critical angle
  • noun the angle at which a ray of light hits a surface and is completely reflected
  • The minimum angle, with respect to the vertical, that will completely reflect an incident electromagnetic wave back into the medium from which it came. If the angle is lesser than this minimum, part or all of the radiation will penetrate or be absorbed by the medium which would have otherwise reflected it. For instance, the minimum angle necessary for radio frequency waves to be totally reflected by the ionosphere.

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