Re: why does not light disperses in glass slab?

Well, Manas, that's a good question, not one I've heard often. It shows a
keen mind on your part, unless it's a question from your teacher.

You ask why "all the seven colors emerge". Some say seven colors while some
say six. Let's say ALL the colors (all the different wavelengths that exist
at the source) come out parallel. Actually the source of light sends light
in many different directions toward the glass and hits the glass in many
different points. So there are many different bundles of those parallel
colors. 

Before you can SEE any of that, the light must enter the pupil of your eye
and be focused on your retina. When that happens, the parallel light is
recombined to nearly the same combination of colors as the original source.
The three types of cones in your retina respond to different ranges of
frequencies and your brain will come up with an image. When the surfaces of
the glass slab are not uniformly parallel, you will see color fringes that
show that the rays did not emerge parallel to each other.

In general, we call any grouping of colors "white" if it is the brightest
combination of frequencies in the general spectral areas of red, green, and
blue. I'm sure you know that TV screens and computer monitors only produce
three colors, Red/Green/Blue. However, their tiny spots are beyond our
limit of resolution at normal viewing distances, so we are always looking
at many tiny PICture ELements (pixel) combined. These pixels will be made up of
varying intensities of light in the red, green, and blue bands of our
visible spectrum. So the thousands or millions of colors advertised by
computer companies indicate the number of combinations of how bright each
little screen spot is made.

As an exercise, make a drawing of a light source and a glass slab. Draw two
different rays striking the glass at different angles. Show each ray
refracting in 3 slightly different directions in the glass (to represent the RGB
scheme) and again as they emerge.  I hope this helps you see what happens.