Cause of normal shift (apparent shift):

Normal shift or apparent shift is the apparent displacement of an object when viewed across a refracting surface, like when a coin appears raised in water.

This happens due to refraction of light at the interface of two media with different refractive indices.

The apparent depth is related to real depth and refractive index by:

Thickness of medium: Directly affects apparent depth (deeper medium → larger shift).

Nature of medium: Refractive index depends on the medium.

Colour of light: Slightly affects due to dispersion (variation of refractive index with wavelength).