Question

For a thin lens with radii of curvatures and $R_{1}$ and $R_{2}$ , refractive index $n$ and focal length $f$ , the factor $\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)$ is equal to

• A. $\frac{1}{f \left(n-1\right)}$
• B. $f (n-1)$
• C. $\frac{\left(n-1\right)}{f}$
• D. $\frac{n}{f \left(n-1\right)}$

Answer 1

Answer: (A)

$\frac{1}{f \left(n-1\right)}$

Answer 2

According to lens maker?s formula
$\frac{1}{f}=\left(n-1\right) \left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)$
$\therefore \left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)=\frac{1}{f \left(n-1\right)}$