To answer this we have look at the smallest scales of matter: atoms.
An atom consists of light, fast-moving electrons orbiting a heavy nucleus. If the atom gains some energy, for example after colliding with another atom, some of this energy will go into exciting one of the electrons, making it jump to another orbit. This state is not very stable, and after a short time the electron will jump back to its original orbit. What happens to the extra energy that the electron had?
This energy is changed to a small amount of light called a `photon' and leaves the atom eventually, perhaps, ending up in an astronomer's telescope!
| Did you know? The photon that is emitted is a single object and thus can be thought of as a particle, like an electron or atom. A photon is light, however, and we normally think of light as waves. This problem of whether light consists of particles or waves confused physicists for many years, until they found that light can be both a wave and a particle at the same time! This strange state of affairs is a result of quantum physics which is one of the great scientific achievements of the 20th century. |
What type of light (X-rays, ultraviolet, infrared, etc.) is emitted depends on what type of transition took place when the electron jumped. If it was big leap then you will get ultraviolet or X-rays, whereas smaller jumps will only give visible light or infrared.