Question

A plane wave front is incident on a thin prism, thin convex lens and a concave mirror separately. The wave front(s) emerging out from the

• A. concave mirror is plane
• B. thin prism is spherical
• C. convex lens and concave mirror are plane
• D. convex lens and prism are plane
• E. convex lens and concave mirror are spherical

Answer 1

Answer: (E)

When a plane wave front is incident on a thin convex lens or a concave mirror, both these optical elements cause the wave front to converge or diverge in a spherical manner. The convex lens focuses the wave front to a point, while the concave mirror converges it to a focal point as well, both of which result in a spherical wave front. Thus, the wave front emerging from both the convex lens and the concave mirror will be spherical in nature.

Therefore, the correct answer is option (E): convex lens and concave mirror are spherical.

Answer 2

Let's understand how a plane wave front transforms after passing through or reflecting from different optical elements:
 
1. Thin Prism:
A prism deviates the light without focusing it. It does not produce a spherical wave front but slightly bends the plane wave front. So, the emerging wavefront is still approximately plane, not spherical.

2. Thin Convex Lens:
A plane wave front incident on a convex lens converges to a point (focus), hence the emerging wavefront becomes a converging spherical wave front.

3. Concave Mirror:
A plane wave front reflects from a concave mirror and converges to the focus, producing a converging spherical wave front.

✅ Correct Option: convex lens and concave mirror are spherical