Question

Define a wavefront. An incident plane wave falls on a convex lens and gets refracted through it. Draw a diagram to show the incident and refracted wavefront.

Hint:

Wavefronts change shape according to Huygens’ principle when passing through optical elements like lenses.

Answer 1

Wavefront and Refraction through a Convex Lens

1. Definition of a Wavefront:

A wavefront is defined as the locus of all the points in a medium that are vibrating in phase. In simple terms, it is an imaginary surface on which all points have the same phase of vibration. For example, in a light wave, the wavefronts are the surfaces of constant phase, where the distance between consecutive wavefronts is the wavelength of the wave.

• Plane Wavefront: A wavefront in which the wavefronts are parallel planes, typically representing a wave traveling in a single direction.
• Spherical Wavefront: A wavefront that expands outward in all directions from a point source, like the ripples on a pond.

2. Refraction through a Convex Lens:

When a plane wave (e.g., light wave) falls on a convex lens, the wavefronts of the incident light are refracted as they pass through the lens. The convex lens bends the light rays such that the refracted wavefronts converge towards a point after passing through the lens.

Diagram of Incident and Refracted Wavefronts:

Explanation of the Diagram:

• The straight lines represent the incident wavefronts approaching the convex lens.
• After refraction through the lens, the refracted wavefronts converge towards a focal point, indicating that the waves are focused after passing through the lens.
• The convex lens causes a change in direction of the wavefronts, focusing the rays to a point (the focal point).

Conclusion:

In the diagram above, the incident plane wavefronts are refracted by the convex lens, causing the refracted wavefronts to converge. This effect is typical of lenses that focus light waves to a focal point.