Antireflection films and interference filters, Physics

Assignment Help:

Q. What are antireflection films and interference filters? 

Ans. Anti 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=nd

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.

Interference Filter: An interference filter, which reflects one of more spectral lines and transmits others, while maintaining a nearly zero coefficient of absorption for all wavelengths of interest. In an optical filter, a semitransparent metal film is deposited on a plate of glass. Then, it is coated with a thin layer of some dielectric material like MgF2, followed by another coating of a thin film of metal. Finally, another plate of glass is kept over the films for mechanical protection. Interference filters have a band pass of 100-150A and peak transmittance of 40-6-%. When a beam of light is incident normally on an interference filter, multiple reflections takes places between the two surface (Layers). If 't' is the separation between the two plates and 'µ' is the refractive index of the dielectric medium, then from the theory of interference in thin film the condition for maxima in transmitted light can be given as

2µt = n¥                                                                                                                                  .....(1)

If we use white light in this case, then different colors (wavelength) are found in transmitted light can be obtained using equation (1)

For example, if µ = 1.5, t = 6 x 10-5 meter, then only two wavelengths of visible region of value of 6000A for n = 4 gives us maxima. Since the wavelengths are widely separated, they can be filter by using a device called interference filter.

If a thin transparent spacer is placed between two semi reflective coatings, multiple reflections and interference can be used to select a narrow frequency band, producing an interference filter. It the spacer is half wavelength for the desired wavelength, then other will be eliminated by destructive interference. If the back layer is totally reflective, then the arrangement is called a dichotic mirror, reflecting only the selected wavelength.

Uses

            Interference filers are widely used in instrumentation for clinical chemistry, environment testing, colorimetric, elemental and laser line separation, flame photometry, fluorescence, immunoassays, etc.

Demerit

            Designing and producing interference filter is a complex procedure requiring thin film expertise and sophisticated instruments.


Related Discussions:- Antireflection films and interference filters

Explain photon, According to Eisenstein's quantum theory light propagates i...

According to Eisenstein's quantum theory light propagates in the bundles of power, every packet being called as photon and possessing energy.

Spin-orbit effect, Spin-orbit effect An influence that causes atomic en...

Spin-orbit effect An influence that causes atomic energy levels to be split since electrons have intrinsic angular momentum (spin) additionally to their extrinsic orbital angul

Michelson inferometre experiment, Why the fringes are in circular form and ...

Why the fringes are in circular form and how does it form?

Define about the set-up of helium-neon laser, Define about the set-up of He...

Define about the set-up of Helium-Neon Laser. The laser radiations along with higher coherence, higher directionality and higher monochromacity can be obtained from this. But,

acceleration, How to find acceleration from succesive speed

How to find acceleration from succesive speed?

Joules laws, Joule's laws (J.P. Joule) Joule's first law The he...

Joule's laws (J.P. Joule) Joule's first law The heat Q generated while a current I pass through a resistance R for a particular time t is given by following Q = I2 R

Michealson interferometer, describe the principle, construction and working...

describe the principle, construction and working of Michelson Interferometer.

Thermistors, Thermistors Insulators and semi-conductors behave in a dif...

Thermistors Insulators and semi-conductors behave in a different way when the temperature increases, because their resistivity decreases. That is: the resistance of an insulato

Dielectric loss, Electrical Properties of Dielectric Material A loss ...

Electrical Properties of Dielectric Material A loss of energy which eventually produces a rise in temperature of a dielectric placed in an alternating electric field is calle

Michelson interferometer, Ask question 15. When the movable mirror of a Mic...

Ask question 15. When the movable mirror of a Michelson''s interferometer is moved through a distance of 0.03mm, 100 fringes are displaced, what is the wavelength and color of the

Write Your Message!

Captcha
Free Assignment Quote

Assured A++ Grade

Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!

All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd