# (a) Define a wavefront. Using Huygen's geometrical construction, explain with the help of a diagram how the plane wavefront travels from the instant t1 and t2 in air.(b) A plane wavefront is incident on a convex lens. Explain, with the help of the diagram, the shape of the refracted wavefront formed.

The surface of constant phase is known as a wavefront.

The Geometrical construction of wavefront:

To determine the wavefront at $t=\tau$ draw spheres of radius $v\tau$ from each point on F1F and draw a common tangent to these spheres to obtain the new position of the wavefront.

(b) The shape refracted wavefront will be as:-

The central part of the incident plane wave traverses the thickest portion of the lens and is delayed the most. The emerging wavefront has a depression at the centre and hence the wavefront is spherical and converges to the point F known as the focus.

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