Download The Replicating Wave The Nature of Light: What causes Electric and

Does a changing Electric Field produce a magnetic
field?
The Replicating Wave
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Maxwell’s equations basically say that a changing magnetic field
produces a changing electric field which in turn produces a
changing magnetic field and so on.
Once you get this going these two kinds of fields would just
continue replicating themselves, moving thru empty space
where both J and are 0 , he then calculated the speed at
which these would propagate to be the know speed of light c.
Implication is that light must just be an electromagnetic wave.
First Transatlantic
communication in 1901
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The Nature of Light: What causes
Electric and Magnetic Fields?
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C= 3 x 108 m/s
Shake a charged rod and you generate
electromagnetic waves. Shake slowly
and you generate radio waves, shake
quickly and you generate ( a million
times per sec) and you can see them.
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Wiggling one charge causes the field
lines attached to it to wiggle, and
after a time the other charge starts to
wiggle!
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Electromagnetic Waves
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Electromagnetic Waves are
Transverse Waves
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Polarization
• The and fields are
perpendicular to each
other
• Both fields are
perpendicular to the
direction of motion
– Therefore, em waves
are transverse waves
Demo: Polaroid
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Charges and Fields, Summary
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Electromagnetic Spectrum
• Stationary charges produce only electric
fields
• Charges in uniform motion (constant
velocity) produce electric and magnetic
fields
• Charges that are accelerated (or oscillate)
produce electric and magnetic fields and
electromagnetic waves
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The EM Spectrum
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How are EM
waves
produced?
Is it correct to say that in every case,
without exception, any radio wave
travels faster than any sound wave?
In glass, Red is scattered the most, violet
the least
1. Yes
2. No
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Check yourself
1.Is it correct to say that a radio
wave is a low-frequency light
wave?
2. Is a radio wave also a sound
wave?
3.What is the wavelength of TV
station broadcasting at 100
MGHertz?
Just as sound can force a sound receiver into
vibration, alight wave can force electrons in
materials into vibration. For visible light this high
frequency will only affect electrons (which basically
have no mass) in the material.
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Speed of Light: Olaus Roemer1675
Earth
Io
Sun
Jupiter
Period of Io is 42.5 hr. period was
shorter by 1000 sec when earth was
moving forward than when it was
moving away.
Extra Distance= 300,000,000,000 m
c=
Extra Time
1000
sec
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Transparent materials
A wave of visible light incident upon a pane of glass
sets up in atoms' vibrations that produce a chain of
absorptions and re-emissions, which pass the light
energy through the material and out the other side.
Because of the time delay between absorptions and reemissions, the light travels through the glass more
slowly than through empty space.
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Light and transparent materials
At lower wave frequencies, such as those of visible
light, electrons in the glass atoms are forced into
vibration, but at less amplitude. The atoms hold the
energy for less time, with less chance of collision with
neighboring atoms, and less energy transformed to
heat. The energy of vibrating electrons is re-emitted as
light. Glass is transparent to all the frequencies of
visible light. The frequency of the re-emitted light that is
passed from atom to atom is identical to the frequency
of the light that produced the vibration in the first place.
However, there is a slight time delay between
absorption and re-emission.
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Only energy having the frequency of blue light is
transmitted; energy of the other frequencies is
absorbed and warms the glass.
Check yourself
1. Why is glass transparent to visible light but
opaque to ultraviolet and infrared?
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Check yourself
1. When red light shines on a red rose, why do the
leaves become warmer than the petals?
Dispersion
2. When green light shines on a rose, why do the petals
look black?
Usually a material absorbs light of some frequencies and
reflects the rest. If a material absorbs most of the visible light
that is incident upon it but reflects red, for example, it appears
red. That's why the petals of a red rose are red and the stem
green. The atoms of the petals absorb all visible light except
red, which they reflect; the stem absorbs all except green,
which it reflects. An object that reflects light of all the visible
frequencies, as the white part of this page does, is the same
color as the light that shines on it. If a material absorbs all the
light that shines on it, it reflects none and is black.
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White Light
White light from the sun
is a composite of all the
visible frequencies is
easily demonstrated by
passing sunlight through
a prism and observing
the rainbow-colored
spectrum. The intensity
of light from the sun
varies with frequency,
being most intense in
the yellow-green part of
the spectrum.
The radiation curve of sunlight is a graph of brightness
versus frequency. Sunlight is brightest in the yellow-green
region, in the middle of the visible range.
