A spherometer is an instrument designed for measuring the radius of curvature of spherical surfaces and the thickness of small objects. Its applicability to the given options is as follows:
Radius of curvature of concave surface: A spherometer can measure the radius of curvature of concave surfaces. This is a primary function. The instrument is placed on the surface, and the curvature is calculated based on the central screw's depression relative to the outer legs.
Specific rotation of liquids: Specific rotation is a property of chiral compounds in solution, measured with a polarimeter, not a spherometer. A spherometer is incapable of measuring this physical quantity.
Thickness of thin plates: A spherometer can effectively measure the thickness of thin plates by determining the difference in height between surfaces with and without the object.
Radius of curvature of convex surface: A spherometer can also measure the radius of curvature of convex surfaces using a method similar to that for concave surfaces, relying on the central screw's displacement to indicate curvature.
Therefore, specific rotation of liquids is the physical quantity that cannot be measured using a spherometer. The other options represent measurements within the spherometer's capabilities, which are primarily focused on determining curvatures and thin thicknesses.
