my question is why light get dispersed …

When a light ray enters into a denser medium from a rarer medium it bends away from the normal at the interface of the two medium because of the decrease in the velocity. Light rays with longer wavelenth bends the least and with sorter wavelength bends the most. Now we all know the white light has seven component light rays of different colours. All of them vary with respective to their wavelength. Red has the longest wavelenth whereas the violet has the shortest. Therefore when a white light enters into the prism all the component light rays bends away from the normal inversely proportional to their wavelength.e.g. red light bends the least and violet bends the most. Therefore we get a spectrum of seven different colours.  

its simple because every colour of light have diffrent velocity and wavelength.So,when a white light enters the prism the light with large wavelength(red) bends the least and colour light with smallest wavelength(violet) bends the the most and similarly it happens for all the colour of light.and we can see all the diffreny colour of white light.