question_answer

Direction for: In each of the following questions a statement of Assertion is given followed by a corresponding statement of Reason just below it. Of the statements mark the correct answer as

Assertion: A biconvex lens of focal length 10 cm is split into two equal parts by a plane parallel to its principal axis. The focal length of the each part will be 20 cm. Reason: Focal length does not depend on the radii of curvature of two surfaces.

A)  If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion

B)  If both Assertion and Reason are true but the Reason is not the correct explanation of the Assertion

C)  If Assertion is true but Reason is false

D)  If both Assertion and Reason are false

Correct Answer: D

Solution :

Since, lens is cut into two equal parts by a plane parallel to its principal axis, so both the part still to be biconvex. hence, focal length of each part will remain as before ie, 10 cm. Focal length of biconvex lens \[\frac{1}{f}=(\mu -1)\left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)\] where \[{{R}_{1}}\] and \[{{R}_{2}}\] be the radii of curvartion of two surfaces.