Minimum deviation
![Light is deflected as it enters a material with refractive index > 1.](/Images/godic/202501/27/Deviation_angle_of_a_beam_of_light_entering_a_medium_with_refractive_index3437.png")
![A ray of light is deflected twice in a prism. The sum of these deflections is the deviation angle.](/Images/godic/202501/27/Deflection_of_light_ray_passing_through_prism3437.png")
![When the entrance and exit angles are equal, the deviation angle of a ray passing through a prism will be a minimum.](/Images/godic/202501/27/Minimum_deflection_of_light_ray_passing_through_prism3437.png")
As a ray of light enters the transparent material, the ray's direction is deflected, based on both the entrance angle (typically measured relative to the perpendicular to the surface) and the material's refractive index, and according to Snell's Law. A beam passing through an object like a prism or water drop is deflected twice: once entering, and again when exiting. The sum of these two deflections is called the deviation angle.