Question

Two plane wavefronts of light, one incident on a thin convex lens and another on the refracting face of a thin prism. After refraction at them, the emerging wavefronts respectively become

• A. plane wavefront and plane wavefront
• B. plane wavefront and spherical wavefront
• C. spherical wavefront and plane wavefront
• D. spherical wavefront and spherical wavefront
• E. elliptical wavefront and spherical wavefront

Hint:

Lens changes curvature; prism only changes direction.

Answer 1

The Correct Option is C

Concept: Wavefront transformation depends on optical element:
• Lens changes curvature of wavefront
• Prism only deviates direction, not curvature

Step 1: Plane wavefront incident on convex lens.
A plane wavefront consists of parallel rays.

Step 2: Effect of convex lens.
A convex lens converges parallel rays to its focus.

Step 3: Nature of emerging wavefront.
Converging rays correspond to a spherical wavefront (centre at focus). \[ \Rightarrow \text{Plane} \rightarrow \text{Spherical} \]

Step 4: Plane wavefront incident on prism.
A prism only changes direction of propagation due to refraction.

Step 5: Effect of prism.
All rays are deviated equally → remain parallel.

Step 6: Nature of emerging wavefront. \[ \Rightarrow \text{Plane} \rightarrow \text{Plane} \]

Step 7: Final conclusion. \[ \boxed{\text{Spherical wavefront and plane wavefront}} \]