UPSC NDA I 2026 General Ability Paper

Concept:Total Internal Reflection (TIR) and geometry of light refraction through a prism.Explanation:1. The light ray falls normally ($90^\circ$) on the first face of the equilateral prism (all angles are $60^\circ$), so it passes through undeviated.2. It strikes the second face. Using the geometry of the triangle formed at the top (angles $90^\circ$, $60^\circ$, and $30^\circ$), the angle between the ray and the second face is $30^\circ$. Thus, the angle of incidence is $90^\circ - 30^\circ = 60^\circ$.3. The critical angle for glass is approximately $42^\circ$. Since the angle of incidence ($60^\circ$) is greater than the critical angle, Total Internal Reflection (TIR) takes place.4. The ray reflects at an angle of $60^\circ$, which means it makes an angle of $30^\circ$ with the second face.5. The reflected ray travels towards the base. In the new triangle formed with the base, the angles are $60^\circ$ (prism base angle) and $30^\circ$. The third angle where the ray hits the base is $180^\circ - (60^\circ + 30^\circ) = 90^\circ$.6. Because the ray strikes the base normally ($90^\circ$), it emerges out of the prism undeviated. Diagram D correctly illustrates this complete path.Answer:Option D