Minimum deviation

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Angle of deviation. The green square represents a transparent object. Red arrows at left and right represent incident and emerging light beams, respectively.
In a prism, the deviation is minimal for the symmetric light path (top trace).

The minimum deviation is the angle between the incident and emerging light ray after transmission through an object such as a prism or a water drop. The angle is also referred to as "the angle of minimum deviation".[1] The direction of the incident beam and the orientation of the object can be varied. The 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.

The angle of minimum deviation depends upon:-

1. Wavelength of the light ray (λ):-The angle of minimum deviation is smaller for longer wavelengths and larger for shorter wavelengths, so the "red" end of the spectrum deviates less than the "violet" end.

2.Material of the prism (μ):-The larger the refractive index of the material, the larger the angle of minimum deviation.

3.Angle of prism(A):-The larger the angle of prism, the larger the angle of minimum deviation.

4.Angle of incidence(i):-The angle of deviation is dependent on the angle of incidence in the form of a U shaped curve

If a line is drawn parallel to the angle of incidence axis (X-axis), it cuts the graph at two points, showing that there are two values of angle of incidence for an angle of deviation. However, at the point of angle of minimum deviation, the line will be tangent to the curve showing that for minimum angle of deviation there is only one angle of incidence.

The refractive index μ, angle of minimum deviation D of light with wavelength λ through a prism with internal angle A are related by the following formula:[2]

\mu_\lambda={\sin ({A+D_\lambda \over 2}) \over \sin ({A \over 2})}


  1. ^ Mark A. Peterson. "Minimum Deviation by a Prism". Mount Holyoke College. 
  2. ^ "Derivation of Angle of Deviation through a Prism".