$(a)$ $A$ ray of light falls normally on a face of a glass slab. What are the values of angle of incidence and angle of refraction of this ray?
$(b)$ Light enters from air to a medium $X$. Its speed in medium $X$ becomes $1.5 \times 10^{8} \ m \ s^{-1}$. Speed of light in air is $3 \times 10^{8} \ m \ s^{-1}$. Find the refractive index of medium $X$.

(N/A) When a ray of light falls normally (perpendicularly) on the surface of a glass slab,the angle between the incident ray and the normal is $0^{\circ}$. Therefore,the angle of incidence is $0^{\circ}$. Since the ray travels along the normal,it does not deviate,and the angle of refraction is also $0^{\circ}$.
$(b)$ Given:
Speed of light in air $(c)$ = $3 \times 10^{8} \ m \ s^{-1}$
Speed of light in medium $X$ $(v)$ = $1.5 \times 10^{8} \ m \ s^{-1}$
The refractive index $(n)$ of a medium is given by the ratio of the speed of light in air to the speed of light in that medium:
$n = \frac{c}{v}$
$n = \frac{3 \times 10^{8} \ m \ s^{-1}}{1.5 \times 10^{8} \ m \ s^{-1}}$
$n = 2$
Thus,the refractive index of medium $X$ is $2$.