WHY ANGLE OF INCIDENCE OF THE PRISM CAN NOT BE LESS THAN 30 DEGREE TO OBTAIN MINIMUM DEVIATION POSITION

There is an angle of incidence at which the sum of the two deflections is a minimum. The deviation angle at this point is called the "minimum deviation" angle, or "angle of minimum deviation’. At the minimum deviation angle, the incidence and exit angles of the ray are identical. This is a consequence of the principle of time reversibility; if the incidence and exit angles were not identical, then reversing the paths (exit becomes entrance, and vice versa) would indicate erroneously that there were two incidence angles resulting in minimum deviation. One of the factors that causes a rainbow is the bunching of light rays at the minimum deviation angle that is close to the rainbow angle.