Question

A beam of light is incident through the holes on side A and emerges out of the holes on the other face of the box as shown in Figure. Which of the following could be inside the box?

Box

• A. Concave lens
• B. Rectangular glass slab
• C. Prism
• D. Convex lens

Answer 1

The correct option is D

Convex lens

Explanation for Correct Option:

Option (D):

1. A convex lens is a lens with an outward curve.
2. The thickness at the center of a convex lens is more than the thickness at the edges of the lens.
3. Convex lenses are converging lenses.
4. They have the ability to converge a parallel beam of light into a point that is known as the focus of the lens.
5. Therefore, as shown in the figure only the convex lens can converge all the parallel beams as shown.
6. The lens found in the human eye is a prime example of a convex lens.
7. Another common example of a convex lens is the magnifying glass that is used to correct Hypermetropia or long-sightedness.

Explanation for Incorrect Option:

Option (A):

1. A concave lens is a lens with an inward curve.
2. The thickness at the center of a convex lens is less than the thickness at the edges of the lens.
3. Convex lenses are diverging lenses.
4. They have the ability to diverge a parallel beam of light in different directions.
5. When the divergent rays are traced back, they intersect at a point that is known as the focus of the lens.
6. Hence, a concave lens is not present inside the box.

Option (B):

1. A rectangular glass slab is a cuboidal piece of glass with three pairs of opposite surfaces.
2. The thickness throughout the glass slab is equal.
3. Therefore, if a parallel beam of light passes through a glass slab, the rays will emerge parallelly and thus option (B) is incorrect.

Option (C):

1. A prism is a pyramidal object made of glass.
2. It has three rectangular and two triangular faces.
3. For monochromatic light, if a parallel beam of light passes through a glass slab, the rays will emerge parallelly and thus option (C) is incorrect.

Hence, the correct answer is option (D).