A beam of monochromatic light falls normally on the surface of a plane-parallel plate of thickness l.

Question:

A beam of monochromatic light falls normally on the surface of a plane-parallel plate of thickness l. The absorption coefficient of the substance the plate is made of varies linearly along the normal to its surface from x1 to x2. The coefficient of reflection at each surface of the plate is equal to p, neglecting the secondary reflections find the transmission coefficient of such a plate.