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On a hot summer night the refractive index of air is smallest just above the ground and increases with height. A light beam is sent horizontally. Using Huygens' principle, as the beam travels it:

1. bends downwards
2. bends upwards
3. becomes narrower
4. travels horizontally without any deflection

Correct answer: bends upwards

Solution

Where n is smaller (near the ground) the wave speed v = c/n is larger, so the lower part of the wavefront outruns the upper part. The wavefront tilts and the ray bends toward the region of higher refractive index, i.e. upward (this is the basis of the mirage forming away from the road on a hot night).

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