Refraction of Light

When a ray of light passes obliquely from one transparent medium into another, it bends at the boundary; this bending is called refraction, and it happens because light travels at a different speed in different media. Refraction obeys two laws: the incident ray, the refracted ray and the normal to the boundary at the point of incidence all lie in one plane, and, for a given pair of media, the ratio $\dfrac{\sin i}{\sin r}$ of the sine of the angle of incidence to the sine of the angle of refraction is a constant -- this is Snell's law, and the constant is called the refractive index of the second medium with respect to the first. The absolute refractive index of a medium, $n=\dfrac{c}{v}$, compares the speed of light in vacuum, $c$, with its (always smaller) speed $v$ inside that medium; a medium with a larger absolute refractive index is called optically denser. When light passes from a rarer medium (smaller $n$) into a denser medium (larger $n$), it bends towards the normal ($r<i$); when it passes from a denser into a rarer medium, it bends away from the normal ($r>i$). Snell's law is more completely stated in terms of absolute refractive indices as $n_1\sin i=n_2\sin r$, which shows immediately that light travelling along the normal itself ($i=0$) passes straight through undeviated, whatever the media. A practical, everyday consequence of refraction at a parallel-sided slab (such as a glass slab or a swimming pool's water surface) is a lateral shift of the emergent ray: the ray le …