Question

Assertion :A convex lens of glass ($\mu =$ 1.5) behave as a diverging lens when immersed in carbon disulphide of higher refractive index $(\mu =1.65)$ Reason: A diverging lens is thinner in the middle and thicker at the edges.

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
• C. Assertion is correct but Reason is incorrect
• D. Both Assertion and Reason are incorrect

Answer 1

The correct option is B Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

We know the focal length of lens is

$\frac{1}{f}=\frac{\mu -{\mu }_o}{{\mu }_o}(\frac{1}{R_1}-\frac{1}{R_2})$

Here $\mu =1.5$ and ${\mu }_0=1.65$, so the focal length of the lens is negative and it will act as diverging lens.

So if you immerse the convex lens in a liquid whose refractive index is greater than refractive index of material of lens then it will change its nature and become concave lens and thus it will start diverging the rays rather than focusing them on a single point.

Reason is wrong as it depends on the surrounding medium.