AIPMT PRELIMS 2004 Physics Refraction MCQ Question

To solve this question, we first need to understand the behavior of light rays as they pass through a medium such as a glass slab, focusing particularly on the phenomenon of refraction.

Explanation of the Correct Answer (A)

When a beam of light composed of different colors, such as red and green, strikes the surface of a rectangular glass slab at an angle, each color will refract differently due to its unique wavelength.

1. Refraction and Wavelength: According to Snell's Law, the refraction of light is given by the equation: $n_1 \sin(\theta_1) = n_2 \sin(\theta_2)$ where $n_1$ and $n_2$ are the refractive indices of the two media (air and glass, in this case), and $\theta_1$ and $\theta_2$ are the angles of incidence and refraction, respectively.

2. Different Refractive Indices: Red light has a longer wavelength than green light, which means it will have a different refractive index in glass (red light is less refracted compared to green light). Therefore, as both rays enter the glass slab at the same angle, they will bend at different angles as they pass through the glass.

3. Emergence from Parallel Faces: When the rays reach the opposite face of the glass slab, they will emerge at different angles due to their different refractive behaviors. Since the faces of the slab are parallel, the angle of emergence will also differ, causing the red and green rays to exit at two distinct points along the edge of the slab, propagating in two parallel directions.

Hence, the correct answer is A) Two points propagating in two different parallel directions.

Explanation of Incorrect Options

• Option B: One point propagating in two different directions through slab.

  • This is incorrect because the two colors of light do not propagate from a single point inside the slab. Instead, they emerge at two separate points because each color has a distinct angle of incidence and refraction.

• Option C: One point propagating in the same direction through slab.

  • This option is incorrect because red and green light do not travel in the same direction after emerging from the slab. They diverge due to different angles of refraction.

• Option D: Two points propagating in two different non-parallel directions.

  • This option is also incorrect because while the rays do emerge from two different points, they propagate in parallel directions upon exiting the slab, contrary to the statement of non-parallel propagation.

Conceptual Summary

In summary, when light of different colors enters a glass slab, the difference in their refractive indices causes them to bend at different angles, resulting in the emergence of red and green rays from two distinct points on the opposite face of the slab, maintaining parallelism as they exit. This demonstrates the principles of refraction and the dependence of light propagation on wavelength.

This understanding of light behavior is crucial for various applications in optics, including lens design and optical instruments, where precise control over light pathways is essential.