Question

Assertion: The focal length of lens does not depend on the medium in which it is submerged.
Reason: $\dfrac{1}{f} = \dfrac{{{\mu _2} - {\mu _1}}}{{{\mu _1}}}\left( {\dfrac{1}{{{R_1}}} - \dfrac{1}{{{R_2}}}} \right)$

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

Answer 1

Hint: Try to recall the equation of focal length of a lens when it is present inside a medium of certain refractive index.

Complete step-by-step answer:
The equation for the focal length of a lens is given by-
$\dfrac{1}{f} = \dfrac{{{\mu _2} - {\mu _1}}}{{{\mu _1}}}\left( {\dfrac{1}{{{R_1}}} - \dfrac{1}{{{R_2}}}} \right)$
Where, f= focal length of lens,
${\mu _1}$=medium of incident rays,
${\mu _2}$=medium of refracted rays,
${R_1}$ and ${R_2}$ are the radius of curvature of the two surfaces of the lens.
Now clearly from the equation of focal length , it shows that the focal length depends upon the medium in which the lens is placed. Hence our assertion is incorrect but the equation shown in reason is correct hence reason is correct.
Therefore, the assertion is incorrect and reason is correct. i.e. option (D) is the correct option.

Note: Try to remember the equation of the focal length. From the equation itself it clears the various factors on which the focal length of a lens depends.