DISPERSION OF LIGHT

The light rays from the sun consist of seven different colors – red, orange, yellow, green, blue, indigo and violet (ROYGBIV). We see seven different colors when these sun are passed through a glass prism.

The splitting of a ray into its component colors is known as dispersion of light. The band of colors into which the light splits is known as a spectrum. White light consists of photons of various wavelengths (or colors). Photons of different wavelength travel with different speed. The red photon has the longest wavelength and the violet photon has the shortest wavelength. Thus when a white light passes through a prism, different photons cross the medium at different speeds and deviating by different angles. The red color appears at the top of the spectrum because it is bent the least or it is refracted the least.. On the other hand, the violet end of the spectrum is bent the most or refracted most, as it takes longer to traverse the glass medium.

           

                                   

When two prisms are placed with their vertices and bases in the opposite direction,  the spectrum of seven colors from the first prism enters the second prism and only a white light comes out.


                           
                                  

                                 REASON FOR RECOMBINATION


When white light is passed through a prism,7 bands of colors are coming out of the prism at angle to the direction of incident white ray,the rays are deviated toward the base of the prism 
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If a plane mirroris placed in the path of the band so that the rays are reflected back,all rays retrace their path and emerge out as white ray. 

From this it is clear that a white ray passing through a prism is split up into a band and is turned toward the base and If a band of light is incident from the base of the prism they emerge out of the prism as white ray and again turned to ward the base of the prism. 

Therefore, when a white ray is passed through the prism they split up into a band and the band, when incident on another prism such that they are incident from the base, they recombine to white ray. In other words, the second prism is inverted so that the band is incident from the base of the second prism.