Given situation

The refractive index of air is:

• smallest near the ground
• increases with height

So, if a horizontal light beam travels close to the ground, the upper part of the wavefront is in air of higher refractive index than the lower part.

Use of Huygens’ principle

By Huygens’ principle, every point on a wavefront acts as a source of secondary wavelets.

The speed of light in a medium is $v = \frac{c}{n}$

Hence:

• where $n$ is smaller, speed is larger
• where $n$ is larger, speed is smaller

Since near the ground $n$ is smaller, the lower part of the wavefront moves faster than the upper part.

Effect on the wavefront

Because the lower portion advances more quickly than the upper portion, the wavefront tilts in such a way that the ray direction changes toward the region of higher refractive index.

Since refractive index increases upward, the beam bends upward.

Option analysis

• A: bends downwards — Incorrect
• B: bends upwards — Correct
• C: becomes narrower — Incorrect
• D: goes horizontally without any deflection — Incorrect

Final answer

The light beam bends toward higher refractive index, i.e. upwards.