The speed of light in the medium is
An initially parallel cylindrical beam travels in a medium of refractive index μ(I) = μ0 + μ2I, where μ0 and μ2 are positive constants and I is the intensity of the light beam. The intensity of the beam is decreasing with increasing radius.
(a) maximum on the axis of the beam
(b) minimum on the axis of the beam
(c) the same everywhere in the beam
(d) directly proportional to the intensity I
