Question

Why should the angle of incidence lie between 35 to 60 degree in april?

Answer 1

If you shine a light beam onto a glossy surface like that of a glass prism using an angle of incidence lower than 30° you will mainly get reflection off the surface of that prism, i.e very little light will actually go through it. If you, on the other hand, make the beam impinge on the prism at an angle between 60°and 90° the resulting refracted beam will indeed be formed but will leave the prism at a very unfavourable, flat angle and most of it will go through the far apex and base of the triangular prism and thus will not be displayed properly on the screen!