Question

A student uses spectacles of focal length $-2.5m$. (a) Name the defect of vision he is suffering from. (b) Which lens is used for the correction of this defect? (c) List two main causes of developing this defect. (d) Compute the power of this lens.

Answer 1

(a) The defect of vision

1. The focal length of the lens is $-2.5m$.
2. Since focal length is negative, the lens is a concave lens.
3. Therefore, the defect of vision is myopic.

(b) Correction of a myopic eye

1. A concave lens is used to correct myopic defect vision.
2. A concave lens is also called a divergent lens.

(c) The two main causes of developing this defect are:

1. Due to elongation of the eyeball.
2. Due to the high converging power of the eye lens.

(d) Calculation of power of lens

Step 1: Given

Focal length of the lens f = $-2.5m$

Step 2: Formula used and calculation

Power )P) of lens is given by

$$\begin{gathered} P=\frac{1}{f} \\ P=\frac{1}{-2.5} \\ =-0.4D \end{gathered}$$

Thus, the power of the lens is $-0.4D$.