When a thick glass slab is placed over printed matter, the letters appear raised. This is due to ___________.

Correct Option: Refraction of light

Explanation:

When a thick glass slab is placed over printed text, the letters appear to be raised slightly above their actual position. This phenomenon occurs due to the refraction of light.

Refraction is the bending of light waves as they pass from one medium into another of a different optical density.

When light rays originating from the printed letters travel from the glass (an optically denser medium) into the air (an optically rarer medium), they bend away from the normal line at the boundary interface.

When these refracted light rays enter the observer's eyes, the human brain traces them straight backward in a straight line. As a result, the rays appear to diverge from a virtual image located slightly higher than the real position of the letters.

This apparent shift upward is known as normal shift or apparent depth effect, and it is a direct consequence of the refraction of light at the glass-air interface.