When an object placed in optically denser medium (glass slab) is viewed from a rarer medium (air), it appears to be at a depth less than the real depth due to refraction. For viewing near the normal direction, the ratio of real depth to the apparent depth of a refracting medium gives the refractive index of the medium.
A travelling microscope can be used to find the real and apparent thickness of the glass slab and hence calculate its refractive index. The video demonstrates an experiment for finding refractive index of a glass slab using travelling microscope.