For a wave travelling from a medium with refractive index n1 to n2 with n2 > n1, the amplitude reflection coefficient is given by: r = (n2 - n1)/(n2 + n1) Consider a beam of light passing through a single pane of glass. There is a reflection at the front surface of the pane, with the reflected amplitude obtainable through the above equation. If using the above equation for the second interface (the back of the pane, travelling from glass to air), r is negative but equal in magnitude to the first interface (travelling from air to glass). Does this make any difference? If calculating the percentage amplitude reduction, would one have to consider both interfaces, or just the first?