Hello,

When a ray of light passing through the prism is parallel to the base of the prism, it suffers a minimum deviation inside the prism and the angle of incidence becomes equal to the angle of emergence. Angle of refraction inside the material of any prism is equal to the half of the angle of prism. Hence, when the angle of refraction at first surface of the prism is 40°, and the angle of incidence is at surface of emergence then the angle of prism is 80 degrees.

Hope this helps,

All the Best!!