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` principal leads us to conclude that as it travels, the light beam:

Options

A. goes horizontally without any deflection
B. bends down wards
C. bends upwards
D. becomes narrower

Answer

To solve the problem, we need to analyze the behavior of light as it travels through air with varying refractive indices. Here’s a step-by-step solution:

### Step 1: Understand the Refractive Index Variation
On a hot summer night, the refractive index of air is smallest near the ground and increases with height. This means that the air near the ground is less dense (rarer medium) compared to the air at higher altitudes (denser medium).

### Step 2: Apply Huygens' Principle
According to Huygens' principle, every point on a wavefront can be considered a source of secondary wavelets. The new wavefront is formed by the envelope of these wavelets. When light travels from a medium of lower refractive index to a medium of higher refractive index, it bends towards the normal.

### Step 3: Analyze the Light Beam's Path
Since the light beam is directed horizontally, we can consider the transition from the rarer medium (lower refractive index near the ground) to the denser medium (higher refractive index at a height). As the light beam moves upward, it encounters air with a higher refractive index.

### Step 4: Determine the Direction of Bending
As the light beam travels from the rarer medium (ground level) to the denser medium (higher altitude), it will bend towards the normal. However, since the light is initially traveling horizontally, this bending will cause the light to deviate slightly upwards.

### Step 5: Conclusion
Thus, as the light beam travels, it bends upwards due to the increase in refractive index with height.

### Final Answer
The light beam bends upwards as it travels.

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