The second law of refraction gives a relationship between the angle of incidence and the angle of refraction. This relationship was discovered by Snell experimentally, so the second law of refraction is called Snell’s law of refraction. According to Snell’s law of refraction of light: The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant, for the light of a given colour and for the given pair of media. If i is the angle of incidence and r is the angle of refraction, then sin i sin r = constant . This constant value is called the refractive index of the second medium with respect to the first.

Define power of a lens. What is its unit?

The power of a lens is a measure of the degree of convergence or divergence of light rays falling on it. The power of a lens is defined as the reciprocal of its focal length in metres. If P denotes the power of a lens and f denotes the focal length of the lens, the power is formulated as; P = 1 f ( in metres ) The SI unit of power of lens is dioptre ( D ).