$(a)$ State Snell's law of refraction.
$(b)$ When a ray of light travelling in air enters obliquely into a glass slab, it is observed that the light ray emerges parallel to the incident ray but it is shifted sideways slightly. Draw a ray diagram to illustrate it.

(N/A) Snell's law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media and for light of a given wavelength. Mathematically, $\frac{\sin i}{\sin r} = \text{constant} = n_{21}$, where $n_{21}$ is the refractive index of the second medium with respect to the first.
$(b)$ When a light ray enters a glass slab obliquely, it bends towards the normal as it travels from a rarer medium (air) to a denser medium (glass). Upon exiting the glass slab, it bends away from the normal as it travels from a denser medium to a rarer medium. The emergent ray is parallel to the incident ray but undergoes a lateral displacement.