Question

A beam of light composed of red and green rays is incident obliquely at a point on the face of a rectangular glass slab. When coming out on the opposite parallel face, the red and green rays emerge from

• A. Two points propagating in two different parallel directions
• B. One point propagating in two different directions through slab
• C. One point propagating in the same direction through slab
• D. Two points propagating in two different non parallel directions

Answer 1

The Correct Option is A

When a beam of light composed of different colors, such as red and green rays, is incident obliquely on the face of a rectangular glass slab, the different colors undergo refraction due to differences in their wavelengths. This phenomenon is due to the dependence of the refractive index on the wavelength of light (dispersion).

Let's break down the process step by step:

1. As the light enters the glass slab, both red and green rays are refracted towards the normal. This is because of the change in speed of light when it moves from air (a rarer medium) to glass (a denser medium).
2. Due to dispersion, red and green light have different refractive indices in glass. Generally, the refractive index for red light is lower than for green light, meaning red light bends less compared to green light.
3. Inside the glass slab, both ray colors travel at different velocities but in parallel directions because the faces of the slab are parallel to each other.
4. Upon emerging from the other side of the slab, refraction occurs again, and the light emerges into the air. Both rays, red and green, will emerge as parallel beams but displaced from each other.

Therefore, the red and green rays emerge from two points propagating in two different parallel directions. This is due to the optical path and the initial oblique entry angle into the parallel-faced slab.

This explanation clarifies why the correct answer to the question is:

Two points propagating in two different parallel directions