Solved: PHYSICS Question for NEET

NEET PHYSICSEasy

A plano-convex lens of unknown material and unknown focal length is given. With the help of a spherometer, we can measure the:

focal length of the lens.

radius of curvature of the curved surface.

aperture of the lens.

refractive index of the material.

Function of Spherometer: A spherometer is a precision instrument primarily designed to measure the radius of curvature ( $R$ ) of spherical surfaces, such as lenses and mirrors.
Measurement Principle: It measures the small height or 'sagitta' ( $h$ ) of the curved surface relative to a plane defined by its three legs. Using the distance between the legs ( $l$ ) and the measured sagitta, the radius of curvature is calculated using the formula: $R = \frac{l^2}{6h} + \frac{h}{2}$ .
Why other options are incorrect:

Focal Length ( $f$ ): Requires knowledge of both the radius of curvature ( $R$ ) and the refractive index ( $\mu$ ) using the Lens Maker's Formula. Since the material (and thus $\mu$ ) is unknown, $f$ cannot be measured directly with just a spherometer.
Refractive Index ( $\mu$ ): Cannot be determined without knowing the focal length.
Aperture: This is the diameter of the lens, which is typically measured with a ruler or vernier calipers, not a spherometer.

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