Phy Chapter 6 Refraction Of Light On Plane Surfaces Caltec S.3

REFRACTION OF LIGHT 14.42 Refraction Refraction is the change in the direction of light when it travels from one medium to another. (a) Laws of Refraction Law I States that: The incident ray and refracted ray are on the opposite sides of the normal at the point of incidence, and all the three are in the same plane. Law II States that: At any boundary between any two given media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for rays of particular wavelength. (b) (i) Refractive Index, n It follows from the second law of refraction that: = Constant The ratio is known as the relative refractive index, 1n2, of the two media. Therefore, Snell’s law can be written as: = 1n2 (ii) Absolute Refractive Index The absolute refractive index of a medium is defined as n = By analogy, the absolute refractive index, n, of medium 1 is given by n1 = ...……1 And the absolute refractive index, n1, of medium 2 is given by n2 = ....……2 Dividing equation 1 by 2 we have: = Or = = 1n2 Therefore, = Or n1Sin i = n2Sine r Note: The absolute refractive index of vacuum is equal to 1 and that of air at normal atmospheric pressure and 20°C is 1.0003. This difference is so small that it is normally ignored; hence the absolute refractive index of air is taken to be 1 as that of vacuum. From the equation n1 Sin i = n2Sine r It follows that: If (i) n2 n1, the ray bends towards the normal as it enters the glass and the angle i r. (ii) n1 n2, the ray bends away from the normal as it leaves the glass., then i r. …