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)

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