Theformulathat describes refraction is also known asSnell's Law. The angle of refraction depends on the angle of incidence of the light, and the indexes of refraction of the two materials. ... As a reminder, the angles inSnell's Laware always measured relative to the normal (perpendicular) to the surface.

Snell's law(also known asSnell–Descartes lawand thelaw of refraction) is aformulaused to describe the relationship between theangles of incidenceandrefraction, when referring to light or otherwavespassing through a boundary between two differentisotropicmedia, such as water, glass, or air.

In optics, the law is used inray tracingto compute the angles of incidence or refraction, and in experimental optics to find therefractive indexof a material. The law is also satisfied inmetamaterials, which allow light to be bent "backward" at a negative angle of refraction with anegative refractive index.

Snell's law states that the ratio of thesinesof the angles of incidence and refraction is equivalent to the ratio ofphase velocitiesin the two media, or equivalent to the reciprocal of the ratio of theindices of refraction:

{\displaystyle {\frac {\sin \theta _{2}}{\sin \theta _{1}}}={\frac {v_{2}}{v_{1}}}={\frac {n_{1}}{n_{2}}}}

with each{\displaystyle \theta }as the angle measured from the normal of the boundary,{\displaystyle v}as the velocity of light in the respective medium (SI units are meters per second, or m/s),{\displaystyle \lambda }as the wavelength of light in the respective medium and{\displaystyle n}as the refractive index (which is unitless) of the respective medium.

The law follows fromFermat'sprinciple of least time, which in turn follows from the propagation of light as waves.

sin i /sin r =constant

sini/sinr =constant is the correct answer of the given question is