Physics 1230: Light and Color
Chuck Rogers
Duane F-631
Charles.Rogers@colorado.edu
(303) 492-4476
http://www.colorado.edu/physics/phys1230
Lecture 3:
Properties of electromagnetic waves (LIGHT!)
HW2 on CULearn: DUE ON FRIDAY, 5PM. (Tomorrow)
HW3 later today: Due on Monday, 5PM
Exam 1 will be Wed. June 9, next week.
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Last lecture
• Electric fields
• Magnetic fields
• Light is an „electromagnetic wave‟
E
B
2
Making sense of the Sine Wave
What does the curve tell you?
-For Water Waves?
-For Sound Wave?
-For E/M Waves?
Use these sims to learn more:
Wave-interference and
Radio-waves
EM radiation often represented by a sinusoidal curve.
OR
What does the curve tell you?
a.The spatial extent of the E-field. At the peaks and
troughs the E-field is covering a larger extent in space
b.The E-field‟s direction and strength along the center
line of the curve
c.The actual path of the light travels
d.more than one of these
e.none of these.
What is light?
Light is an electromagnetic wave.
l is the wavelength or distance between crests.
side view
end view
Blue arrow is electric field direction and amplitude,
Red arrow is magnetic field direction and amplitude.
Electric fields make charges “q” move, creating a current.
The current can be in your optic nerve, or a solar cell.
5
True or False: I have used the
metric system
A) TRUE
B) FALSE
True or False: I am comfortable
with the metric system
A) TRUE
B) FALSE
6
“SI” units of measure
(the metric system)
•
•
•
•
Length: meters (m)
Time: seconds (s)
Velocity: meters/second (m/s)
Frequency: cycles per second or Hertz (Hz)
Hz has units 1/seconds
Always state the units for your answers!
7
I have used „scientific notation‟
A) Yes
B) No
C) What?!
True or False: I am comfortable
with scientific notation.
A) TRUE
B) FALSE
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Scientific notation – powers of 10
1,000 = 1.0 x 103
2,100,000 = 2.1 x 106
1/(1,000) = 0.001 = 10-3
0.1 = 10-1
1 = 100
10 = 101
Calculator notation: 2.1 x 106 = 2.1E6
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Scientific notation – prefixes
m
m
n
milli = 10-3
micro = 10-6
nano = 10-9
k
M
G
T
kilo = 103
mega = 106
giga = 109
Tera = 1012
Common usage:
wavelength in mm, mm or nm
frequency in kHz, MHz, GHz or THz
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Frequency and Period
• Frequency is the number of
full cycles per second
• Period is the time to swing
forth and back
• If f = 2 Hz,
then 2 swings per second,
and period = 0.5 s.
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http://phet.colorado.edu/sims/string-wave/string-wave_en.html
Some radio wave frequencies
•
•
•
•
•
•
Submarine communication, 76 Hz
AM radio, 520 – 1610 kHz
FM radio, 87.5 – 108 MHz
Wireless internet, 2.4 GHz
GPS, 1575 MHz or 1.575 GHz
Security scanner: 1000 GHz = 1 THz
The frequency of light is very high, ~1015
Hz, and is off this scale.
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Wavelengths
nanometers
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Visible spectrum (for humans)
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We see color when waves of different
wavelengths enter enter our eyes!
Light with wavelength of 650 nm
appears red when it enters a viewers eye
Light with wavelength of 520 nm
appears green when it enters a viewers eye
Light with wavelength of 470 nm
appears blue when it enters a viewers eye
Clicker question
•
The wavelength of green
light is around 500 nm.
How many wavelengths
of green light fit into one
cm (0.4 inches, or a
fingertip)?
a)
b)
c)
d)
e)
20 thousand
50 thousand
Two million
Two billion
5 billion
• wavelength = 500 nm =
5 x 102 x10-9 m =
5 x 10-7 m
• Hence 1 m has
(1 m)/(5 x 10-7 m) = 107/5
wavelengths, OR
2 million wavelengths
• Since 1 cm is 1/100 of a
meter there are 2 x
106/100 = 20,000
wavelengths in a cm
Speed of light
• Measurements based on
distance = speed x time
• speed = distance / time
Early observation: lightning precedes thunder.
(Sound goes 1000 ft. / sec, or 1 mi. in 5 sec.)
19
Early measurements of c
• 1600s: Galileo, lanterns on hilltops
(“null” result)
• 1800s: Roemer, light crosses earth‟s orbit
(16 mins)
• 1800s: Fizeau, rotating gear
• 1900s: Michaelson, rotating mirror
• c = 300,000 km/s = 3 x 108 m/s
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The frequency-wavelength formula
Speed = (distance traveled)/(Time it took)
= (wavelength)/(period)
c=l/T
c=lf
f =c/l
l=c/f
Four versions of
exactly the same
statement.
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Clicker Question
An FM radio station transmits at a frequency
of: f = 100 MHz = 108 Hz
then the wavelength is :
A) 1 m B) 0.3 m C) 3 m D) 100 m
E) None of these.
c=lf
f=c/l
l=c/f
l = [3 x 108 m/s] / [108 /s] = 3 m
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Made it to here Lecture 3
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