incident wavefront touches the reflecting surface, the point B is yet to travel a distance BB ′ to touch the reflecting surface at ′ B . When the point B touches the reflecting surface at ′ B , the point A would have reached ′ A . This is applicable to all the points on the wavefront. Thus, the reflected wavefront ′ ′ A B emanates as a plane wavefront.
The two normals N and ′ N are considered at the points where the rays L and M fall on the reflecting surface. As reflection happens in the same medium, the speed of light is same before and after M' M L' L B' N' N A' i i r r X Y Figure . Laws of reflection - - - - Unit wave optics of medium ( ) is, c / v = n and that of medium ( ) is, c / v = n . In ratio form, sin sin i r ( .
) In product form, n sin i = n sin r ( . ) Hence, the laws of refraction are proved. In the same way the laws of refraction can be proved for wavefront travelling from denser to rarer medium also. The speed of light is inversely proportional to the refrative index of the medium ( µ and also directly proportional to wavelength of light ( v ∝λ .
Hence, λ λ = n n ( . ) If light of a particular frequency travels through different media, then, its frequency remains unchanged in all the media. Only the wavelength changes according to speed of light in that medium. Note EXAMPLE .
The wavelength of light from sodium source in vacuum is Å .What are its (a) wavelength, (b) speed and (c) frequency when this light travels in water which has a refractive index of . . Solution The refractive index of vacuum, n = The wavelength in vacuum, λ = Å. The speed in vacuum, c = v =