Question

What happens when ligth travels from glass to air.and air to glass

Answer 1

Glass is a Denser medium. and Air is Rarer medium
Velocity of light is less in denser medium compare to rarer medium.

So, when light enters from glass to air, the velocity of light increases. ( Denser to Rarer)
and also
when enters from air to glass. the velocity of light decreases. (Rarer to Denser)

I hope it is clear to you..