Problem 5.216

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 \(x_{1}\) to \(x_{2} .\) The coefficient of reflection at each surface of the plate is equal to o. Neglecting the secondary reflections, find the transmission coefficient of such a plate.