It is then refracted away from the normal at the boundary between glass and air as it leaves the prism.

The incident ray is refracted towards the normal as it enters the glass prism from air. It is then refracted away from the normal at the boundary between glass and air as it leaves the prism.

The more accurate method of measuring the index of refraction is to fashion a prism with the medium under consideration, and to measure the angle of deviation of a parallel monochromatic light beam.