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Interference in Thin Films Interference effects are commonly observed in thin films Interference effects are commonly observed in thin films Examples are.

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Chapter 9 Light as a Wave Interference Light waves interfere with each other much like mechanical waves do Light waves interfere with each other.

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Presentation on theme: "Interference in Thin Films Interference effects are commonly observed in thin films Interference effects are commonly observed in thin films Examples are."— Presentation transcript:

1 Interference in Thin Films Interference effects are commonly observed in thin films Interference effects are commonly observed in thin films Examples are soap bubbles and oil on water Examples are soap bubbles and oil on water Assume the light rays are traveling in air nearly normal to the two surfaces of the film Assume the light rays are traveling in air nearly normal to the two surfaces of the film

2 Interference in Thin Films, 2 Rules to remember Rules to remember An electromagnetic wave traveling from a medium of index of refraction n 1 toward a medium of index of refraction n 2 undergoes a 180° phase change on reflection when n 2 > n 1 An electromagnetic wave traveling from a medium of index of refraction n 1 toward a medium of index of refraction n 2 undergoes a 180° phase change on reflection when n 2 > n 1 There is no phase change in the reflected wave if n 2 < n 1 There is no phase change in the reflected wave if n 2 < n 1 The wavelength of light λ n in a medium with index of refraction n is λ n = λ/n, where λ is the wavelength of light in vacuum The wavelength of light λ n in a medium with index of refraction n is λ n = λ/n, where λ is the wavelength of light in vacuum

3 Phase Changes Due To Reflection An electromagnetic wave undergoes a phase change of 180° upon reflection from a medium of higher index of refraction than the one in which it was traveling An electromagnetic wave undergoes a phase change of 180° upon reflection from a medium of higher index of refraction than the one in which it was traveling Analogous to a reflected pulse on a string Analogous to a reflected pulse on a string

4 Phase Changes Due To Reflection, cont. There is no phase change when the wave is reflected from a boundary leading to a medium of lower index of refraction There is no phase change when the wave is reflected from a boundary leading to a medium of lower index of refraction Analogous to a pulse in a string reflecting from a free support Analogous to a pulse in a string reflecting from a free support

5 Interference in Thin Films, 3 Ray 1 undergoes a phase change of 180° with respect to the incident ray Ray 1 undergoes a phase change of 180° with respect to the incident ray Ray 2, which is reflected from the lower surface, undergoes no phase change with respect to the incident wave Ray 2, which is reflected from the lower surface, undergoes no phase change with respect to the incident wave 180 phase change iiii

6 Interference in Thin Films, 4 Very Thin Films: Destructive interference (t<< λ) Very Thin Films: Destructive interference (t<< λ) Since the top reflected wave is inverted and the bottom one is not, the result is out of phase by 1/2 λ Since the top reflected wave is inverted and the bottom one is not, the result is out of phase by 1/2 λ Thin Films: Constructive Interference (t= λ/4) Thin Films: Constructive Interference (t= λ/4) Path difference is λ/2 Path difference is λ/2 Since the top reflected wave is inverted and the bottom one is not, the result is now in phase Since the top reflected wave is inverted and the bottom one is not, the result is now in phase

7 Interference in Thin Films, 5 Thin Films: Destructive interference (t= λ/2) Thin Films: Destructive interference (t= λ/2) Path difference is λ Path difference is λ Since the top reflected wave is inverted and the bottom one is not, the result is in now one half wavelength out of phase Since the top reflected wave is inverted and the bottom one is not, the result is in now one half wavelength out of phase The conditions are valid if the medium above the top surface is the same as the medium below the bottom surface The conditions are valid if the medium above the top surface is the same as the medium below the bottom surface

8 Interference in Thin Films - Coatings To form a nonreflecting coating a thin-film on glass with a minimum film thickness of λ /(4n 1 ) is required. To form a nonreflecting coating a thin-film on glass with a minimum film thickness of λ /(4n 1 ) is required. Destructive interference when: 2t=λ/(2n 1 ) Minimum thickness for nonreflecting surfaces: t=λ/(4n 1 )

9 Newtons Rings Another method for viewing interference is to place a planoconvex lens on top of a flat glass surface Another method for viewing interference is to place a planoconvex lens on top of a flat glass surface The air film between the glass surfaces varies in thickness from zero at the point of contact to some thickness t The air film between the glass surfaces varies in thickness from zero at the point of contact to some thickness t A pattern of light and dark rings is observed A pattern of light and dark rings is observed This rings are called Newtons Rings This rings are called Newtons Rings The particle model of light could not explain the origin of the rings The particle model of light could not explain the origin of the rings Newtons Rings can be used to test optical lenses Newtons Rings can be used to test optical lenses

10 Newtons rings, cont.

11 Ray 1 undergoes a phase change of 180 on reflection, whereas ray 2 undergoes no phase change

12 Problem Solving Strategy with Thin Films, 1 Identify the thin film causing the interference Identify the thin film causing the interference The type of interference – constructive or destructive – that occurs is determined by the phase relationship between the upper and lower surfaces The type of interference – constructive or destructive – that occurs is determined by the phase relationship between the upper and lower surfaces

13 Problem Solving with Thin Films, 2 Phase differences have two causes Phase differences have two causes differences in the distances traveled differences in the distances traveled phase changes occurring on reflection phase changes occurring on reflection Both must be considered when determining constructive or destructive interference Both must be considered when determining constructive or destructive interference The interference is constructive if the path difference is an integral multiple of λ and destructive if the path difference is an odd half multiple of λ The interference is constructive if the path difference is an integral multiple of λ and destructive if the path difference is an odd half multiple of λ The conditions are reversed if one of the waves undergoes a phase change on reflection The conditions are reversed if one of the waves undergoes a phase change on reflection


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