As light beam goes horizontally ,there is no change in refractive index as the light passes through,so there must be no deviation..But the answer is not that ..Please explain..

It really relies on upon point of occurrence however before that we should know why it goes astray? why light curves when it flies out from one medium to the next? to know the response for this question. 

why refraction? why bearing of wave changes? 

You can clarify this utilizing the Principle of slightest activity or equally Fermat's rule. In the event that you need light to get from place A to place B it will take the speediest course. As the paces vary in the two distinctive medium the quickest course ends up being unified with a twist at the limit.