On a hot summer night, the refractive index of air is smallest near the ground and increases with height from the ground. When a light beam is directed horizontally, the Huygens' principle leads us to conclude that as it travels, the light beam:
- (A)goes horizontally without any deflection
- (B)bends downwards
- (C)bends upwards✓ Correct
- (D)becomes narrower
Answer: (C)
Solution
Picture a wavefront standing vertically and marching horizontally. Light travels at , so wherever is smaller the wave moves faster. Here is least at the ground and grows with height, so the lower edge of the wavefront outruns the upper edge. A wavefront that advances faster at its bottom tips over, and because the ray always stays perpendicular to the wavefront, the beam curves towards the slower, higher-index region above -- it bends upwards. This is exactly the reverse of the daytime desert mirage, where the hot rarer layer lies at the ground.
JEE Main · 2015 · 04 Apr 2015 · Offline · Single correct option