LO9

1. Thin Film Interference!
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True or False!
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1. Constructive or destructive interference can occur at the surface of a thin film, depending on
the properties of the film.!
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2. Light will reflect from the boundary surface when there is a difference in the index of
refraction.!
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3. Variation in thickness in a film of soapy water results in different colours being dominant in
destructive interference in various regions of the film.!
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Matching!
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1. Thin film interference ____ 2. Hard reflection ____ 3. Soft reflection ____!
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a. Important in antireflection coatings in high quality optics and in energy-saving technologies
for windows.!
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b. Reflected light experiences no phase change!
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c. Reflected light experiences a 180 degrees phase change!
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Example!
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From 2011 Final exam:!
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Visible light (400-750 nm) is incident normally on a piece of glass (n= 1.5, thickness 3.0 cm) that
with a thin film of water (n=1.3, thickness 700 nm) on top. For which wavelength(s) will there be
no reflected light?!
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Answers!
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(True and False)!
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2. 1. F - Constructive or destructive interference can occur at the surface of a thin film, depending
on the thickness of the film and any phase changes that occur during reflection at the
surfaces of the film.!
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2. T!
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3. F - Variation in thickness in a film of soapy water results in different colours being dominant in
constructive interference in various regions of the film.!
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(Matching)!
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1. a 2. c 3. b!
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(Example)!
In this question, we use the formula 2t= [(m+0.5)λ]/n (destructive interference) as we want
destructive interference.!
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Rearranging the equation for λ:!
2nt= (m+0.5)λ!
λ= 2nt/(m+0.5)!
λ= (2*1.3 * 700)/(m+0.5)!
λ= 1820/(m+0.5)!
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For m=0, λ= 1820/(0+0.5)= 3640!
For m=1, λ= 1213!
For m= 2, λ= 728!
For m=3, 520!
For m= 4, 404!
For m=5, 331!
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Reject the values we get for m=0, m=2, and m≥5 and therefore our answers are!
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728 nm, 520 nm, 404 nm