Question

A lens behaves as a converging lens in air and a diverging lens in water $(\mu =1.33)$. The refractive index of the material is

• A. equal of unity
• B. equal to 1.33
• C. between unity and 1.33
• D. grater than 1.33

Answer 1

Answer: (C)

between unity and 1.33

$\frac{1}{f}=(\frac{{\mu }_{lens}}{{\mu }_{medium}}-1)\times (\frac{1}{R_1}-\frac{1}{R_2})$

Lens is converging if f is positive and diverging if f is negative

$R_1,R_2$ are constant

${\mu }_{air}=1and{\mu }_{water}=1.33$

So, ${\mu }_{lens}$ has to be between ${\mu }_{air}=1and{\mu }_{water}=1.33$ for f to change sign.