Question

during refraction of light through the glass slab incident ray and emergency ray are paralle to each other explain

Answer 1

Here is your answer.
Light rays are reversible. They travel exactly the same path if they are going in the opposite direction. So if light is refracted some angle going from, say, air to glass, it would refract exactly the same angle in the opposite direction going from glass to air.
The bottom side of the slab is the reversed situation of going from glass to air.
If the two sides if the glass slab are parallel, then no new angles are introduced. The two refractions cancel out the changes in direction of the ray. But as you can see, the emergent ray is displaced laterally, although it’s parallel to the incident ray.

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