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Electromagnetism and LightMonday – Properties of waves (sound and light) – interference, diffraction [Hewitt 12]
Tuesday – Light waves, diffraction, refraction, Snell's Law. [Hewitt 13, 14]
Wednesday – Lenses, polarization [Hewitt 14]Thurday/Friday – Magnetic fields forces and induction [Hewitt 11]
TUESDAY
This morning –how electromagnetic radiation is made, interference, diffraction, spectra, doppler effect, interference
Afternoon – Refraction, snell's law.
This lecture will help you understand:
Electromagnetic Spectrum Spectrum of Gasses Doppler Effect Bow Waves/Shock Waves and the Sonic Boom Why the Sky is Blue, Sunsets are Red, and Clouds
are White Diffraction Interference of Light
Light Waves
“The light of stars that were extinguished ages ago still reaches us. So it is with great men who died centuries ago, but still reach us with the radiations of their personalities.”
—Kahlil Gibran
Wave Speeds
Speed of light (in vacuum/air)
Speed of sound depends on wind conditions, temperature, humidity
speed in dry air at 0°C is about 330 m/s in water vapor slightly faster in warm air faster than cold air
speed in water about 4 times speed in air speed in steel about 15 times its speed in air
c=3×108 m /s
Consider a person attending a concert that is being broadcast over the radio. The broadcast microphone is sitting right on the stage.The person sits about 45 m from the stage and listens to the radio broadcast with one ear while listening to the concert inperson with the other ear. Further suppose that the radio signal is sent around the Earth before reaching the listeners radio.
HINT: Don't guess on this one. You can do two calculations which will tell you the answer.
A situation to ponder…
Which signal will be heard first?
A. radio signalB. the “live” sound signalC. both at the same timeD. none of the above
A situation to ponder…CLICKER QUESTION
Which signal will be heard first?
A. radio signalB. the “live” sound signalA. both at the same timeD. none of the above
Explanation:A radio signal travels at the speed of light—3 × 108 m/s. Time to travel 45 m at 340 m/s 0.13 s. ≈
Time to travel 4 × 107 m (Earth’s circumference) at 3 × 108 m/s 0.13 s. Therefore, if you sit farther back at the concert, ≈the radio signal would reach you first!
A situation to ponder…CHECK YOUR ANSWER
Light WavesLight is the only thing we can see originates from the accelerated motion of
electrons electromagnetic phenomenon
Electromagnetic Spectrum
Electromagnetic wave made up of vibrating electric and magnetic
fields
k pHeT
If an electron vibrates up and down 1000 times each second, it generates an electromagnetic wave with a
A. period of 1000 seconds.B. speed of 1000 m/s. C. wavelength of 1000 m.D. none of the above
Electromagnetic SpectrumCHECK YOUR NEIGHBOR
If an electron vibrates up and down 1000 times each second, it generates an electromagnetic wave with a
A. period of 1000 seconds.B. speed of 1000 m/s. C. wavelength of 1000 m.D. none of the above
Explanation:The vibrating electron would emit a wave with a frequency of 1000 Hz, which is not in the list above.
Electromagnetic SpectrumCHECK YOUR ANSWER
Electromagnetic SpectrumElectromagnetic spectrum classification of electromagnetic waves according to
frequency lowest frequency of light we can see appears red highest frequency of light we can see appears violet higher frequency of light is ultraviolet—more energetic and causes
sunburns beyond are Xray and gamma ray
Electromagnetic SpectrumElectromagnetic spectrum
Below Radio? … more radio Above Gamma? … more gamma
Some gamma rays have as much
energy as a fastball … no one knows how.
Transparent and Opaque Materials
Light is transmitted similar to sound light incident on matter forces some electrons in
matter to vibrate
Transparent and Opaque Materials
How light penetrates transparent material such as glass
Transparent and Opaque Materials
How light penetrates transparent material such as glass (continued)
• electrons or molecules in the glass are forced into vibration• energy is momentarily absorbed and vibrates the electrons in the
glass• this vibrating electron either emits a photon or transfers the energy
as heat
• Time delay between absorption and reemission of energy of vibrating electrons results in a lower average speed of light through a transparent material
Transparent and Opaque Materials
Average speed of light through different materials• vacuum—c (300,000,000 m/s)• atmosphere—slightly less than c (but rounded off to c)• water—0.75 c• glass—0.67 c, depending on material• diamond—0.41 c•
Refraction (the bending of light as it enters a material) is CAUSED BY this speed change
Compared with the frequency of illuminating light on a sheet of transparent plastic, the frequency of light that is transmitted
A. is slightly less.B. is the same. C. is slightly higher.D. depends on the type of plastic.
CLICKER QUESTION
Compared with the frequency of illuminating light on a sheet of transparent plastic, the frequency of light that is transmitted
A. is slightly less.B. is the same. C. is slightly higher.D. depends on the type of plastic
Explanation:Speed of light in plastic may vary, but the frequency transmitted doesn’t.
