Transcript Light PPT - Paso Robles High School

Light
•Wave Vs. Particles
•Electromagnetic Waves
•Frequency and Wavelength
•The Electromagnetic Spectrum
•Planck’s Constant
•Photo Electric Effect
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Light
Objectives:
I can describe the dual nature of light.
I can explain experimental evidence of the
wave and photon nature of light.
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decompressor
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Light: Introduction
Robert Hooke
In the 17th century, Isaac
Newton proposed the
“corpuscular theory” stating
that light is composed of
particles. Other scientists, like
Robert Hooke and Christian
Huygens, believed light to be
a wave.
Christian Huygens
Isaac Newton
Light: Introduction
Today we know that light behaves
as both a wave and as a particle.
Light undergoes interference and
diffraction, as all waves do, but
whenever light is emitted, it is
always done so in discreet packets
called photons. These photons
have momentum, but not mass.
Wave Vs. Particles
Light is an electromagnetic wave. As
light travels through space an electric
field and a magnetic field oscillate
perpendicular to the wave direction
and perpendicular to each other. A
light wave is transverse since each field
oscillates in a plane perpendicular to
the direction of the wave motion.
Light requires no medium!
Electromagnetic Waves
A positive charge has and
electric field shown by
electric field lines. When
the positive charge
oacillates it creates
oscilations in the electric
field surrounding the
charge.
Electromagnetic Waves
The charge also creates a
magnetic field that follows
the right hand rule. Light is
an electric field coupled with
a magnetic field. The two
fields oscillate together but in
different planes.
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Electromagnetic Waves
Light is an electric field coupled
with a magnetic field. The two
fields oscillate together but in
different planes.
Above is a set of 3-D coordinate
axes. The z -axis is vertical, the
y-axis is horizontal, and the x axis is coming out toward you.
Electromagnetic Waves (cont.)
The red wave represents an
oscillating electric field in the y-z
plane. It is a snapshot in time. At
the crests and troughs, the electric
field will exert the greatest force on
a charge, but in opposite
directions.
Electromagnetic Waves (cont.)
The blue wave represents an
oscillating magnetic field in the x-y
plane. Like the electric field, the
magnetic field is strongest at the
crests and troughs.
Electromagnetic Waves (cont.)
An electric and a magnetic field oscillating
together. The fields travel through space
together. They have the same period and
wavelength, but they oscillate in two
different planes that are perpendicular to
each other. The electric field, the magnetic
field, and the wave direction are all mutually
perpendicular.
Electromagnetic Waves (cont.)
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Frequency and Wavelength
The frequency of a light wave corresponds to the
color we see. The amplitude corresponds to
brightness.
Light
Sound
Frequency Color
Pitch
Amplitude Brightness Loudness
Red light has a frequency of over 400 trillion
Hertz. The frequency of violet light is even
higher—over 750 trillion Hz. Other types of
electromagnetic radiation, like X-rays, have even
higher frequencies, and some have lower
frequencies, like radio waves.
Frequency and Wavelength (cont.)
The wave formula is v =  f
(wave speed = wavelength 
frequency). Light has a constant
speed in a vacuum. Thus, the
bigger f is, the smaller  must
be. Red light has a wavelength of
700 nm. (1 nm = 1 nanometer =
10-9 m)
The Electromagnetic Spectrum
The electromagnetic spectrum covers a wide
range of wavelengths and photon energies.
Visible light ranges from 400 to 700
nanometers. Only a small portion of the
electromagnetic spectrum is visible to us.
The smaller the wavelength, the more energy
each photons of the light has.
Electromagnetic Spectrum (cont.)
Wavelengths other that visible light serve useful purposes:
Radio waves are very long (a few
centimeters to 6 football fields) and
can be used to send signals. These
signals are transmitted by radio
stations. They transmit information
and music via amplitude modulation
(AM) and frequency modulation (FM).
Microwaves (a few millimeters long)
are also used in communications.
Microwave ovens are great for
heating food since food is primarily
water, and microwaves have just the
right frequency to get water
molecules vibrating.
Electromagnetic Spectrum (cont.)
Infrared (micrometers in length) are
used in remote controls to change the
channel, and they are also radiated by
objects that are warmer than their
surrounding (like your body). They
make night vision equipment possible.
Ultraviolet light is harmful to our bodies
because its wavelength is so small.
Short wavelength mean high energy for
photons. UV causes our skin to tan and
burn. Fortunately, the ozone layer
blocks most UV radiation, but
prolonged exposure to the sun should
be avoided, since UV rays can cause
skin cancer. On the positive side UV
radiation helps people to produce their
own vitamin D.
Electromagnetic Spectrum (cont.)
X-rays are even more energetic, and
hence more dangerous, than UV
rays, but luckily they cannot
penetrate our ozone layer. They are
produced in space and of course are
used by doctors to get pictures of
your bones.
Gamma rays are the most energetic of
the light waves and little is known
about them other than they are very
harmful to living cells and are used by
doctors to kill certain cells and for
other operations. They are produced
in nuclear explosions. Like other high
energy rays, our atmosphere protects
us from gamma rays.
Electromagnetic Spectrum (cont.)
Astronomers have many different types of
telescopes at their disposal to observe the
universe in all parts of electromagnetic
spectrum. Some telescopes are groundbased; others
are space-based:
Quantum Mechanic--Background
Recall that a black body is an ideal absorber of all incident radiation. A hot
black body is also a perfect emitter--radiation is the result of its temperature,
and since none of this is absorbed, it is a perfect emitter of radiation. A black
body emits all wavelengths of light but not equally; there is always a
wavelength in which the radiation peaks. The hotter the black body, the smaller
the peak wavelength.
Quantum Mechanic--Background
Objects around you are cool, so their peak is in the
infrared. The sun is hot enough to peak in the visible
spectrum (all other wavelengths are emitted too but
at lower intensities).
In the late 19th century classical physics had
predicted something impossible: as the temperature
rises, the intensity of the peak radiation approaches
infinity (red dashed line). The theory did match
experimental data for large wavelengths but failed for
small ones. This was known as the “ultraviolet
catastrophe.”
Black Body Radiation
• Phet: Black Body Radiation Simulation
Planck’s Constant
In 1900 Max Planck came up with a revolutionary way to
resolve the problem by assuming that energy came in
discrete amounts (quanta). This was the beginning of
quantum mechanics. Each quantum of light is called a
photon, and its energy is given by E = h f, where f is the
frequency of the radiation and h is the constant of
proportionality called Plank’s constant. The formula states
that higher frequency light has proportionally more energy
per photon. Einstein lent credence to Plank’s ideas by
explaining the photoelectric effect in a similar manner.
Robert Millikan did a series of experiments involving the
photoelectric effect and calculated the constant:
h = 6.626  10-34 J s.
Max Plank
Planck’s Constant
Before Planck light was considered to be a
wave. Today we know it can be interpreted
as either a particle or a wave. As a wave,
bright light can be explained as a large
amplitude in the electric and magnetic fields.
As a particle, bright light would be explained
by a large number of photons.
Wave Particle Duality
Mechanical Universe:
Wave Particle Duality
Phet: Fourier
Transformation Making
Waves
A great follow up on this
presentation
would be to watch the
Mechanical Universe: Wave
Particle Duality that is part of
this Bundle.
28
Credits
Numerous Images as well as information were obtained from the following sources:
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Credits
(cont.)
http://www.worldlights.com/world/candela.html
http://www.westsidesystems.com/rays.html
http://www.electro-optical.com/bb_rad/emspect.htm
http://violet.pha.jhu.edu/~wpb/spectroscopy/em_spec.html
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