Identify the physical quantity that cannot be measured using spherometer :

Solution

Physics sir · 3 steps

A spherometer only measures small heights with a screw, so ask whether each quantity is a length.

1. What it does. A spherometer is a screw gauge on three legs. Its central screw measures a small vertical height h very precisely, using the pitch and the circular scale.
2. Curved surfaces. Place the legs on a curved surface and find the sagitta h of the central screw. With l the distance between the legs, R = l²/6h + h/2 works for a convex or a concave surface.

   Quantity	What is measured	Spherometer?
   Radius of curvature, concave	sagitta h of the surface	yes
   Radius of curvature, convex	sagitta h of the surface	yes
   Thickness of a thin plate	height of the plate on a flat glass sheet	yes
   Specific rotation of a liquid	angle through which plane-polarised light turns	no

3. So. Specific rotation is an optical angle measured with a polarimeter, not a length, so a spherometer cannot measure it. The answer is (b).

The idea to keep. A spherometer is a screw gauge for small heights. Anything it gives, such as thickness or radius of curvature, must come from one tiny length h.

Answer from the NTA final answer key. Solution by PrepWiser.