Question

When a parallel beam of light enters water surface obliquely at some angle, what is the effect on the width of the beam?

Hint:

The width of a light beam reduces when it enters a denser medium at an oblique angle due to the refraction of light.

Answer 1

When a parallel light beam strikes a water surface at an angle, it refracts because the speed of light changes from air to water. This refraction affects the beam's width as follows:

1. Refraction at the Water Surface:

Light transitions from air (refractive index ≈ 1) to water (refractive index ≈ 1.33). This speed change causes bending, governed by Snell's law:

\[ n_1 \sin(\theta_1) = n_2 \sin(\theta_2) \]

where:

• \( n_1 \) is the refractive index of air (1),
• \( n_2 \) is the refractive index of water (1.33),
• \( \theta_1 \) is the angle of incidence in air, and
• \( \theta_2 \) is the angle of refraction in water.

2. Impact on Beam Width:

Oblique entry causes the beam to bend. As light slows in water, rays bend towards the normal, resulting in a narrower beam in its propagation direction.

• The beam's width reduces upon entering water due to ray bending toward the normal. This narrowing occurs because slower light in water leads to closer ray spacing post-refraction.
• This effect is amplified with larger angles of incidence, where bending is more pronounced. Smaller angles of incidence yield less noticeable width changes.

3. Conclusion:

When a parallel light beam enters water obliquely, its width decreases due to refraction. The light slows down and bends toward the normal. The greater the angle of incidence, the more the beam narrows.