Gerard DeAngelis
Physics Trimester 3
Waves, Sound and Light
Waves
1. Wave- a means by which energy is transported through a medium
a. Medium- the “stuff” in which the wave is occurring
i.
Ex: Water, air (sound), string, silky
b. Waves need a medium through which to travel*
2. In the 16th Century, scientist thought everything in the universe was either “matter-like”
or “wave-like”
a. In some ways, matter and waves act differently in different phenomenons
b. But, they could not agree on if light was a matter or a wave
i.
Became a huge fight in the scientific community
3. Mechanical Wave- requires a medium to travel through. There are two types:
a. Transverse Waves- an wave in which particles of the medium (here the dot in the
picture) work perpendicularly (think “up and down”) to the direction of the velocity
as the wave moves past the point
i.
ii.
iii.
iv.
v.
vi.
Equilibrium Position- where the medium would if with no waves running
trough it
1. Medium not transmitting energy
Crest- top of sine curve
Trough- bottom of sine curve
1. Distance of the top of the crest and the bottom of the trough from
the equilibrium position is equal
Wavelength- distance between two same points on a curve
1. Usually between a crest and crest and through or trough
2. Abbreviation= λ
a. Measured in m
Amplitude- height the crest (or trough) reaches above the equilibrium
position
1. The more energy you put in a wave, the higher the amplitude
a. The louder the sound the higher the amplitude
Exhibited in water and light
b. Longitudinal Waves- an wave in which particles of the medium (here the dot in
the picture) work parallel (think “back and forth”) to the direction of the velocity as
the wave moves past the point
i.
4.
5.
6.
7.
8.
2.
Compression- denser region
1. When studies represented by a crest
ii.
Expansions/ Ratifications- spread out region
1. When studies represented by a trough
iii.
Exhibited by sound
Period (T)- amount of time it takes or a single wavelength to pass a certain point
a. Measured in (s)
i.
So really just a form of time
Frequency (f)- the number of wavelengths that pass a certain point per second
a. Inverse of period
b. Measured in Hertz (Hz)
i.
1 Hz= 1 cycle/ second
ii.
1 Hz= 1/s = s-1
1. Cycle- a whole “something”
a. Ex: whole swing of pendulum, whole vibration, etc.
b. Since it is not a unit is can be substituted as 1
Pulse- when there is only one input of energy
a. Usually either a single crest or a crest and a trough
Periodic wave- a continuous input of energy
a. Repeating of crests and troughs
For mechanical waves the amplitude, frequency, and wavelength do not affect the speed
a. All mechanical waves travel the same speed in the same medium
i.
If mediums change (or even something about the medium), however, the
speed of your wave will change
1. Example: deep ocean waves= slow; shallow ocean waves= faster
b. Note: Unlike mechanical object, even if you put more energy into the wave
(higher amplitude) it does not go faster
However, for light, all the above information that is true for mechanical waves does not
hold true
a. Amplitude has nothing to do with energy
i.
Higher frequency= higher energy
1. Ex: X-rays have a lot of energy
Wave Phenomenon
1. Reflection- characteristic of waves in which an incident wave strikes a surface and
bounces off
a. A wave bouncing off a surface
b. Law of Reflection- the angle at which a wave hits a surface is the angle it
bounces off at
c. When a wave is reflected it “changes phase”
i.
The crests become troughs and the troughs become crest
d. Is a 180o change in direction (and phase)
2. Refraction- when a wave travels from one medium to a second medium it changes
direction (bends)
a. Exception- if the light approaches the border between mediums (hits the object
straight on”) is will not be refracted
i.
Waves are only reflected when they strike at an angle
b. *The reason for this bending is that when waves travel through a different
medium their speed and wavelength change
i.
Note: frequency never changes though
ii.
iii.
Normal line- perpendicular to boundary of mediums
Red Line
1. When a wave refracts towards the normal line it slows down
a. “Slow down, bend toward”
iv.
Green Line
1. When a wave reflects away from the normal line it speeds up
a. “Speed up, bend away”
v.
Blue Line
1. Exception to reflection rule
2. When a wave does not reflect (At an angle) it does not speed up
or slow down, it travels at the same speed
c. But light acts differently than all this
3. Diffraction- ability of a wave to bend around a corner (go around obstacles) or spread
out in a region beyond the gap
a. The amount a wave bends depend on its wavelength and the size of the gab it is
trying to get though
b. Longer wavelength= more diffraction
i.
