Minimum deviation

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Angle of minimum deviation. Blue square represents an object (usually transparent) . Red arrows at left and right represent incident and emerging light beams, respectively.

The minimum deviation, if it exists, of the angle between the incident light ray and the emerging one after transmission through an object such as a prism or a water drop. The angle is also called angle of minimum deviation. Direction of the incident beam and the orientation of the object can be varied. Value of D can be larger than 90 degrees. One of the factors that rainbow can be observed is due to bunching of light rays at the minimum deviation that is close to the rainbow angle.