while studying diffraction........through glass slab why the light ray, that enters from the normal, not bend towards any side while others bend...

I believe you are actually referring to refraction - the
bending of light when it goes from one medium to
another.

Light is a form of energy which travels as waves
with a certain frequency, wavelength, and velocity.  When a wave hits an interface (in
your case when the wave goes from air into the glass) the velocity of the wave changes
depending on the density of the substance.

When a wave
strikes the interface from the normal, the wave front reaches the glass all at the same
time.  However, when the wave strikes the surface at an angle, part of the wave front
enters the glass sooner than other parts of the wave. As each portion of the wave enters
the glass it slows down in sequence and you get the bending effect that you
see.

Diffraction is the slight bending of a wave as it goes
around an object. (see second referrence link below)