This is why you only hear the bass of a car blasting music that is far away
4. Dispersion- special medium in which waves of different frequencies
travel at different speeds
a. Possible in mechanic waves and common in light
b. Is how a glass prism separates the different colors of the
rainbow from white light
5. Superposition- process of how to determine the amplitude when two
waves arrive at the same point simultaneously
a. To find the amplitude of the resulting wave just add the amplitudes of the two that
are overlapping
i.
Note- troughs have a negative amplitude
b. Constructive Interference- occurs when final wave has an amplitude greater than
any of the individual waves
i.
Example: Waves of an amplitude of 3 and 5 meet
1. If they combine to be 8,7or 6 it is constructive interference
2. However, if they add up to be 5 or less it is not
ii.
Total Constructive Interference- occurs when two waves perfectly line up
so it is crest to crest and trough to trough exactly
1. When they line up as such we call it in phase
c. Destructive Interference- two waves that arrive at the same point and result in a
lower amplitude
i.
When a crest lines up with a trough and a trough lines up with a crest
ii.
Total Destructive Interference- two waves line up perfectly to negate the
effect of both waves and leave no waves behind
1. Three conditions must be met. The waves must have:
a. Same amplitude
b. Same frequency
c. Completely out of phase
i.
180o (2π) out of phase
2. Leaves behind no wave (straight line)
Sound
1. Humans can only hear a certain range: 20Hz- 20,000Hz
a. As you age your ear drum becomes less elastic and cannot vibrate fast enough
to pick up the higher notes
2. Resonance- all object have a natural vibration that they always make when they are hit
(no matter how hard or soft)
3. Sympathetic Vibration- getting another object to resonate at the same frequency as
another
a. When objects are forced to vibrate at their natural frequency they easily begin to
vibrate. This new vibration that is called is a sympathetic vibration.
4. Beats- a series of alternate reinforcements and cancellations (loudness and faintness)
produced by the interference of two waves of slightly different frequencies
a. Switching between constructive and destructive interference of music notes
b. Beats per second= difference in two frequencies
5. Doppler Effect- apparent change in frequency as the gap between the emitter of the
wave and the receiver of the wave change
a. AKA “eeeeeeeeeeeeeeeeeeeeeeeeeeeeooooooooooooowwwwwwwwwwww”
b. If the gap between the emitter and receiver:
i.
Stays the same (even if two are moving)
1. The frequency remains at its true frequency
ii.
Shortens
1. Waves are compressed
a. Wavelength shorten
b. Frequency goes up
iii.
Enlarges
1. Waves stretch out
a. Wavelength increases
b. Frequency goes down
c. Note: the instant that a moving object is right in front of you (gap is 0) you hear
the true frequency
d. This happens for light as well
i.
Red shift= galaxies that move away from us look red
ii.
Blue shift= galaxies that come closer to us look blue (really violet)
iii.
But this will only happen if you are traveling near the speed of light
What in Tarnation is Light?
Question of the Ages
1. The real answer is that we are not 100% sure what it is still
2. The Particle Model Basis
a. Ancient Greeks (400 B.C.)— Aristotle and others (but not all!):
i.
Thought light emanated as “streams from the eye” and “streams from the
objects”
ii.
Thought it traveled at an infinite speed and appeared everywhere at once.
b. Ole Rømer (1676)
i.
First to measure the speed of light as a finite quantity
1. Showed it’s speed was not infinite
2. Measurement was pretty accurate
ii.
Determined by measuring the time light takes to get to us from Jupiter 6
months apart
1. Found it takes 16 mins to cross Earth’s orbit
3. 1700: Newton (Particle/ “Corpuscular” Theory) v. Christian Huygens (Wave Theory)
a. Newton’s Particle Model (1704 Opticks)
i.
Newton was asked by scientists to do research on this
ii.
Made 2 assumptions
1. Light particles are very very small
a. Since when two light beams are crossed and there is no
collisions between the light particles, the particles must be
extremely small
2. Light travels really fast
a. This is why gravity does not seem to ever pull it down
iii.
In his book explains all phenomena of light in terms of his particles theory
1. Shadows
a. Shadow edges are crisp because light is a particle
b. If it was a wave it would be blurry (diffraction)
i.
We now know, however, in reality shadows are a
bit fuzzy if we look close to them
2. Reflection
3. Refraction (bending of light as it enters water)
a. Light travels straight in one medium because a light
particle deep within a medium experiences no net force
(force equal on all sides)
b. However, when the particle gets close to the water, the
light particles experience an attractive force towards the
more massive water molecules (pulls on light particle
more) and creates a net down force
i.
