What is a maser?

What is a maser?
The word maser is an acronym for Microwave Amplification by Stimulated Emission of Radiation.
The word laser stands for Light Amplification by Stimulated Emission of Radiation - its the same process, but lasers work with visible light, whereas masers work with microwave frequency radiation.

Okay, so a maser is an invisible laser, working as microwave frequencies (frequencies quite a bit lower than those of visible light). Whats with the "Amplification by Stimulated Emission of Radiation" bit?

Well, lets start off by looking at what stimulated emission is. Say you have an atom or molecule - it has lots of energy levels (remember drawing electrons in the different energy levels of an atom at school?). Usually the lowest energy levels are all full, and the upper levels are maybe a bit full, but not much.

So we start off in a low energy state... Then a photon of light comes along - a photon of just the right frequency for the molecule - and the molecule absorbs the photon. Now it has more energy, and the higher energy level is more full. This is called absorbtion. Thats the first was atoms or molecules (particles, for the sake of brevity!) can interact with radiation, or light. The picture below illustrates two energy levels of a particle. The wave is a photon, that gets absorbed by the particle, moving it to a higher energy state.



The second way particles interact with radiation is called spontaneous emission. When our particle has absorbed a photon it is in a higher energy state, but its not really stable. It wants things to be back to normal, comfortable with all the lowest energy levels full. So the particle spontaneously emits a photon of the same special frequency for the atom, and drops back to a lower energy.

This is actually how scientists can tell what the sun and stars and the Milky Way are made of. We look out into the sky and capture the radiation from a particular object. Then we look for special frequencies that ony come from certain particles - we call emission or absorbtion of these special frequencies spectral lines. If we see those special frequencies, we know we are seeing that element or molecule. For example, I look at silicon monoxide, SiO, which is at a frequency of 43122.03 MHz. When we see emission at that frequency, we know it is SiO.

But I've got side-tracked from the main question - what is a maser? Tune in to the next post for the answer!