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I Love Lucy Airs for First Time (1951) READING: reread chapter 7 HOMEWORK – DUE TUESDAY 10/20/15
HW-BW 7.1 (Bookwork) CH 7 #’s 5, 7-12 all, 14, 15, 20, 21, 24, 28-31 all, 34 HW-WS 12 (Worksheet) (from course website)
HOMEWORK – DUE THURSDAY 10/22/15 HW-BW 7.2 (Bookwork) CH 7 #’s 39, 42, 48-52 all, 55-60 all, 64, 69, 71, 72,
78, 90 HW-WS 13 (Worksheet) (from course website)
Lab Next Monday/Tuesday – EXP 9
Prelab
Next Wednesday/Thursday – EXP 10
The Nature of Light: Its Wave NatureLight is a form of electromagnetic radiation
made of perpendicular waves, one for the electric field and one for the magnetic field
Light is a form of electromagnetic radiationmade of perpendicular waves, one for the electric field
and one for the magnetic fieldAll electromagnetic waves move through space at
the same, constant speed2.998 x 108 m/s in a vacuum = the speed of light, c
The Nature of Light: Its Wave Nature
Characterizing WavesThe amplitude is the height of the wave
the distance from node to crest or node to trough
Characterizing Waves
Node
Characterizing WavesThe amplitude is the height of the wave
the distance from node to crest or node to troughthe amplitude is a measure of how intense the light is –
the larger the amplitude, the brighter the light
Characterizing Waves
Characterizing WavesThe wavelength (l) is a measure of the distance
covered by the wavethe 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 WavesThe wavelength (l) is a measure of the distance
covered by the wavethe 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
Characterizing Waves
The frequency (n) is the number of waves that pass a point in a given period of timethe number of waves = number of cyclesunits are hertz (Hz) or cycles/second = s−1
1 Hz = 1 s−1
Characterizing Waves
LIGHT!!!wavelength and frequency are INVERSLY proportional
wavelength frequency
LIGHT!!!wavelength and energy are INVERSLY proportional
wavelength energy
LIGHT!!!energy and frequency are DIRECTLY proportional
energy frequency
Wavelength and FrequencyWavelength and frequency of electromagnetic
waves are inversely proportional because the speed of light is constant, if we know
wavelength we can find the frequency, and vice versa
c
Calculate the wavelength of red light (nm) with a frequency of 4.62 x 1014 s−1
649 nm
8
14 92.998 10 1 1
1 4.62 10 1 10
m n s m
s m
Calculate the wavelength (m) of a radio signal with a frequency of 106.5 MHz
2.815 m
8 12.998 101 ms
Hz11 s
1 MHz61 10 Hz
1106.5
MHz
Color The color of light is determined by its
wavelength or frequency White light is a mixture of all the
colors of visible light a spectrum RedOrangeYellowGreenBlueViolet
When an object absorbs some of the wavelengths of white light and reflects others, it appears colored the observed color is predominantly
the colors reflected
Types of Electromagnetic Radiation
low frequency and energy
high frequency and energy
• Electromagnetic waves are classified by their wavelengthRadio waves = > 0.01 m
4 2Microwaves = 1 10 m < < 1 10 m Infrared (IR)
5 4far IR = 1 10 m < < 1 10 m 6 5middle IR = 1 10 m < < 1 10 m
7 6near IR = 1 10 m < < 1 10 m 7 7Visible light = 4 10 m < < 8 10 m
.YO GR BIV
Ultraviolet (UV)7 7near UV = 2 10 m < < 4 10 m
8 7far UV = 1 10 m < < 2 10 m 10 8X-rays = 1 10 m < < 1 10 m
10Gamma rays = < 1 10 m
Electromagnetic Spectrum
InterferenceThe interaction between waves is called
interferenceWhen waves interact so that they add to make a
larger wave it is called constructive interferencewaves are in-phase
InterferenceThe interaction between waves is called
interferenceWhen waves interact so they cancel each other it is
called destructive interferencewaves are out-of-phase
Diffraction 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 traveling particles do not diffract
Diffraction 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 traveling particles do not diffract
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
2-Slit Interference
https://www.youtube.com/watch?v=hRFQd_fkzws
The Photoelectric Effect Many metals emit electrons when a light shines on them.
called the photoelectric effect
The Photoelectric Effect
The Photoelectric Effect Many metals emit electrons when a light shines on them.
called the photoelectric effect Classic wave theory said this effect was due to the light
energy being transferred to the electron. 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
ejectedLight waves’ intensity made brighter, more electrons should be ejected
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
Experiments showed that a minimum frequency was needed before electrons would be emittedcalled the threshold frequencyno dependence on intensity
It was observed that high-frequency light from a dim source caused electron emission without any lag time
The Photoelectric Effect: The Problem
chE h
Einstein’s ExplanationEinstein proposed that the light energy was
delivered to the atoms in packets, called quanta or photonsThe energy of a photon of light is directly proportional to
its frequency and inversely to wavelengththe proportionality constant is called Planck’s Constant,
(h) and has the value
cE h
h
346.626 10 J sphoton
h cE h
3 92
34 8
1 1 10 1 1 103.83 3.37 10
6.626 10 1 3.00 10 1
nmnm
J
J
photonmJ
m
m
J m
s
s
Calculate the number of photons in a laser pulse with wavelength 337 nm and total energy 3.83 mJ
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?
2
34 27
1 1.0 10 1
6.626 10 1 8.5 10
photo n
ph
J
s nJ oto
1.8x107 s-1 or 1.8x107 Hz or 18 MHz
Ejected ElectronsOne photon at the threshold frequency gives the
electron just enough energy for it to escape the atombinding 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