15.2 The Dual Nature of Light
(Electromagnetic Waves)
1. Wave Model of Light
a. Light can be reflected (like waves)
i. mirrors
ii. everything you see
iii. how color works (more later)
b. Light can be refracted (like waves)
i. prism produces the spectrum
ii. glass rod looks bent in water
iii. light changes speeds in different media.
c. Light is diffracted (like waves)
i. diffraction grating separates it into
colors.
d. Light waves interfere
i. double slit will cause interference
patterns
ii. Thomas Young’s Double Slit Experiment.
iii. Remember the color patterns on the
bubbles?)
e. Light waves are modeled as a transverse
electromagnetic wave: combination of an
electric field oscillating around a magnetic
field
2. Particle Model of Light
a. wave model does not explain how some light
will displace electrons and others won’t.
b. particle theory explains these atomic or
quantum interactions.
c. photons are particle like bundles of energy
that travel with a wavelength and
frequency.
d. quantum physics – energy comes in units or
quanta.
3. Wave-Particle duality - Because light has
properties of both waves and particles it is
often referred to as both (duality means having
two characteristic natures)
4. Frequency determines the energy of light:
higher frequency = more energy
5. The speed of light depends
upon the medium.
a. 3 x 108 m/s in a vacuum
b. symbol is c
c. light slows down when it
passes from vacuum to
medium. (opposite of
mechanical wave)
d. changes in speed cause refraction.
(bending)
6. Intensity (brightness) of light
a. the rate at which energy flows through a
given area of space.
b. intensity decreases as photons spread out
form the source.
7. The Electromagnetic Spectrum has been
divided into 7 categories based on its
wavelength.
a. visible light: 400 – 700 nm
b. Recall that a nanometer (nm) is 1 x 10-9m
Frequency and Wavelength of Energy in the
Electromagnetic Spectrum
Energy
Frequency in hertz Wavelength in meters
gamma-rays
1020-1024
<10-12 m
x-rays
1017-1020
1 x10-9 - 1 x 10- 12
ultraviolet
1015-1017
400 x10-9 to 1 x10-9
visible
4-7.5x1014
750 x10-9 - 400 x10-9
near-infrared
1x1014-4x1014
2.5 x10-6 -750 x10-9
infrared
1013-1014
25 x10-6 -2.5 x10-6
microwaves
3x1011-1013
1 x10-3 -25 x10-6
radio waves
<3x1011
>1 mm
8. UV light (ultraviolet) three types or bands UVA, UVB, and UVC.
a. ozone layer absorbs some, but not all
b. UVA (400 – 320 nm): longest λ Not
absorbed by the ozone layer, not harmful,
backlight range.
c. UVB (320 – 290 nm): some absorbed by ozone
layer, but much reaches the Earth's
surface; most harmful we are exposed to;
causes sunburn
d. UVC (100 – 290nm): Completely absorbed by
the ozone layer and oxygen; artificially
generated as germicide, highly dangerous.
9.
X rays – ( 1 nm – 1pm (1 x 10 –12m)
a. very high energy, highly penetrating,
b. 1895 by Wilhelm C. Roentgen, (German);
found them by accident when experimenting
w/vacuum tubes.
c. Cannot penetrate atmosphere.
d. medical X-rays – dense material absorbs
rays and appears white on film.
e. Astronomers use x-rays to study stars &
black holes.
10. gamma rays (– 10-12 to 10-14 m)
a. emitted by radioactive atoms
b. travel across universe
c. astronomers also collect these for star
data.
d. kill living cells; used to kill cancer cells
Now let’s look at
waves with long λ.
11. Infrared
(1x 10-6 to 10-3 m)
divided into regions
a. Near Infrared: About 10-6 m used in remote
sensing; not hot
b. Far Infrared (10-3): longer λ; thermal
radiation; HEAT
12. Microwaves (cm – mm range)
a. water, fat and sugar absorb waves at this
range so useful for cooking.
b. communication in TV, cell phones, and
satellites
c. Doppler Radar and other short wave remote
sensing applications
d. cosmic microwave radiation believed to be
left over from the big bang.
e. radar – uses reflecte waves to determine
the velocity and location of objects.
i. Doppler effect remember will change
perceived λ.
13. Radio waves (1 m – 10,000m) : AM, FM, CB,
Band name Abbr
ITU
band
Frequency
and
Wavelength in air
Example uses
Extremely low
ELF
frequency
1
3–30 Hz
Communication with submarines
100,000 km – 10,000 km
Super low
frequency
SLF
2
30–300 Hz
10,000 km – 1000 km
Communication with submarines
Ultra low
frequency
ULF
3
300–3000 Hz
1000 km – 100 km
Communication within mines
Very low
frequency
VLF
4
3–30 kHz
100 km – 10 km
Submarine communication, avalanche beacons,
wireless heart rate monitors, geophysics
Low
frequency
LF
5
30–300 kHz
10 km – 1 km
Navigation, time signals, AM longwave
broadcasting
Medium
frequency
MF
6
300–3000 kHz
1 km – 100 m
AM (Medium-wave) broadcasts
High
frequency
HF
7
3–30 MHz
100 m – 10 m
Shortwave broadcasts, amateur radio and overthe-horizon aviation communications
Very high
frequency
VHF
8
30–300 MHz
10 m – 1 m
FM, television broadcasts and line-of-sight
ground-to-aircraft and aircraft-to-aircraft
communications
Ultra high
frequency
UHF
9
300–3000 MHz
1 m – 100 mm
television broadcasts, microwave ovens, mobile
phones, wireless LAN, Bluetooth, GPS and TwoWay Radios such as FRS and GMRS Radios
Super high
frequency
SHF
10
3–30 GHz
100 mm – 10 mm
microwave devices, wireless LAN, most modern
Radars
Extremely
high
frequency
EHF
11
30–300 GHz
10 mm – 1 mm
Radio astronomy, high-speed microwave radio
relay