A thin transparent coating can be deposited on a spectacle lens in a vacuum. If the thickness of this layer is equal to one fourth of the wavelength of light (actually slightly less than one fourth of the wavelength to allow for the decreased velocity of light when traveling through the coating), and if the index of refraction of the coating is chosen properly, light waves reflected from the interface between the coating and the glass are one-half wavelength out of step with the waves reflected from the outer surface of the coating (Fig. 17). The two wave trains cancel each other (destructive interference), and no reflections are seen.