Download Chapter 33: Electromagnetic Waves

Physics 4C
An Electromagnetic Wave
Chapter 33:
Electromagnetic Waves
The EM Spectrum
Polarization
Refraction
Apparent Depth
Chromatic Dispersion
Rainbows
Green Flash
Total Internal Reflection
The Visible Spectrum
⇒ An electromagnetic wave is a transverse
wave consisting of oscillating electric and
magnetic fields.
The Electromagnetic Spectrum
⇒ Imagine if you can the world view of little
creatures who could see only a very tiny portion
of the visible spectrum, creatures who are color
blind to all other parts. Their world view would
be very limited.
⇒ Guess what? We are like those little creatures,
in that the spectrum of colors we can see are a tiny
portion of the electromagnetic spectrum!
⇒ The electromagnetic spectrum consists of
electromagnetic waves of all frequencies.
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The Electromagnetic Spectrum
Polarization
⇒ For an EM wave, the
plane containing the
electric field is called the
plane of oscillation.
⇒ Visible light makes up a very small portion (less
than one millionth of 1%) of the measured
electromagnetic spectrum.
Polarization
⇒ Polarization of EM
wave is represented by
showing a “head-on” view
of plane of oscillation.
Polarization
⇒ Transverse wave
polarized in vertical plane
⇒ Transverse wave
polarized in horizontal
plane
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Refraction of Light
Refraction of Light
⇒ To understand why light
bends when passing from
one transparent material to
another, imagine a cartwheel
rolling from a sidewalk onto
grass.
Refraction: The bending of light when it passes
from one transparent material to another.
Refraction of Light
⇒ Because the wheels roll slower in the grass,
the left wheel will slow down first causing the
cartwheel to change directions.
Apparent Depth
⇒ Similarly, when light hits
a transparent material like
water where the speed of
light is slower, one side of
the wavefront slows down
before the other side.
⇒ Because light travels slower in the water, the
left side of the wavefront will slow down first,
causing the light to bend.
⇒ Because of refraction, the apparent depth of
objects in water is less than their actual depth.
(Objects are deeper than they appear.)
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Chromatic Dispersion
Place a glass test tube in water and you
can see the tube. Place it in clear soybean
oil, and you may not be able to see it.
What does this tell you about the speed of
light in the oil and in the glass?
⇒ The index of refraction (n) in a transparent material
depends upon the frequency of the light.
⇒ In general, n is greater for higher frequencies (violet)
than for lower frequencies (red). Therefore violet light is
bent more than red light.
⇒ The speeds in both glass and soybean
oil are the same, so there is no reflection
or refraction between the glass and oil.
Chromatic Dispersion
Rainbows
⇒ Dispersion occurs
when sunlight hits a
water droplet. This is
what causes rainbows.
⇒ The incoming sunlight enters the water droplet and different
frequencies are bent (refracted) by different amounts.
Chromatic Dispersion: The separation of
light into different colors arranged according
to their frequency.
⇒ When this refracted light hits the back of the water droplet,
some of it is transmitted back into the air (not shown) and some of
it is reflected back into the water droplet.
⇒ The light reflected from the back of the water droplet then gets
bent again (refracted) as it leaves the water droplet.
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Rainbows
Rainbows
⇒ Although each water drop disperses a full spectrum of
⇒ The different colors of a rainbow are the result of the
dispersion of sunlight by millions of water droplets that
act like tiny prisms.
colors, a person can only see one color of light from each
drop.
⇒ Because of this, two people in different positions do
not see the “same” rainbow.
Rainbows
Rainbows
⇒ Rainbows are
⇒ Sunlight can sometimes be
actually complete circles,
however the ground cuts
off the bottom half.
reflected twice within a single
water droplet.
Single reflection –
single rainbow.
Double reflection –
double rainbow.
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Rainbows
Green Flash
⇒ Sometimes a momentary flash (lasting 1
or 2 seconds) of green light can be seen
when the sun sets.
Green Flash
Green Flash
⇒ A simplified explanation of green
flashes is that our atmosphere acts like
an upside down prism. White light is
dispersed with blue on top, green near
the top, and red on the bottom.
⇒ At the moment of setting, the red is cut off by the
Earth, the blue is removed because of atmospheric
scattering, and the green survives to give the green flash.
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Blue Flash
Total Internal Reflection
⇒ Under ideal atmospheric conditions, it is possible to
see a blue flash.
Total Internal Reflection
⇒ When light tries to go from one transparent material into
another transparent material where the speed of light is
greater, the light will get totally internally reflected (no
light gets refracted out) if the angle the light hits at is greater
than a certain angle called the critical angle.
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