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Anti-reflection Coated Lens 87

Fig. 8.4: Constructive interference

With anti-reflection coating the optical effect that is used to reduce the
surface reflection is based on destructive interference. It works by
interposing a transparent layer, i.e. the coating; we cause two reflections –
the first from the surface layer itself and the second from the lens surface.
If the thickness of the layer is made a quarter of a wavelength thick, the
two reflected rays will be out of the phase with one another by half a
wavelength. This is because the rays reflected from the lens surface have
to travel through the layer and then back again. Being out of phase with
one another they effectively cancel or eliminate each other, thus destroying
the reflection.

SINGLE LAYER ANTI-REFLECTION COATING
The simple principle of single layer anti-reflection coating is that the
substrate (lens) is coated with a thin layer of material so that reflection
from the outer surface of the film and the outer surface of the substrate
cancel each other by destructive interference. To achieve this we take the
advantage of the undulatory nature of light by creating opposition of phases
between reflected waves.
If we place a thin coating on the lens, the light beam, which is a series of
waves, hits the coating and breaks up into reflected waves and refracted
waves. The refracted waves then hit the lens and split second time into
reflected and refracted waves. By a careful calculation of the thickness and
index, in such a way that the first series of reflected waves and the second
are super imposed while being out of the phase by half a wavelength, we
obtain mutual cancellation. Light which is not reflected is refracted.