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

Magnification in scanning electron microscope, How is magnification in scan...

How is magnification in scanning electron microscope controlled? Magnification in scanning electron microscope: Magnification into a SEM (scanning electron microscope) can b

Carnots theorem, Carnot's theorem (S. Carnot) The theorem which states...

Carnot's theorem (S. Carnot) The theorem which states that no engine operating among two temperatures can be more capable than a reversible engine.

Constraint relation : wedge and block, A block goes on a wedge which in tu...

A block goes on a wedge which in turns goes on a horizontal table, as given in figure. The wedge angle is Φ. As long as the wedge is in met with table, we have the trivial const

Charge on the outer surface of a parallel plate capacitor, Why does the cha...

Why does the charge on the outer surface of a parallel plate capacitor doesn't change when it is connected by a battery. only inner surface charge gets affected. Why??

What action reaction pair is responsible for pushing a car, Q. What action ...

Q. What action reaction pair is responsible for pushing a car forward? Ans. The wheels push back on the ground using friction while the ground then pushes forward on the

Pressure, The input piston of a hydraulic jack has cross sectiona

The input piston of a hydraulic jack has cross sectiona

Define current density, Q. Define: current density. Give its unit. The ...

Q. Define: current density. Give its unit. The amount of charge passing per unit time through unit area taken perpendicular to the direction of flow of charge at that point is

New potential difference, A parallel plate capacitor has plates of area 200...

A parallel plate capacitor has plates of area 200 cm 2 and separation among the plates 1.00 mm. What potential difference will be manufactured if the charge of 1 nC is given to th

Tension in a string, two blocks of mass 2kg and 4 kg are held together by a...

two blocks of mass 2kg and 4 kg are held together by a string force of 10N is applied on 2kg and 20N on 4kg find the tension in the string connected between two blocks

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