Physics 202, Lecture 27 Today’s Topics ! Wave Nature of Waves: Interference ! Breakdown of ray approximation ! Huygen’s principle ! Light as Waves ! Two-Slit Interference ! Thin Film Interference ! Change of Phase at Boundaries ! Exercise on Thin Film Interference ! Exercise on Non Reflective Coating 1 Reminder: Light and Optics " Nature of Lights # Lights as rays # Lights as EM waves: f, !, ", v, A, interference … # Lights as group of photons " Optics: Physics of lights # Geometric Optics: Treat light as rays (Ch. 31,32) ! Ray approximation. # Wave Optics: Wave properties becomes important Interference, diffraction…(Ch. 33,34) 2 Ray Approximation " When the wavelength of the light is much smaller than the size of the optical objects it encounters, it can be treated as (colored) rays. Ray approximation is valid when !<<d Ray approximation is not valid near the gap when !~d. OK elsewhere 3 Huygens’ Principle " Every point on a wave front can be considered as a secondary source of waves that spread out in the forward direction. The new wave is the result of the superposition of these secondary waves 4
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Physics 202, Lecture 27 Reminder: Light and Optics · Physics 202, Lecture 27 Today’s Topics ! Wave Nature of Waves: Interference ! Breakdown of ray approximation ! Huygen’s principle
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Physics 202, Lecture 27 Today’s Topics
! Wave Nature of Waves: Interference ! Breakdown of ray approximation ! Huygen’s principle ! Light as Waves ! Two-Slit Interference
! Thin Film Interference ! Change of Phase at Boundaries ! Exercise on Thin Film Interference ! Exercise on Non Reflective Coating
1
Reminder: Light and Optics " Nature of Lights
# Lights as rays # Lights as EM waves: f, !, ", v, A, interference … # Lights as group of photons
" Optics: Physics of lights # Geometric Optics: Treat light as rays (Ch. 31,32) ! Ray approximation.
# Wave Optics: Wave properties becomes important Interference, diffraction…(Ch. 33,34)
2
Ray Approximation " When the wavelength of the light is much smaller than
the size of the optical objects it encounters, it can be treated as (colored) rays.
Ray approximation is valid when !<<d
Ray approximation is not valid near the gap when !~d. OK elsewhere 3
Huygens’ Principle " Every point on a wave front can be considered as a
secondary source of waves that spread out in the forward direction. The new wave is the result of the superposition of these secondary waves
" When a light traveling in medium 1 of n1 is reaches at a boundary with medium 2 of n2: # The reflected light has a 180o(%) phase shift if n1<n2 # There is no phase change for reflected light if n1>n2
# In any change, no phase shift for refracted light n1<n2:
180o(%) phase shift n1>n2:
0o phase shift
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Thin Film Interference " Thin film splits light ! split lights then interfere
lights 3,4 also interfere
lights 1,2 interfere lights 1,2 interfere
n!!n=!/n
$"12 ~ 2%/!n (2t) + %'
phase change %'for light 1
$"34 ~ 2%/!n (2t) '
Quiz: Constructive/destructive
Conditions?
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Exercise: Non Reflective Coating " Determine the minimum thickness (t) of SiO coating
so a light of 550nm is non-reflective at the surface.
t
Solution (see board): Non “reflective” !1 and 2 cancel each other (destructive interference)