinctively, we might think that when we turn on a light switch the room lights up instantly, but this is not true. Light takes time to travel.
Virtual Time Travel
So what can we see from our vantage point in the Universe? We can see every object whose light has had time to reach us. Light does not travel instantaneously; it takes time to leave an object and get to us. The farther away an object, the longer the light takes. When we see the light, we see the object as it was very long ago: looking farther out in space is looking back in time!
In addition to enabling us to see back in time, light can tell us much more about the Universe. One of the most important aspects of light is that it serves as an astronomical ruler. Scientists can use the relationship between how bright a star appears to be as observed from earth and a measure of its intrinsic brightness, or luminosity, to calculate the distance to the star.