(a) Laws of refraction of light: (i) Incident ray travelling from one medium to another, bend in such a way that the incident ray, refracted ray and the normal ray, at the point of incidence, all lie in the same plane.(ii) The ratio of sine of angle of incidence (i) to the sine of angle of refraction $(r)$ is constant for a given pair of media.i.e. $\;\frac{\sin i}{\sin r} =$ constantAbsolute refractive index of a medium is the ratio of $\; \sin i$ to $\; \sin r$.Absolute refractive index$= \;\frac{\;\text{Speed of light in vacuum}\;}{\;\text{Speed of light in medium}\;}$ (b) Given: $n_{A}=2.0, n_{B}=1.5$, speed of light in medium $B (v_B) = 2 \times 10^8 \text{ m/s}$, speed of light in medium $v_{A}=$ ?(i) $n_{B} \;=\; \frac{c}{\; \text{Speed of light in a medium} \; (v_{B})}$$\;=\; \frac{c}{v_{B}}$$c \;= n_{B} \times v_{B} = 1.5 \times 2 \times 10^8 = 3 \times 10^8 \text{ m/s} .$(ii) $n_{A} = \; \frac{c}{v_{A}}$$v_{A} = \; \frac{c}{n_{A}} = \; \frac{3 \times 10^8}{2}=1.5 \times 10^8 \text{ m/s}$