Question

Light emerges out of a convex lens when a source of light kept at its focus. The shape of wavefront of the light is:

• A. Both spherical and cylindrical
• B. Cylindrical
• C. Spherical
• D. Plane

Answer 1

The Correct Option is D

When light emerges out of a convex lens with a source placed at its focus, the rays emerge parallel to the principal axis. This results in a plane wavefront due to the parallel nature of the emergent rays.

Answer 2

Step 1: Recall the concept of a wavefront
A wavefront is the locus of all points in a medium that oscillate in the same phase at a given instant. In the case of light waves, wavefronts represent surfaces perpendicular to the direction of propagation of light rays.

Step 2: Understand what happens at the focus of a convex lens
When a point source of light is placed at the focus of a convex lens, the rays of light emerging from the lens become parallel to the principal axis after refraction. This happens because a convex lens converts diverging rays from the focal point into parallel rays — an essential property of converging lenses.

Step 3: Relation between rays and wavefronts
Parallel rays correspond to a plane wavefront, because all the points on the wavefront are equidistant from the source in the direction of propagation. Thus, the spherical wavefronts emitted by the point source become plane after refraction through the convex lens.

Step 4: Conclusion
Therefore, the shape of the wavefront of the light emerging from the convex lens, when the source is placed at its focus, is a plane wavefront.

Final answer

Plane