![]() It can be argued that, a prism colours the white light and the light by itself is white. Spectrum: The band of seven colours obtained on the screen, when a white light splits into its component colours is called the spectrum. The dispersive power of flint glass is more than that of crown glass.Dispersion: The phenomenon due to which a white light splits into its component colours, when passed through a prism is called dispersion. A prism of larger dispersive power will produce a spectrum having a wider span than a prism of lower dispersive power. The dispersive power depends only on the material of the prism and nothing else. Hence for dispersive power, the following relation is internationally accepted- ω = (µ F – µ C)/(µ D – 1) The mean refractive index for this part is nearly equal to the refractive index for the D-line (Yellow, λ = 5893 Å) of sodium. Our eye is most sensitive to the range of wavelength which lies between the F-line (sky-green, λ = 4861 Å) and C-line (red, λ = 6563 Å) of the hydrogen spectrum. On substituting in equation (i) the dispersive power of the medium is found as- ω = / (µ Y – 1) A = (µ V – µ R)/(µ Y – 1) -(ii)Įquation (ii) itself may be taken as the definition of dispersive power. When white light passes through such a prism, the deviation of violet, yellow and red light are given by- δ V = (µ V – 1) A For a prism of small angle A at a small angle of incidence i, the deviation produced is given by- δ = (µ – 1) A We may express ω in terms of refractive index (µ). Dispersive power ω = angular dispersion/average deviation = (δ V – δ R)/δ Y -(i) (This is also the position of minimum deviation for this wavelength). These quantities are measured in the position of the prism when it is passing the mean wavelength symmetrically through it. The property of producing dispersion by a medium is expressed in terms of dispersive power ω which is the ratio of angular dispersion to the average deviation. Both of these will be different for a different medium. Further (δ V – δ R) gives the angular dispersion caused by the medium. Since the yellow colour lies almost near the middle, its deviation δ Y is used to express the mean deviation produced by a particular medium. When white light enters a transparent medium (say in form of a transparent prism) the light of different colours has deviated through different angles. The angular dispersion produced by the prism depends upon. The unit of angular dispersion is degree. (i)įor violet colour, δ V = (µ V – 1) A -(iii) Cause of Dispersion of Light:Īccording to Cauchy’s formula, the refractive index (µ) of material depends on the wavelength (λ) as written below. The phenomenon of separation of light of different frequencies (or wavelengths) into its separate components is called dispersion of light. The higher frequencies (smaller wavelengths) have deviated most and cause a violet sensation while lower frequencies (longer wavelengths) are deviated least and produce a red sensation in our eyes. Hence a prism deviates light of different colours by different amounts. Since the refractive index of a medium is given by the ratio of the speed of light in a vacuum to the speed of light in that medium, it is clear that a medium has different refractive indices for different colours corresponding to different frequencies of light. ![]() ![]() In a medium like glass, light rays of different colours corresponding to different frequencies travel at different speeds. ![]() In a vacuum, all frequencies of light travel with the same speed c = 3 x 108 ms-1, but it is not so in a physical medium. We know that light travels in waves and the components that produce different sensations of colours are infact, correspond to waves of different frequencies (or wavelengths). When a narrow beam of white light is allowed to pass through a prism, a beautiful pattern of colours appears on the screen, this shows that white light consists of a number of components each of which causes a particular sensation of colour. ![]()
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