A Travelling microscope is used during the experiment to measure theA. focal length of a convex mirrorB. focal length of a convex lensC. refractive index of glass slabD. resistivity of a given wire

A Travelling microscope is typically used in physics experiments to measure small distances and displacements with high precision. In the context of the options you provided:

C. refractive index of glass slab

A Travelling microscope can be used to measure the thickness of a glass slab and, with additional measurements, can be used to determine the refractive index of the glass slab. This is done by measuring the thickness of the glass slab and then using it in conjunction with other optical measurements involving light passing through the slab.

It is not typically used for the other options:

A. focal length of a convex mirror - Focal length of a mirror can be measured using other methods like the mirror formula and the method of obtaining a sharp image of a distant object.

B. focal length of a convex lens - Focal length of a lens can also be measured using various methods such as the lens formula, the displacement method, or the autocollimation method.

D. resistivity of a given wire - Measuring the resistivity of a wire typically involves electrical measurements and is not the primary function of a travelling microscope.