College Physics (7th Edition)

It is always dark because of the destructive interference. This is because of the $180^{\circ}$ phase shift. If there had not been the $180^{\circ}$ phase shift, zero thickness would have corresponded to constructive interference.
It is always dark because of the destructive interference. This is because of the $180^{\circ}$ phase shift. If there had not been the $180^{\circ}$ phase shift, zero thickness would have corresponded to constructive interference.