Le·nard rays n. Physics. Rays emanating from the outer surface of a plate composed of any material permeable by cathode rays, as aluminium, which forms a portion of a wall of a vacuum tube, or which is mounted within the tube and exposed to radiation from the cathode. Lenard rays are similar in all their known properties to cathode rays. So called from the German physicist Philipp Lenard (b. 1862), who first described them.

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