# What is Laser?

The term "laser" originated as an acronym for "light amplification by stimulated emission of radiation". The above left is a photo of US Air Force laser experiment, and the above right is a simplified structure of laser ($$1.$$ Gain medium, $$2.$$ laser pumping energy, $$3.$$ High reflector, $$4.$$ Output coupler, $$5.$$ laser beam). Which of following explanations is NOT correct?

a) The high reflector reflects all the incident lights, and the output coupler is partially transparent.

b) If we supply proper energy to the gain medium, the electron in the gain medium is excited from a lower to a higher energy level.

c) The wavelength and output power of laser are determined by the gain medium.

d) Only solid state or plasma state materials can be the gain medium.

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