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Light is a WAVE
5.2.1 EM Spectrum & EM Waves
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EM Waves• Charged objects generate
electric fields.– When they move, the field changes.
• Changing electric fields produce MAGNETIC FIELDS.
• Changing magnetic fields produce ELECTRIC FIELDS.
• The E-fields and B-fields are perpendicular to each other.
• Waving a charge back and forth generates an EM WAVE.
Require a medium
Do not require a medium
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The Speed of Light (well, EM Waves really)• EM Waves travel at 3.0 x 108 m/s (in a vacuum).
• They are slower in denser mediums.
• Speed of a medium depends on INDEX OF REFRACTION.
• The closer the index is to 1.0 the faster the medium.
Equation
v
cn
Index of Refraction Speed of Lightin vacuum
Speed of Lightin medium
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What is the speed of light in a medium with an index of refraction of 2.0?
n = c / vv = c / n
v = (3 x 108 m/s) / (2.0)
v= 1.5 x 108 m/s
Example
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EM Wave Types
An EM Wave’s type depends on its frequency/wavelength
103 10-2 10-5 10-7 10-8 10-10 10-12Wavelength (m)
Freq (Hz)
Temperature of bodies emitting the wavelength 1 K 100 K 10000 K 10 million K
104 108 1012 1015 1016 1018 1020
About the size of…
Building Human Honey Bee Pinpoint Protozoan Molecules Atoms Atomic Nuclei
Radio Microwave Infrared Visible Ultraviolet X-ray Gamma Ray
All objects emit EM radiation
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Practice1. The constellation of Orion is a
highly recognizable arrangement of stars. There are two very distinct stars in this constellation, Betelgeuse which is a red star and Rigel which is a bluish star. The color red has a wavelength of about 750 nm and the color blue has a wavelength of about 400 nm.
a. Which color has the higher frequency?
b. Which star is hotter, Betelgeuse or Rigel?
BLUE
RIGEL
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Practice2. An EM wave is traveling through outer space with a
frequency of 8.0 x 1013 hertz.a. What is the wavelength of this wave?
v = fλ λ= v / f
λ = (3.0 x 108 m/s) / (8.0 x 1013 Hz)
λ = (3.8 x 10-6 m)
b. What type of wave is this?
Infrared
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Practice3. Listed below are the properties of several EM Waves.
State whether the property given is a wavelength or frequency, then state which type of EM Wave it is.
a. 500000 hertz _______________ ______________
b. 5.0 x 10-9 meter _______________ ______________
c. 3.0 x 1013 hertz _______________ ______________
d. 4.5 x 1014 hertz _______________ ______________
e. 200 meters _______________ ______________
frequency
wavelength
wavelength
frequency
frequency
radio
X ray / UV
infrared (IR)
visible light (red)
radio
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End of 5.2.1