Each photon's momentum in the light beam is p=h=632. 8106. The first gas laser was the helium-neon laser.
Although the first laser transitions were in the near infrared, the red laser, with a wavelength of 632.8 nm, is now the most widely employed transition. This laser is offered in lengths ranging from around 10 cm to more than 100. For longer wavelength lines, helium-neon lasers with a 1- to 10-m W output at 543 nm (green), 595 nm (yellow), or 633 nm provide an option to krypton ion lasers (red). A helium-neon laser can produce light with a power output of 1 mW at a wavelength of 632.8 nm.
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