CBSE Class 10 Science 2017 · Paper 531 · Question 24

(a) The law is Snell’s law of refraction (the second law of refraction): the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant, for light of a given colour and for the given pair of media. Mathematically, $\frac{\sin i}{\sin r}=n_{21}$, where the constant $n_{21}$ is the refractive index of the second medium with respect to the first. In terms of speeds, $n_{21}=\frac{v_1}{v_2}$: the speed of light in medium 1 divided by the speed of light in medium 2. The absolute refractive index of a medium is $n_m=\frac{c}{v}$, where $c$ is the speed of light in vacuum and $v$ its speed in the medium. (b) Turpentine oil ($n=1.47$) is optically denser than water ($n=1.33$), so a ray passing from turpentine oil into water bends away from the normal. In the ray diagram the refracted ray in the water makes a larger angle with the normal than the incident ray in the oil ($r>i$; for $i=40^{\circ }$, $\sin r=\frac{1.47}{1.33}\sin 40^{\circ }$ gives $r\approx 45.3^{\circ }$).