what happens when angle of incidence is 90 degree?'

 then angle of incidence would also be 90 degree

that means the light will reflect back through the same path

I Agree !!!

if the ray is perpendicular to the surface

then it will go back with same path

 Miss asmi the genius 

I agree with U 
 

 angle of degree 90 degree means it is the normal,so it will deflect back in the same path

u r right in my view

i was here 2 gve de same answer!

WHEN ANGLE OF INCIDENCE IS 90 DEGREE THEN ANGLE OF REFLECTION WILL ALSO BE 90 DEGREE,WHICH MEANS THAT ANGLE OF INCIDENCE AND ANGLE OF REFLECTION WILL BE SUPPLEMENTARY ie;THEIR SUM WILL BE 180 DEGREE.IT DOES NOT MEAN THAT THE LIGHT RAY WILL GET REFLECTED ALONG THE SAME PATH BUT IT MEANS THAT THE INCIDENT RAY AND REFLECTED RAY WILL JOIN TO FORM A STRAIGHT LINE.

 hey arun

their is no difference in my answer and that of urs

 Angle of incidence is the angle between incident ray and normal at that point and since it is told in the question that the angle of incidence is 90⁰ so this means the light is not falling normally on the surface but is moving along the surface so it will continue its path and move straight. This will not violate laws of reflection also as the angle of reflection will also be 90⁰.

 thumbs up please !!!!!!!!

 in case of refraction?

 see..when the angle of incidence is 90 degree...itsimply means that it is overlaping normal ray..and then its angle with the normal ray will become 0 degree...so as per law of reflection - angle of incidence will be equal to angle of reflection. so angle of reflection will be 0 degree and so it will return back to the same path. now understood....