Question

Readings of glass slab experiment if angle of incidence is 90 degres

Answer 1

You must mean incident normally. If you mean so, then the angle of incidence is 0 deg.

Then the ray of light will go undeviated i.e. the path of light will not change.

If you don't mean the above. What's the point of having a light rat incident at an angle of 90 deg?