How Can Thin Film Interference Cause Complete Destructive Interference?

[maccha]

I've learned that when a wave is partially transmitted and partially reflected, it loses some of it's amplitude in order to conserve energy. How then, in thin film interference, can one wave produce complete destructive interference with the other if it has been partially transmitted? Wouldn't the two waves have different amplitudes?

 

[ehild]

For complete destructive interference, the two waves need to have equal amplitudes. Such complete destructive interference can happen in the reflected light when a thin film of appropriate thickness and refractive index covers a substrate. One ray is reflected directly from the front surface of the film. The remaining light is transmitted into the layer, and partly reflected at the back surface. This ray travels back, and partially transmitted at the film-air interface. This ray interferes with the first one.

ehild