Download waves

The Godfather is Released (1972)
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Reading for Monday
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HOMEWORK – DUE Monday 3/20/17
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WS 11 (Worksheet): (from course website)
Lab Today/Tomorrow
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WS 10 (Worksheet): (from course website)
CH 6 Extra Credit
HOMEWORK – DUE Wednesday 3/22/17
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Chapter 8 sections 1-2
EXP 9
Collect EXP 10 data
Lab Wednesday/Thursday
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Finish EXP 10
The Nature of Light: Its Wave Nature
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Light is a form of electromagnetic radiation
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made of perpendicular waves, one for the electric field and one for the
magnetic field
The Nature of Light: Its Wave Nature
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Light is a form of electromagnetic radiation
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made of perpendicular waves, one for the electric field and one for the
magnetic field
All electromagnetic waves move through space at the same, constant speed
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2.998 x 108 m/s in a vacuum = the speed of light, c
Characterizing Waves
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The amplitude is the height of the wave
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the distance from node to crest or node to trough
Characterizing Waves
Node
Characterizing Waves
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The amplitude is the height of the wave
the distance from node to crest or node to trough
 the amplitude is a measure of how intense the light is – the larger the
amplitude, the brighter the light
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Characterizing Waves
Characterizing Waves
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The wavelength (l) is a measure of the distance covered by the wave
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the distance from one crest to the next (or the distance from one trough to
the next, or the distance between alternate nodes)
Characterizing Waves
Node
Characterizing Waves
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The wavelength (l) is a measure of the distance covered by the wave
the distance from one crest to the next (or the distance from one trough to
the next, or the distance between alternate nodes)
 For visible light, the wavelength is related to the color of light
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Characterizing Waves
Characterizing Waves
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The frequency (n) is the number of waves that pass a point in a
given period of time
the number of waves = number of cycles
 units are hertz (Hz) or cycles/second = s−1
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1
Hz = 1 s−1
LIGHT!!!
wavelength and energy are INVERSELY proportional
wavelength
energy
LIGHT!!!
wavelength and energy are INVERSELY proportional
wavelength
energy
LIGHT!!!
energy and frequency are DIRECTLY proportional
energy
frequency
Wavelength and Frequency
 Wavelength
and frequency of electromagnetic waves are
inversely proportional
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because the speed of light is constant, if we know wavelength we can find
the frequency, and vice versa
c  nl
Calculate the wavelength of red light (nm) with a frequency of 4.62x1014 s−1
2.998 108 m
1s
1 nm



1s
4.621014 1109 m
649 nm
Calculate the wavelength (m) of a radio signal with a frequency of 106.5 MHz
2.998 108 m 1 Hz 1 MHz
1
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


6

1
1s
110 Hz 106.5 MHz
1s
2.815 m
Color
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The color of light is determined by its wavelength or
frequency
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White light is a mixture of all the colors of visible light
a spectrum
 RedOrangeYellowGreenBlueViolet
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When an object absorbs some of the wavelengths of
white light and reflects others, it appears colored
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the observed color is predominantly the colors reflected
Types of Electromagnetic Radiation
• Electromagnetic waves are classified by their wavelength
Radio waves = l > 0.01 m
Microwaves = 1 104 m < l < 1 102 m
Infrared (IR)
far IR = 1 105 m < l < 1104 m
middle IR = 1 106 m < l < 1105 m
near IR = 1 107 m < l < 1106 m
Visible light = 4  107 m < l < 8 107 m
ROY G. BIV
Ultraviolet (UV)
near UV = 2  107 m < l < 4 107 m
far UV = 1 108 m < l < 2  107 m
X-rays = 1 10
10
8
m < l < 110 m
Gamma rays = l < 11010 m
low frequency
and energy
high frequency
and energy
Electromagnetic Spectrum
Interference
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The interaction between waves is called interference
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When waves interact so that they add to make a larger wave it is
called constructive interference
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waves are in-phase
Interference
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The interaction between waves is called interference
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When waves interact so they cancel each other it is called
destructive interference
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waves are out-of-phase
Diffraction
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When traveling waves encounter an obstacle or opening in a barrier
that is about the same size as the wavelength, they bend around it –
this is called diffraction
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Traveling particles do not diffract
2-Slit Interference
The diffraction of light
through two slits
separated by a distance
comparable to the
wavelength results in an
interference pattern of
the diffracted waves
An interference pattern
is a characteristic of all
light waves
https://www.youtube.com/watch?v=hRFQd_fkzws
The Photoelectric Effect
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Many metals emit electrons when a light shines on them.
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called the photoelectric effect
The Photoelectric Effect
The Photoelectric Effect
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Many metals emit electrons when a light shines on them.
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called the photoelectric effect
Classic wave theory said this effect was due to the light energy being
transferred to the electron.
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The energy of a wave is directly proportional to its amplitude and its
frequency
If the wavelength of light is made shorter, more electrons should be ejected
 Light waves’ intensity made brighter, more electrons should be ejected
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Predicts that if a dim light were used there would be a lag time before
electrons were emitted to give the electrons time to absorb enough energy
The Photoelectric Effect: The Problem
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Experiments showed that a minimum frequency was needed before
electrons would be emitted
called the threshold frequency
 no dependence on intensity
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It was observed that high-frequency light from a dim source caused
electron emission without any lag time
Einstein’s Explanation
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Einstein proposed that the light energy was delivered to the atoms in
packets, called quanta or photons
The energy of a photon of light is directly proportional to its frequency and
inversely to wavelength
 the proportionality constant is called Planck’s Constant, (h) and has the
value 6.6261034 J s
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photon
E  hn 
hc
l
Calculate the number of photons in a laser pulse with wavelength 337
nm and total energy 3.83 mJ
1 photon
1103 J
1s
1109 m

 3.83 m J 

 3.37  102 nm
34
8
6.626 10 J s
1 mJ
3.00 10 m
1 nm
6.49x1015 photons
What is the frequency of radiation required to supply 1.0 x 102 J of
energy from 8.5 x 1027 photons?
1 photon
1.0 102 J
1


6.626 1034 J s
1
8.5 1027 photon
1.8x107 s-1 or 1.8x107 Hz or 18 MHz
Ejected Electrons
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One photon at the threshold frequency gives the electron just enough
energy for it to escape the atom
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binding energy, f
When irradiated with a shorter wavelength photon, the electron
absorbs more energy than is necessary to escape
 This excess energy becomes kinetic energy of the ejected electron
Kinetic Energy = Ephoton – Ebinding
KE = hn − f
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Suppose a metal will eject electrons from its surface when struck by yellow light.
What will happen if the surface is struck with ultraviolet light?
1. No electrons would be ejected.
2. Electrons would be ejected, and they would have the same kinetic
energy as those ejected by yellow light.
3. Electrons would be ejected, and they would have greater kinetic energy
than those ejected by yellow light.
4. Electrons would be ejected, and they would have lower kinetic energy
than those ejected by yellow light.