NDA General Ability Question (2026) — Solution

Step-by-step solution

When the light ray falls perpendicularly on the first face of the equilateral glass prism, its angle of incidence is 0^ . Thus, it enters the prism undeviated. In the triangle formed at the top vertex, the angle of the prism is 60^ , and the angle at the first face is 90^ . The angle between the ray inside the prism and the second face is 180^ - (90^ + 60^ ) = 30^ . The normal to the second face makes an angle of 90^ with the surface. The angle of incidence at the second face is 90^ - 30^ = 60^ . The critical angle for a glass-air interface is approximately 42^ . Since the angle of incidence (60^ ) is greater than the critical angle, the ray undergoes total internal reflection. By the law of reflection, the angle of reflection is 60^ , so the reflected ray makes an angle of 30^ with the second face. In the triangle formed by the reflected ray, the second face, and the base, the prism's rightmost angle is 60^ . The angle the ray makes with the second face is 30^ . Therefore, the angle at which the ray strikes the base is 180^ - (60^ + 30^ ) = 90^ . Since the reflected ray strikes the base perpendicularly, its angle of incidence at the base is 0^ , and it emerges from the prism undeviated. This complete path is correctly represented in the diagram corresponding to option (4).