Q. What do you mean by Ant reflecting Films?

When a beam of light travels normally from a medium of refractive index µ₂ to a medium of refractive index µ₁, a part of light is refracted and a part of light is transmitted, then reflecting and transmitting coefficients can be given as Such types of film are called Anti-Reflecting Film.

Consider Fig. given, an incident light reaches to an air-film interface. Some of it reflects back to the air and some of it transmits into the film. The transmitted part refracts in the films and reaches to a film substrate interface. Some of it reflects back to the air and some of it transmits into the substrate. The two reflected waves, which are generated at the film air interface and the substrate film interface, interact and produce a resultant reflected light waves.

The intensity of the net reflected light depends on the optical thickness of the film. Optical thickness of a film is given by,

T = n d

When n is the refractive index and d is the physical thickness of the film. If the optical thickness of the film is a quarter waves, the phase difference of the two reflected waves is π. They become out of phase and interfere destructively. Then, the net intensity of the reflected wave is at minimum. If the optical thickness of the film is a half wave, the phase difference of the two reflected waves is 2π. They become in phase and interfere constructively. Then, the net intensity of the reflected wave is at maximum.