Wave Theory, Wavefronts & Doppler Effect
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:
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Answer: (b)
- Refractive index increases with height, so the speed is smaller higher up.
- The upper part of each wavefront moves slower than the lower part, so the wavefront tilts.
- The beam (normal to the wavefront) turns towards the region of higher .
- So the answer is (b) bends upwards.