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Light is a WAVE
5.2.1 EM Spectrum & EM Waves
EM Waves
• Charged objects generate
electric fields.
– When they move, the field changes.
• Changing electric fields produce
MAGNETIC FIELDS.
Require a medium
• 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.
Do not require a medium
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.
Index of Refraction
Equation
c
n
v
Speed of Light
in vacuum
Speed of Light
in medium
Example
What is the speed of light in a medium with an index
of refraction of 2.0?
n=c/v
v=c/n
v = (3 x 108 m/s) / (2.0)
v= 1.5 x 108 m/s
EM Wave Types
An EM Wave’s type depends on its frequency/wavelength
Radio
Microwave
Infrared
Visible
Ultraviolet
X-ray
Gamma Ray
103
10-2
10-5
10-7
10-8
10-10
10-12
Wavelength (m)
About the size of…
Building
Human Honey Bee Pinpoint Protozoan
Molecules
Atoms
Atomic Nuclei
Freq (Hz)
104
Temperature of
bodies emitting
the wavelength
108
1012
1K
1015
100 K 10000 K
1016
1018
10 million K
All objects emit EM radiation
1020
Practice
1. 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? BLUE
b. Which star is hotter, Betelgeuse
or Rigel?
RIGEL
Practice
2. 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
Practice
3. 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
frequency
_______________
radio
______________
b. 5.0 x 10-9 meter
wavelength
_______________
X ray / UV
______________
3.0 x 1013 hertz
frequency
_______________
infrared (IR)
______________
d. 4.5 x 1014 hertz
frequency
_______________
visible
light (red)
______________
e. 200 meters
wavelength
_______________
radio
______________
c.
End of 5.2.1