The difference between masers and lasers

The main difference between a maser and a laser is that masers use microwaves. Hence the letter m. Although this is considered more of historical term.

Laser stands for light amplification by stimulated emission of radiation.

A hydrogen maser diagram. image: wikipedia

It is now suggested (by Charles H . Townes) that the the term Maser stand for molecular amplification by stimulated emission of radiation because the typical masers produce not just microwaves but other wavelengths. 

Although light in the case of laser is often visible light it could be other light from any wavelength such as infrared or ultra violet.

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