CHECK YOUR ANSWER
The average speed of light is less in
A. air before entering glass.B. glass. C. air after emerging from glass.D. none of the above
CLICKER QUESTION
The average speed of light is less in
A. air before entering glass.B. glass.C. air after emerging from glass.D. none of the above
CHECK YOUR ANSWER
InterferenceTwo patterns of interference constructive interference
increased amplitude when the crest of one wave overlaps the crest of another wave
destructive interference reduced amplitude when the crest of one wave overlaps
the trough of another wave
Double slit constructive interference(diffraction)
m is aninteger, 0, 1, 2, 3 etc.
dsin=m
dyL=m
ymax=mLd
Single slit diffraction
Light from differentParts of a singleslit interfereswith itselfDestructive Condition:
asinθ=mλ
Single slit diffraction(Wide bright area in middle)
Diffraction
Diffraction bending of waves by means other than reflection
and refraction property of all kinds of waves seen around edges of many shadows
DiffractionWaves diffract after passing through a narrow opening.
Plane waves passing through openings of various sizes. The smaller the opening, the greater the bending of the waves at the edges.
Diffraction
Amount of diffraction depends on wavelength of the wave compared to the size of the obstruction that casts the shadow.
Animation … Huyghen's principle.
Huyghen's Principle –
Every piece of a slit makes circular waves …
Their interference makes what we see .
Doubles slit diffraction is superposition of single and double slit
Dot spacing = Wavelength * Distance to wall / d
dsin=m
Diffraction gratingSame formula as two slits … but sharper dots.
dsin=m
Condition for constructive interference between slits separated by “d”.
Condition for destructive interference for single slit of width “a”.
“Rayleigh Condition”Minimum resolvable angularseparation for lens of diameter D.
min=1.22
D
Summary: Interference & Diffraction
asin=m
Single slit interference leads toDiffraction limit
Destructive Condition slit:
For circular aperture – Theta is full width
To see a 100 kLy galaxy 2 M Ly away
asinθ=mθ
θmin≃λ/a
θmin≃1.22λ/D
min=1.22
D
min=1.220.1 m1220m
=10−4 rad=3×108m/s3×109 Hz
Very Large Array
=2.8 m
Air-borne laser, laser missile defense
min=1.22
D
Death Star … Large Mirror …Good for destroying planets!
min=1.22
D
Use Diffraction gratings to identify gasses.
Identifying atomic energy levels
E=hf
E=hc
h=6.62×10−34 Joule⋅sec
Interference at thin filmsInterference at thin films
Reflection and Refraction of Sound
Refraction
bending of waves—caused by changes in speed affected by
wind variations temperature variations
Roswell!
The Roswell incident has been explained.
It WAS a top secret government program.
It wasn't “just a weatherballoon” that crashed.
Sonnenfeld will tell all.
Doppler Effectchange of frequency due to the motion of the wave
sourcemost evident in change of pitch
frequency of waves increases as the siren approaches (hear higher pitch)
frequency of waves decreases as siren moves away (hear lower pitch)
http://en.wikipedia.org/wiki/Doppler_effect (Nice animations)
When a fire engine approaches you, the
A. speed of its sound increases.B. frequency of sound increases. C. wavelength of its sound increases.D. all of the above increase
CLICKER QUESTION
When a fire engine approaches you, the
A. speed of its sound increases.B. frequency of sound increases.C. wavelength of its sound increases.D. all of the above increase
Comment:Be sure you distinguish between sound speed, and sound frequency.
CHECK YOUR ANSWER
Doppler Effect
Doppler effect also applies to light increase in light frequency (blue shift) when light source approaches
you decrease in light frequency (red shift) when light source moves away
from you The expansion and the age of the Universe is entirely measured by
the Doppler Effect. v = H * D
v= speed of galaxy relative to Earth H= Hubble constant D=distance to galaxy
Doppler EffectThe panel on the left arespectral lines from the sun.
The panel at right arespectral lines of a distantstar.
The star is [A] A different composition of gassesas our sun[B] Moving away from Earth[C] Moving toward Earth[D] Not enough information.
CLICKER QUESTION
Doppler Effect
The star is [A] A different composition of gassesas our sun[B] Moving away from Earth[C] Moving toward Earth[D] Not enough information.
CHECK YOUR ANSWER
Explanation:The lines are in correct relative
orientation, so are same gasses, but they move into the red. Thus star is moving away.
Wave Barriers and Bow Waves
Wave barrier waves superimpose directly on top of one another
producing a “wall”example: bug swimming as fast as the wave it makes
Wave Barriers and Bow Waves
Supersonic aircraft flying faster than the speed of soundBow wave Vshape form of overlapping waves when object
travels faster than wave speed an increase in speed will produce a narrower V
shape of overlapping waves.
Shock Waves and the Sonic Boom
Shock wave pattern of overlapping spheres that form a cone from
objects traveling faster than the speed of sound
Sonic BoomShock wave consists of two cones
a highpressure cone generated at the bow of the supersonic aircraft
a lowpressure cone that follows toward (or at) the tail of the aircraft
it is not required that a moving source be noisy
Why does the air condense?
Cerenkov Radiation and Tachyons
Particles traveling faster than light make a shock wave too!
Fast charged particles inthe water bath of a nuclear reactor make a blue glow.
Tachyons would glow in a vacuum
Hewitt Problem 5An electric field does 12 J of work on a charge of 0.0001 C as it moves from point A to point B. What is the voltage change between point A and point B?
How much work does this same field do on a charge of 0.0002 Coulombs?
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