A point source of monochromatic light emitting a luminous flux Ф is positioned at the centre of

Question:

A point source of monochromatic light emitting a luminous flux Ф is positioned at the centre of a spherical layer of substance. The inside radius of the layer is a, the outside one is b. The coefficient of linear absorption of the substance is equal to x, the reflection coefficient of the surfaces is equal to p, neglecting the secondary reflections, find the intensity of light that passes through that layer.