For refraction at an interface between two media, Snell's law is correctly written as 
A$\dfrac{\sin r}{\sin i}=n_{21}$
B$\dfrac{\sin i}{\sin r}=n_{21}$
C$\dfrac{\tan i}{\tan r}=n_{21}$
D$\sin i+\sin r=n_{21}$
Answer & Solution
Correct answer: B. $\dfrac{\sin i}{\sin r}=n_{21}$
Snell's law states that for a given pair of media, $\dfrac{\sin i}{\sin r}=n_{21}$, where $n_{21}$ is the refractive index of medium 2 with respect to medium 1. The ratio is independent of the angle of incidence for the given wavelength.
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