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 the surface encompassing all points in a medium vibrating in unison. Essentially, it is an imaginary surface where all points share the same vibration phase. For instance, in light waves, wavefronts are surfaces of constant phase, with the separation between successive wavefronts equalling the wavelength.

• Plane Wavefront: Characterized by parallel plane wavefronts, typically indicating a wave propagating in one direction.
• Spherical Wavefront: Originating from a point source, this wavefront expands radially outwards, similar to ripples on water.

2. Refraction Through a Convex Lens:

Upon encountering a convex lens, plane wavefronts (such as those of light) undergo refraction. The convex lens redirects light rays, causing the refracted wavefronts to converge at a focal point after passing through the lens.

Diagram of Incident and Refracted Wavefronts:

Explanation of the Diagram:

• The parallel lines depict the incident wavefronts approaching the convex lens.
• Post-refraction, the refracted wavefronts converge towards a focal point, signifying that the waves are concentrated at this point.
• The convex lens alters the direction of the wavefronts, focusing the rays onto a single point, the focal point.

Conclusion:

The diagram illustrates how incident plane wavefronts are refracted by the convex lens, resulting in the convergence of the refracted wavefronts. This is a characteristic behavior of lenses designed to focus light waves at a focal point.