Question

Why does the light ray not change direction travelling from air to glass?

Answer 1

As long as the light strikes the glass with an angle of incidence greater than 0o, then yes, the direction will change. This occurs anytime you have 2 materials with different indices of refraction.

As light passes from air (with a low index of refraction) to glass (with a higher index of refraction), it will slow down, which bends the light toward the normal.

If the light passes from glass to air, it will speed up, which bends the light away from the normal.

The angle of refraction can be calculated using Snell's Law (n1sinΘ1=n2sinΘ2), where Θ1 is the angle of incidence and you know the two indices of refraction.

Answer 2

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REFRACTION occurs when light rays enters glass from air because air is rarer medium and glass is denser medium.