Becoz, in this case the emergent ray( the final light ray coming out of the slab is such that it gets parallel to the incident ray produced.
The Brainliest Answer!

This Is a Certified Answer

Certified answers contain reliable, trustworthy information vouched for by a hand-picked team of experts. Brainly has millions of high quality answers, all of them carefully moderated by our most trusted community members, but certified answers are the finest of the finest.
Let us say the rectangular glass slab has two surfaces S1 and S2. They are parallel. When light rays are incident on S1 from air at angle α (not 90), refraction takes place with an angle β and the colors are separated by a little angle as per the law of refraction.

When the refracted ray falls on the other surface S2, the angle of incidence now becomes β and on using the formula we find that angle of emergence is α in air. The situation is symmetric at S1 and S2. So the light colors combine again to align  themselves at angle of emergence = α and propagate as a parallel beam of rays together.

At S2, violet color bends less towards normal and compensates for more bend at S1. At S2 red color bends.

Actually there is a lateral shift in the position of rays, that is different for different colors. But is very small and if there are a lot of rays, then it will not noticeable.

2 5 2
click on thank you and on stars.