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  4. Assertion A biconvex lens of focal length 10 cm is split into two equals 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.

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Q. Assertion A biconvex lens of focal length 10 cm is split into two equals 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.

Ray Optics and Optical Instruments

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.
image
Hence, focal length of each part will remain as before ie, 10 cm.
Focal length of biconvex lens
$ \frac{1}{f} = (\mu_g -1) \bigg(\frac{1}{R_1}-\frac{1}{R_2}\bigg)$
where $R_1$ and $R_2$ be the radii of curvature of two surfaces.