M-rays are a form of radiation that can be used to negate areas in force fields.

In 2267, the self-proclaimed god Apollo surrounded the USS Enterprise with a force field contrived to resemble an enormous hand. This field blocked communications, transporters, and weapons fire. It was too powerful for the ship to pull free from; the tractor beam proved equally ineffective. Ultimately, Spock directed the nuclear electronics lab to work on a plan to generate strong pinpoint charges of M-rays, on selected wave lengths, powered by the combined output of all the ships engines. The plan was successful and by shooting through the resulting holes in the field, Spock was able to destroy Apollo's temple, the source of his power, and free the ship. (TOS: "Who Mourns for Adonais?")

In 2268, a Melkotian buoy self-destructed by emiting M-rays of highly unstable nature in energy output levels beyond the capability of the Enterprise's sensors to measure. Captain Kirk initially assumed the M-ray emissions to be preparations for an attack. (TOS: "Spectre of the Gun")

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