Transcript Electromagnetic Waves - Natomas Unified School District

The electromagnetic spectrum
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Wave-particle duality of light.
Huygen’s diffraction experiment—wave
behavior.
Einstein’s photovoltaic effect—particle
behavior.
These chapters only look at wave
behavior.
Light experiments
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Olaus Roemer—observed difference in
times of Io’s revolution of Jupiter.
Time difference of 22 minutes between
shortest and longest appearance.
Christian Huygen’s—knew Earth orbit
distance and size of Sun, calculated speed
of light.
Light experiments
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Albert Michelson—used mirror 35 km away
and revolving octagonal mirror to calculate
speed of light.
Used 35 km distant mirror on mountainside
and rotating mirror.
Adjust speed on mirror until he could see
reflected light and calculated speed as
299,920 km/sec or 3 x 108 m/s.
Electromagnetic Waves
What Are Electromagnetic
Waves?
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Waves that propagate (move) through
space
Can move in vacuum
Carry energy
Move at 3.0 x 108 m/s in vacuum
Consist of oscillating (varying) electric
and magnetic fields at right angles
Examples
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Visible Light
Infra Red
Ultra Violet
Radio waves
Microwaves
X-rays
Gamma Rays
In order of increasing wavelength
Visible Light Portion
In what order?
Increasing frequency
Why EM Waves exist
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Composed of electric
field which produces
magnetic field, which
produces the electric
field.
They create each other
and move on forever.
Time to Calculate Wavelength
and Frequency
Use c = lf
wave speed equation
Problem
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Find the wavelength of a 93.3 MHz FM
radio wave
l = c/f
= 3.00 x 108 m/s / 93.3 x 106 Hz
= 3.22 m
Problem
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Find the frequency of a 4 x 10-8 meter
long wave.
Find the wavelength of a wave with a
frequency of 5 x 109 Hz.
Transparent materials
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Visible light has a frequency range that
matches that of clear glass.
When visible light hits the glass,
electrons in the glass vibrate at their
resonant frequency, which allows light to
pass.
Higher frequency UV light causes very
fast vibration and loss of energy as heat.
Transparent materials
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Lower frequency infrared light produces
less vibration, which produces fewer
collisions than UV.
This allows some energy to get through
the glass allowing heat in the window.
Opaque materials
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Opaque materials absorb light and
become warmer.
Metals have free electrons on their
surfaces and these may reflect some of
the light that shines on them.
Shadows
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Umbra—area of complete shadow.
Penumbra—area of partial shadow.
Sharp shadows occur for small sources
of light nearby, or large sources further
away.
What causes these partial shadows?
Polarization
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A polarizing filter allows only one
orientation of light through.
Crossing two polarizing filters allows no
light through.
What effect does a third filter have?
Why?
Reflection
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Mirrors reflect light.
Water and glass also reflect light.
They only reflect a significant amount
when light shines at a low angle.
The critical angle is the angle where light
begin to completely reflect back from
glass or water.
Reflection
Creating an image
Reflection
Diffuse reflection
Concave mirrors
Convex mirrors
Refraction
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Light refracts, or bends, when it passes
between two media having different
indices of refraction-n.
Whether light refracts toward or away
from normal line depends on the index of
refraction of the two media.
Refraction and n
Snell’s law
Common indices of refraction
SUBSTANCE
INDEX OF REFRACTION
AIR
1.00
WATER
1.33
QUARTZ
1.46
GLASS
1.52
ZIRCONIUM
1.92
DIAMOND
2.42
Using Snell’s law
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Example: Light shines from air to water
at an incident angle of 400. Calculate the
refracted angle.
Practice: Light shines from air to
diamond at an incident angle of 350.
Calculate the refracted angle.
Using Snell’s law
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Practice: Calculate the 3 angles for the
situation below;
AIR
400
GLASS
ZIRCONIUM
AIR
Dispersion
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Dispersion occurs due to the fact that
longer wavelengths bend more than
shorter wavelengths on refraction.
Dispersion and rainbows
Dispersion and rainbows
Dispersion in diamonds
Total internal reflection
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Total internal reflection occurs in fiber
optic cables used in communication.
Diffraction
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Term given to bending of light by any
method other than refraction and
reflection.
Diffraction gratings bend light around
very narrow gaps.
They allow us to study composition of
the light.