The power of a lens being +4 dioptres suggests that it is a

Options

A. convex lens
B. plano-convex lens
C. concave lens
D. none of these

Answer

To determine the type of lens based on its power, we can follow these steps:

### Step 1: Understand the Concept of Lens Power
The power of a lens (P) is defined as the reciprocal of its focal length (f) in meters:
\[ P = \frac{1}{f} \]
The unit of power is diopters (D).

### Step 2: Analyze the Given Power
In this case, the power of the lens is given as +4 diopters:
\[ P = +4 \, \text{D} \]

### Step 3: Calculate the Focal Length
Using the formula for power, we can find the focal length:
\[ P = \frac{1}{f} \]
Rearranging gives:
\[ f = \frac{1}{P} \]
Substituting the value of P:
\[ f = \frac{1}{4} = 0.25 \, \text{m} \]
This means the focal length of the lens is +0.25 meters.

### Step 4: Identify the Type of Lens
A lens with a positive focal length is a **convex lens** (also known as a converging lens). Therefore, since the power is +4 diopters, we conclude that the lens is a convex lens.

### Final Answer
The power of a lens being +4 dioptres suggests that it is a **convex lens**.

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