Question

Assertion (A) The focal length of lens does not change, when red light is replaced by blue light.
Reason (R) The focal length of lens does not depends on colour of light used.

• A. Both assertion and reason are true and reason is the correct explanation of assertion
• B. Both assertion and reason are true but reason is not the correct explanation of assertion
• C. Assertion is true but reason is false
• D. Both assertion and reason are false

Answer 1

Answer: (D)

Focal length of the lens depends upon its refractive index as $\frac{1}{f} \propto(\mu-1)$.
Since, $\mu_{0}>\mu_{r}$
So, $ f_{b} < f_{r}$
Therefore, the focal length of a lens decreases when red light is replaced by blue light.