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I have been reading about how lasers function : A photon is used for stimulated emission of electrons from the metastable state to a lower energy state. What I don't understand is : How can "giving" energy (in the form of photons) to electrons stimulate them to come to a lower energy state ? After stimulated emission , the old photon exists along with the new one and with the same energy as earlier, so how did it actually stimulate the electrons to fall ?

PS : I have just begun to learn phy, so pls ans as simply as possible :)

biogirl
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I guess your question is: how the electrons "know" they should jump down when the photons come in? Well, stimulated emission is a theoretical discovery by Einstein. The strict explanation requires quantum mechanism. A simple explanation is the electron at the upper level is too lazy to think about where should it jump. So it just follow the incoming photon, release the exact same energy and fall. If there is no incoming photon, it actually releases a random energy, which is called "spontaneous emission". If the electron at the lower level wants to jump back, it has to capture a photon, which is called "absorption". To amply the light, you will want to have more electrons at the upper level, which is called "population inversion".

Hope this helps.

Yuchao Jiang
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