Thin film interference occurs when light waves are reflected off the surface of a thin film and interfere with each other. When the light waves reflected off the top and bottom surfaces of the film are in phase, they constructively interfere and certain wavelengths of light are amplified, producing colors. This explains the rainbow patterns seen on surfaces like oil slicks and beetles. Anti-reflection films are designed to have a thickness that causes the reflected light waves to be out of phase and cancel each other out, reducing reflection.

1. Thin Film Interference
What is Thin Film Interference?
To understand thin film interference it is important that one first has a good
understanding of
-Reflection/Refraction and,
-Wave Interference
as these are the two main components of thin film interference!
If you have noticed rainbows on the back of CD's, on beetles, on bubbles or on oil
slicks, then you have seen thin film interference!!!
Definition
Thin film interference is the constructive and deconstructive interference of light
waves that have been reflected off the surface of a thin film. It is best described
using diagrams and in 2 parts!
Part 1. Reflection
Fig, 1

2. When a light wave reaches the boundary between two media part of the wave is
reflected off the surface of the boundary while the rest of the light travels into the
medium. If the film is thin enough this light wave will be reflected off of the bottom
of the film and will exit the second medium back into the first.
Part 2. Interference
Conditions for interference:
1. The two reflected light waves must be close enough together to be able to interact
and cause constructive or deconstructive interference. This requires incident angles
close to the normal.
2. For complete constructive or deconstructive interference, the film must be just
thick enough so that the wave that exits the film has traveled enough to be perfectly
in or out of phase with the first reflected wave.
Depending on the thickness of the film and the wavelength of the incident light, the
reflected waves will be shifted so that they are either in or out of phase, or a little bit
of both. Due to gravity and other varying factors, the thickness of the film is not
uniform throughout. So some points of the film may be the perfect thickness to
cause yellow light rays (wavelength =570–590 nm) to leave the film in phase. When
the waves are in phase they interfere constructively and cause the light to appear
brighter (larger amplitude). Other points along the film may cause blue or red light
to be amplified and this is how we see a "rainbow" on various surfaces!
Antireflection Films
Anti reflection films are often applied to lenses and glass to allow us to be able to see
clearly though the glass. These films are designed to be exactly thick enough to
cause the reflected light rays to be 180 degrees out of phase so that they completely
cancel each other out so that there is no reflection!