Question

Consider an equiconvex lens of radius of curvature R and focal length f. If f > R, the refractive index $\mu$ of the material of the lens.

• A. is greater than zero but less than 1.5
• B. is greater than 1.5 but lens than 2.0
• C. is greater than 1.0 but less than 1.5
• D. none of these

Answer 1

The correct option is C is greater than 1.0 but less than 1.5

$\frac{1}{f}=(\mu -1)\left(\frac{2}{R}\right)$

$orf=\frac{R}{2(\mu -1)}$

$Now,f>R$

$\therefore \frac{R}{2(\mu -1)}>R$

$or\frac{1}{2(\mu -1)}>1or2(\mu -1)<1$

$or\mu -1<\frac{1}{2}or\mu <(1+\frac{1}{2})or\mu <1.5$

Why this question? Caution: Refractive index for any medium cannot be less than 1.