What Is Light?

What Is Light?

Light is electromagnetic radiation, a type of energy that provides sustenance to plants on Earth and gives us the ability to see the world around us.

Light can travel through space without any need for a physical medium. In fact, it travels at its highest speed in empty space; any form of matter that light interacts with slows down its journey. In a complete vacuum, the speed of light, 299,742,458 meters per second (around 670 million miles per hour), is the upper limit of all motion. Nothing can travel faster than it!

Note that, despite how quickly light travels in space, it can take billions of years for the light from distant stars and galaxies to reach us; think about how incredibly far these objects must be from us for that to be true! Because of the vast distances between objects in space, the light from many of the celestial objects you can see through a telescope is from the distant past, and, in many ways, can be viewed as a time capsule from the year in which the light was first generated.

Fun fact: Astronomers use light to define the primary unit of measurement used in astronomy, the light-year! A light-year is the distance that light can travel in one year.
How Is Light Produced?



The light that is most important in astronomy is produced mainly by two processes: incandescence and ionized gas emission. Incandescence is the light generated by hot, dense bodies like the Sun. When any material is heated to a sufficiently high temperature (above around 500°C), its constituent atoms and ions vibrate so violently that they spontaneously emit light.



Ionized gas emission, on the other hand, is light generated by hot, diffuse ionized gasses. New stars form in large clouds of gas known as emission nebulae. These young stars give off intense radiation, exciting hydrogen atoms and molecules in the cloud. Sometimes electrons become so excited that they are stripped from their atom, in which case we say that the atom is ionized. When these highly excited electrons return to a lower energy level, they give off the additional energy in the form of light.