The minimum deviation is the angle between the incident ray and light rays possing through an object such as a prism or a water dropafter refraction. The angle is also referred to as "the angle of minimum deviation". The direction of the incident rays and the orientation of the object can be varied. The value of deviation 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 wavelength of the light ray.The angle of minimum deviation is smaller for shorter wavelengths and larger for longer wavelengths .