Question

Determine the power of the resulting half lens if equi-convex lens of focal length $20\text{cm}$ is cut into $2$ equal half about line parallel to principal axis & line perpendicular to principal axis

• A. $+2.5\text{D},+5\text{D}$
• B. $+5\text{D},+5\text{D}$
• C. $+2.5\text{D},+2.5\text{D}$
• D. $+5\text{D},+2.5\text{D}$

Answer 1

The correct option is D $+5\text{D},+2.5\text{D}$

As we know that power of lens is defined as

$P=\frac{1}{f\text{(in metre)}}$

When equiconvex lens is cut about line parallel to the principal axis:


$P=\frac{1}{f}=\frac{1}{20}\times 100=+5\text{D}$

When equiconvex lens is cut about line perpendicular to the principal axis:

$P^{\prime }=\frac{P}{2}=\frac{+5}{2}=+2.5\text{D}$


Hence, option $\left(d\right)$ is correct.

Why this question ? Tip: If an equiconvex of focal length $f$ is cut into equal parts about a line parallel to the principal axis then its $\mu ,R_1$ and $R_2$ remains the same. Therefore, focal length of each part will be equal to $f$ only, but if an equiconvex of focal length $f$ is cut into equal parts about a line perpendicular to the principal axis, then focal length of each part will be equal to $2f$.