Refractive index is defined as the ratio of velocity of light in first medium to the velocity of light in second medium when a ray of light emerges at the point of incidence......Refractive index of glass slab is more than air because of air is in gaseous state n glass slab is in solid satte,,,,,when light emerges through glass slab there is more deviation in velocity of light inside yhe glass slab,,,,but in case of air there is slight deviation in velocity of light..
Change in the angle when a ray of light enters from one medium to other