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Unit 13: EM Radiation and Waves
The Wave Nature of Light
Unit 13 Outline
• Waves Versus Particles
•EM Waves and EM Spectrum
•Scattering of Light by the Atmosphere
• The Visible Spectrum and Dispersion,
Rainbows
•Interference by Thin Films
•Polarization
•
Do Now
Double Slit Interference:
A pair of narrow, parallel slits separated by
0.250mm are illuminated by the green
component from a mercury vapor lamp (λ =
546.1nm). The interference pattern is
observed on a screen 1.20 m from the plane
of the parallel slits. Calculate the distance
from the central maximum to the first bright
fringe.
Given:
L=1.20m
m=1
Find: y-?
Solution:
θ=0.125°
md 310250.0
m910546
tanLybright
00218.010250.0
105461sin
3
9
dm
mLybright 00262.0)0021.0(20.1tan
Thin Film Interference
Wave Nature of Light
• In the early nineteenth century, Thomas
Young showed that the interference of
light passing through two slits produces an
interference pattern when projected on a
screen.
• Now we will investigate another example
of interference that provides further
evidence in support of the wavelike
behavior of light.
Thin Film Interference • The bright color reflected from soap
bubbles or streaks of color in a thin film of
oil resting upon a water puddle or concrete
driveway - thin film interference.
• Thin film interference provides another
evidence for the wave behavior of light.
• Thin film interference present only if the
film thickness is greater than a few
wavelengths.
Interference by Thin Films
• Fast ->slow phase
change
• Slow->fast no phase
change
• Reflected rays 1 and
2 are out of phase.
11 n
5.12 n
13n
Я R
Interference by Thin Films
• Reflected rays 1 and 2
are out of phase.
• Path length difference is
multiple of wavelength –
destructive interference.
• t- thickness of the film
• m=1,2,3…
• - wavelength in the
film
11 n
5.12 n
13n
Я R
t
nmt 2
n
Interference by Thin Films
- wavelength in air
- wavelength in
soap water
n – index of refraction
f – frequency (is fixed)
Destructive
interference (dark)
Constrictive
interference (bright)
f
cair
nair
f
vn
v
cn
n
airn
nmt air
2
nmt air
)2
1(2
24.8 Interference by Thin Films
The wavelength of the
light will be different
in the oil and the air,
and the reflections at
points A and B may or
may not involve
reflection.
24.8 Interference by Thin Films
A similar effect takes place when a shallowly
curved piece of glass is placed on a flat one.
When viewed from above, concentric circles
appear that are called Newton’s rings.
24.8 Interference by Thin Films
One can also create a thin film of air by creating
a wedge-shaped gap between two pieces of
glass.
24.8 Interference by Thin Films
Problem Solving: Interference
1. Interference occurs when two or more waves
arrive simultaneously at the same point in
space.
2. Constructive interference occurs when the
waves are in phase.
3. Destructive interference occurs when the
waves are out of phase.
4. An extra half-wavelength shift occurs when
light reflects from a medium with higher
refractive index.
Example
A soap bubble appears green (λ=540nm) at
the point of its front surface nearest the
viewer. What is the smallest thickness the
soap bubble could have? Assume n=1.35.
Solution: Phase change at only one surface
is equivalent to a path difference (m=0). 2
1
nmt air
)2
1(2 nm
nm
nt air 100
)35.1)(4(
540
4