Presentation on theme: "by Bhaskar Department of Physics"— Presentation transcript:
1by Bhaskar Department of Physics Engineering PhysicsbyBhaskarDepartment of PhysicsK L University
2Interference in Thin Films Lecture 4&5 (06,07 & 13 Aug)Interference in Thin Films
3Interference in thin films Types of Interference: The phenomenon is divided into two classes based on the mode of production of interference. These areInterference produced by the division of wave frontYoung’s double slit experiment.Interference produced by the division of amplitudeBut interference pattern can also be produced by division of amplitude of a single wave.Lloyd’s mirror.But the positions of the dark and bright fringes are reversed here when compared to the patterns of Young’s double slit experiment.
4Interference in thin films Stocks Principle: According to stocks principle an wave undergoes a phase change of 180 degrees on reflection from a medium of higher index of refraction than the one in which it is travelling.
5Interference in thin films Reflection of a Transverse Wave from a Fixed End:Wave exerts upward force on support=> downward force on rope=> inversion of original wave
6Interference in thin films Reflection of a Transverse Wave from a Free End:If end is free to move wave is not invertedNo force on free endSo wave is just reflected
7Interference in thin films Reflection by denser mediumReflection by less dense medium
8Interference in thin films This reflection produces a phase difference of ½λ from the original wave and is called a fixed-end reflection.
9Interference in thin films The reflection produces no phase difference with the original wave, this is called a free-end reflection.
10B denser medium rarer A C O ar at a ar2 D art A denser medium rarer A att|atr|M VENKAT
11Interference in thin films Optical Path:The optical path travelled by a light ray in a medium of refractive index 'μ' is not equal to actual path travel led by the light ray.Optical path travelled by light beam ǂ Actual path travelled by light
12Interference in thin films Thin film planes can be parallel to each other or inclined. That is why, the concept of interference in thin films can be studied under two categories, namely,Interference in parallel plate film andInterference in wedge-shaped films.Soap Film – Why Color?
13Interference in thin films We observes colors in such thin films as soap bubbles, coatings on camera lenses and in a butterfly's wings or peacock's feathers. When ray 1 strikes the top interface, some of the light is partially reflected, ray 2, and the rest is refracted, ray 3. When ray 3 strikes the bottom interface, some of it is reflected, ray 4, and the remainder is refracted, ray 6.When ray 4 strikes the top interface from underneath, some is reflected (not shown) and some is refracted, ray 5. It is the interference between rays 2 and 5 that produces a thin film's color when the film is viewed from above.
14Interference in thin films Interference in plane parallel films due to reflection of light:Ray 2 undergoes a phase change of 180° with respect to the incident rayRay 1, which is reflected from the lower surface, undergoes no phase change with respect to the incident wave
15Interference in thin films Interference in plane parallel films due to reflection of light:TNSQiiairiCrArtMEBtrP
16Interference in thin films Interference in plane parallel films due to reflection of light:For constructive interference2 μ t cos r =(2n±1) λ/2 n = 0, 1, 2 …For normal incidence r=002 μ t = (2n±1) λ/2 n = 0, 1, 2 …For destruction interference2 μ t cos r = n λ n = 0, 1, 2 …For normal incidence r=0°2 μ t = n λ n = 0, 1, 2 …
17Interference in thin films Two factors influence interferencePossible phase reversals on reflectionDifferences in travel distanceThe conditions are valid if the medium above the top surface is the same as the medium below the bottom surfaceIf the thin film is between two different media, one of lower index than the film and one of higher index, the conditions for constructive and destructive interference are reversed
18Interference in thin films Interference in plane parallel films due to transmitted of light:The conditions for bright is2 μ d cos r = n λ n = 0, 1, 2 …The conditions for dark is2 μ d cos r =(2n±1) λ/2n = 0, 1, 2 …We can say the interference pattern due to reflected and transmitted rays are complementary each other.
19Interference in thin films Interference in wedge-shaped films:
20Interference in thin films Interference in wedge-shaped films:For constructive interference2 μ t cos (r + α) =(2n±1) λ/2 n = 0, 1, 2 …For normal incidence r=002 μ t cos α = (2n±1) λ/2 n = 0, 1, 2 …For destruction interference2 μ t cos (r + α) = n λ n = 0, 1, 2 …2 μ t cos α = n λ n = 0, 1, 2 …
21Interference in thin films Applications of Wedge method:Determination of thickness of a paper or diameter of a wire/hair.2) Verification of flatness of the given transparent surface.
22Interference in thin films Determination of thickness of paper or thin film:
23Interference in thin films Determination of thickness of paper or thin film:Thickness of paper:
24Interference in thin films Ray 2: No phase change from internal reflection BUT wave travels extra distance 2 t during which wavelength isnumber of “extra” wavelengths from travel through the film is• Ray 1: 180° (i.e. π) phase change from external reflectionequivalent to a path difference of
25Interference in thin films NON-REFLECTIVE COATINGS:If film is between layers with higher and lower refractive index, conditions reverseconstructive interference for2 μ t = n λ n = 0, 1, 2 …Get 180° phase change at both reflections
26Interference in thin films NON-REFLECTIVE COATINGS:Non-reflecting medium can prepared by coating the thin films on to the mediums.These films were useful to prevent the reflection of light(still some light can reflects but reflected ratio decreases).Here the condition for reflection minimum requires a path difference is λ/2.