Though laser action usually involves a transition between two atomic or molecular energy levels and it therefore essentially monochromatic light, there may be more than one set of levels which can be utilized in a given medium. For example, the helium-neon laser can produce green at 543.5 nm and infrared at 1523 nm as well as the familiar red 632.8 nm laser action. The dye lasers have so many possible laser transitions which are broad that some of them can be tuned over and almost continuous range.
One convenient way to tune a laser is to place a prism or grating in the path to one of the mirrors as illustrated below. The prism is oriented so that the desired wavelength will be normally incident on the mirror at the back of the prism and therefore retrace its path back into the laser cavity. Other wavelengths with different indices of refraction will strike the mirror at a different angle and will be taken out of the beam. Since a grating is like a "super prism" with more dispersive power, more precise tuning could be obtained with a grating.
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