(a) The intensity of scattered light varies inversely as fourth power of wavelength (i. e., I∝ 1/λ4). In visible light blue colour has minimum wavelength, so it is scattered most, that is why bluish colour predominates in a clear sky.
(b) While light consists of infinite wavelengths starting from 400nm (violet) to 750nm (red). The refractive index of proton is maximum for violet and minimum for red; so prism separates constituent colours of white light and causes maximum deviation for violet colour. That is why violet colour is seen at the bottom of spectrum when white light is dispersed through a prism
