Incident angle in one media (air) and the refraction is taking place in the glass medium both has different refractive indices hence angle incidence and angle diffraction are different. Velocity of light is different in two different media. read more
i=incidence angle, r=angle of refraction Refractive index= sini/sinr Since for different materials value of refractive index is different hence both angles are not equal. read more
Not that the angle of incidence does not equal the angle of reflection in all cases, such as a diffraction grating. QED explains that, too. Some of the paths are impossible, because the diffraction grating is dark there. So that biases the highest probability path to a different angle. read more