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Reflection of Light
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Mixing Colors
Most of the objects around us reflect rather than emit
light. They reflect only part of the light that is incident
upon them, the part that gives them their color
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Why is the sky blue
The tinier the
particle, the greater
amount of higherfrequency light it
will re-emit
Why sunsets are red
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A sunbeam must
travel through
more atmosphere
at sunset than at
noon. As a result,
more blue is
scattered from the
beam at sunset
than at noon. By
the time a beam of
initially white
light gets to the
ground, only light
of the lower
frequencies
survives to
produce a red
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sunset.
The spaceship at S wishes to touch the surface of the giant
planet and proceed to point X in the shortest distance
possible. To what point P on the planet surface should the
spaceship travel?
A beam of light falls on an atom and increases the vibrational
motion of electrons in the atom. The vibrating electrons re-emit
the light in various directions. Light is scattered. Of the visible
frequencies of sunlight, violet is scattered the most by nitrogen
and oxygen in the atmosphere, followed by blue, green, yellow,
orange, and red, in that order. Red is scattered only a tenth as
much as violet. Although violet light is scattered more than
blue, our eyes are not very sensitive to violet light. Therefore
the blue scattered light is what predominates in our vision, and
we see a blue sky!
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Fermat’s Principal
This idea was formulated by the French scientist Pierre
Fermat in about 1650, and it is called Fermat's principle
of least time. His idea is this: Out of all possible paths
that light might take to get from one point to another, it
takes the path that requires the shortest time .
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Reflection
1. Point a
2. Point b
3. Point c
4. Actually, all yield the same total distance.
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Why?
Paper
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Law of reflection
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Plane Mirrors
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In order that you are able to see a full-length view of
yourself, the minimum size for a plane mirror must be
In order that you are able to see a full-length view of
yourself, the minimum size for a plane mirror must be
1. one-quarter your height.
2. one-half your height.
3. three-quarters your
height.
4. your full height.
5. … depends on your
distance.
1. one-quarter your height.
2. one-half your height.
3. three-quarters your
height.
4. your full height.
5. … depends on your
distance.
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Curved Mirrors
Curved Mirrors
a ) The virtual image formed by a convex mirror (a mirror
that curves outward) is smaller and closer to the mirror than
the object. ( b ) When the object is close to a concave mirror
(a mirror that curves inward like a “cave”), the virtual image is
larger and farther away than the object. In either case the
law of reflection holds for each ray.
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Curved Mirror
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Object Located Beyond the
Center of Curvature
Focal Length Shown by Parallel
Rays
•A parallel ray goes thru focal point after hitting miror
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•A ray thru focal point emerges parallel
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Object Located Between the
Center of Curvature and the
Focal Point
Object Located At the
Center of Curvature
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Object Located Between the
Focal Point and the Mirror
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Curved Mirrors
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Why we see the penny
A Straw in water looks bent
Demo: Penny in bowl
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Refraction of Light
Why light bends
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Why Light Bends Demo
Note that when the pulse reaches the end of the
medium, a portion of its energy is transmitted into
the more dense medium (in the form of a transmitted
pulse), a portion of its energy remains in the less
dense medium (in the form of a reflected pulse).
Second, observe that the transmitted pulse has a
smaller speed and a smaller wavelength than the
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incident pulse.
Fermat’s Principal
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Refraction
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Snell’s Law
Imagine that you are a lifeguard
at a beach and you spot a person
in distress in the water. We show
the relative positions of you, the
shoreline, and the person in
distress in Figure 28.13 . You are
at point A, and the person is at
point B. You can run faster than
you can swim. Should you travel
in a straight line to get to B?
The angle of incidence is larger than the angle of
refraction by an amount that depends on the relative
speeds of light in air and in water.
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Check Yourself
Suppose our lifeguard in the preceding example were a
seal instead of a human being. How would its path of
least time from A to B differ?
Check your answer
The seal can swim
faster than it can run,
and its path would
bend as shown;
likewise with light
emerging from the
bottom of a piece of
glass into air.
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Index of Refraction
Speed of light in vacuum
n= Speed of light in medium
n=index of
refraction
For most
transparent
materials n is
between 1-2
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Light rays bend as they pass from air
into water at a non–90 degree angle.
This is refraction. Which quantity
doesn’t change when light refracts?
Light rays bend as they pass from air
into water at a non–90 degree angle.
This is refraction. Which quantity
doesn’t change when light refracts?
1. Average speed of light
2. Material’s index
of refraction
3. Frequency of light
4. Wavelength of light
1. Average speed of light
2. Material’s index
of refraction
3. Frequency of light
4. Wavelength of light
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Physics of Rainbow
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When your eye is located between the sun (not
shown off to the left) and a water-drop region,
the rainbow you see is the edge of a threedimensional cone that extends through the
water-drop region.
