A plane wave front falls on a convex lens. The emergent wave front is

Options

A. plane
B. diverging spherical
C. converging spherical
D. none of these.

Answer

To solve the problem of determining the type of emergent wave front when a plane wave front falls on a convex lens, we can follow these steps:

### Step 1: Understand the Nature of the Incoming Wave Front
A plane wave front consists of parallel wavefronts that are uniformly spaced. This means that the wavefronts are flat and extend infinitely in one direction.

**Hint:** Remember that a plane wavefront is characterized by its uniformity and parallelism.

### Step 2: Analyze the Effect of the Convex Lens
When a plane wave front encounters a convex lens, the lens causes the light rays to converge. The lens has a varying thickness, being thicker at the center than at the edges, which affects the speed of light passing through it.

**Hint:** Consider how the lens shape influences the path of light rays.

### Step 3: Consider the Speed of Light in Different Media
As the plane wavefront passes through the lens, the light rays traveling through the glass (the lens material) will slow down compared to those traveling through air. This difference in speed causes the rays to bend towards the normal when entering the lens and bend away when exiting.

**Hint:** Think about Snell's law and how light behaves at the interface of different media.

### Step 4: Determine the Emergent Wave Front
Due to the convergence of the light rays as they pass through the convex lens, the emergent wave front will no longer be flat. Instead, it will take the shape of a spherical wavefront that converges at a point known as the focus of the lens.

**Hint:** Visualize how the rays converge at a point after passing through the lens.

### Step 5: Conclude the Type of Emergent Wave Front
Since the light rays are converging after passing through the lens, the emergent wave front is classified as a converging wave front, which is spherical in nature.

**Hint:** Remember that converging wave fronts are typically associated with lenses that focus light.

### Final Answer
The emergent wave front from a convex lens when a plane wave front falls on it is a **converging wave front**.