The angles of incidence and refraction of a monochromatic ray of light of wavelength `lamda` at an air-glass interface are i and r, respectively. A parallel beam of light with a small spread `delta lamda` in wavelength about a mean wavelength `lamda` is refracted at the same air-glass interface. The refractive index `mu` of glass depends on the wavelength `lamda` as `mu(lamda) = a + b//lamda2` where a and b are constants. Then the angular spread in the angle of refraction of the beam is