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When light slows down in going from one medium to
another, like going from air to water, it refracts toward the
normal. When it speeds up in traveling from one medium to
another, like going from water to air, it refracts away from
the normal.
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Refraction through glass.
Although dashed line AC is
the shortest path, light
goes a slightly longer path
through the air from A to a,
than a shorter path through
the glass to c, and then to
C. The emerging light is
displaced but parallel to
the incident light.
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Atmospheric Refraction
Because of atmospheric refraction, when the sun is
near the horizon it appears higher in the sky
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Mirage
Light from the sky picks up speed in air near the ground
because that air is warmer and less dense than the air
above. When the light grazes the surface and bends
upward, the observer sees a mirage.
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Fish in Water
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Jose wishes to “spear” a fish with a
laser. In order to make a direct hit, he
should aim the laser beam
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Jose wishes to “spear” a fish with a
laser. In order to make a direct hit, he
should aim the laser beam
1. above
2. below
3. directly at
the observed fish.
1. above
2. below
3. directly at
the observed fish.
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Critical Angle
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Total Internal Reflection
Light emitted in the water is partly refracted and partly
reflected at the surface. The blue dashes show the
direction of light and the length of the arrows indicates
the proportions refracted and reflected. Beyond the
critical angle the beam is totally internally reflected.
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An observer under water sees a circle of light at the still
surface. Beyond a cone of 96｡ (twice the critical angle)
an observer sees a reflection of the water interior or
bottom.
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Fiber Optics
Total internal reflection occurs in glass surrounded by air,
for the speed of light in glass is less than in air. The critical
angle for glass is about 43｡, depending on the type of glass.
So light in the glass that is incident at angles greater than
43｡ to the surface is totally internally reflected. No light
escapes beyond this angle; instead, all of it is reflected back
into the glass
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Focal Length of a Converging
Lens
Focal Length of a Diverging Lens
• The parallel rays pass through the lens and
converge at the focal point
• The parallel rays can come from the left or right of
the lens
• The parallel rays diverge after passing through the
diverging lens
• The focal point is the point where the rays appear
to have originated
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Diverging lens
•A parallel ray, when extrapolated backwards goes thru fp
•A ray heading at fp point on other side, comes out parallel
•A ray going toward C, goes undeflected
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Optical Bench
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The Eye
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The Eye
• The normal eye focuses
light and produces a sharp
image
• Essential parts of the eye
– Cornea – light passes
through this transparent
structure
– Aqueous Humor – clear
liquid behind the cornea
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The Eye – Near Point
Far Point
• distance farthest from the eye at which
an object can be clearly focused is
called the far point. If your far point is
not very far away, you are nearsighted
and you need corrective lenses.
• The near point is the closest distance for which the
lens can accommodate to focus light on the retina
– Typically at age 10, this is about 18 cm
– It increases with age
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Conditions of the Eye
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Farsightedness
• Eyes may suffer a mismatch between the focusing power
of the lens-cornea system and the length of the eye
• Eyes may be
– Farsighted
• Light rays reach the retina before they converge to form an image
– Nearsighted
• Also called hyperopia
• The image focuses behind the retina
• Can usually see far away objects clearly, but not
nearby objects
• Person can focus on nearby objects but not those far away
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Correcting Farsightedness
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Compound Microscope
• A converging lens placed in front of the eye can correct the
condition
• The lens refracts the incoming rays more toward the
principle axis before entering the eye
– This allows the rays to converge and focus on the retina
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Telescopes
Refracting Telescope
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• Two fundamental types of telescopes
– Refracting telescope uses a combination of lenses to form an
image
•
– Reflecting telescope uses a curved mirror and a lens to form an
image
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• Telescopes can be analyzed by considering them to be two
optical elements in a row
– The image of the first element becomes the object of the second
element
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The two lenses are arranged so that the
objective forms a real, inverted image of
a distant object
The image is near the focal point of the
eyepiece
The two lenses are separated by the
distance ƒ o + ƒ e which corresponds to
the length of the tube
The eyepiece forms an enlarged,
inverted image of the first image
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Why we think light is a wave:
Diffraction
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Two Slit Interference
Amount of bending depends on wavelength and slit width
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Spectrum
The wavelength of laser light
δ = r2 – r1 = d tan θ = d y/L
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Spectra of Helium Gas
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Absorption Lines
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Doppler Shift for light
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The Hubble Graph
The Age of the Universe
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