***So, Newton must assume that light travels faster
in water
1. This is the only major disparity between his
book and Huygens
4. Dispersion of light through a prison
a. Newton is credited with showing that white light is made up
of all the different colors
b. Says that colors are different masses, so they move at
different speeds as they enter the prism (pulled on at
different amount because of their differing mass)
iv.
Newton, admittedly, had some doubts, however
1. He was just a bit more convinced it was a particle
b. Huygens’ Wave Model
i.
Newton objects to light being a wave because a wave needs a medium
1. How could light travel through space (a vacuum with no medium)
if it is a wave?
2. Huygens answers this saying that an “luminiferous ether” exists
everywhere in the universe, it is just undetectable
a. This is his medium for his wave
ii.
Like Newton, explains all light phenomenon, but this time in light of his
wave theory
1. Refraction
a. Since light bends toward the normal when it goes from air
to water, it must move slower in water than air (According
to wave rules)
i.
So, says light must travels slower in water
2. Diffraction
a. Says that shadows are indeed a bit fuzzy
3. Doesn’t explain color and why no interference
4. Wavelets- Each wave front can be thought of as many small
wavelets coming from previous front
c. There books were both just about equal so scientists took the side of Newton
because:
i.
His fame
ii.
Straight line v. true shadows
iii.
Medium????
iv.
No interference noticed
1. So, from 1700 on, many scientists just took Newton on his fame
4. After 1700, experiment after experiment showed that light was probably a wave
a. Despite that fact, Newton’s fame had scientists still holding on to believing that
light was a particle
i.
Bit by bit, however, they chipped away at the foundation
5. 1800: Thomas Young
a. Believed that light was a wave
b. Double-Slit experiment
i.
One of the most important experiments ever done
ii.
Tries to achieve destructive interference of light, proving it is a wave
1. While with particles you may have constructive interference, you
can never have destructive interference where two particles
cancel each other out
iii.
Gets conditions for a wave to constrict interfere:
1. Frequency is made the same my a monochromatic filter (color
filter)
2. Amplitude is made the same by having it diffract from a single
point
iv.
Result:
1. Bright bands of colored light with nothing in between them
2. The nothing in between= complete destructive interference
c. Effectively proves that light is made of waves
i.
Many people still however refuse to believe
6. Augustin Fresnel and Thomas Young (1815’s)
a. Polarization- Eliminating all the random vibrations of a wave except for those in
one direction.
i.
All vibrations are restricted to a single plane.
b. They are able to polarize light
i.
Shows that light is a transverse wave
ii.
Longitudinal waves cannot be polarized
7. 1850: Léon Foucault
a. Measures speed of light in water and finds that light travels slower in water
i.
The denser that material, the slower light travels in it
1. Ex: light travels in diamonds much slower
b. Now, Huygens is proved right and the entire scientific community agrees with him
8. The Electromagnetic Model—James C. Maxwell (1873)
a. Theoretical physicist- a physicist who logically and mathematically discovers new
theories, but does not do any hands-on experiments.
b. Never intended to study light, but discovered the speed of light when working
………with electromagnetism
i.
Worked with Michael Faraday
c. Realized that by charging an electric field to can charge a magnetic field which
……….can charge an electric field and so on
i.
Wanted to find out what condition would be needed to make this cycle go
on forever
d. Comes up with 4 equations to get answer (Maxwell’s equations)
i.
Concludes that the condition needed would be:
1. v2= 9 x 1016 m2/s2
a. Which simplifies to…
2. v= 3 x 108 m/s
a. The speed of light!!!
e. This shows that light is an electromagnetic wave
i.
It travel through space by warping the electric and magnetic fields around
it
1. Is not a mechanical wave so does not need a medium
f. Electromagnetic Spectrum
i.
Note: For light energy does not correspond with amplitude
1. Frequency has to do with energy
a. Why gamma rays are so dangerous
g. Accelerated charges produce EM waves
a. Problems with theory:
i.
All atoms would radiate energy and everything would glow
ii.
All electrons would spiral into nucleus and all atoms would collapse
iii.
Therefore, the universe can’t exist.
1. So, we know Maxwell’s theory is not entirely true...only half of the
picture
b. Confirmed by Heinrich Hertz who was the first to generate and detect EM waves
2. Michelson–Morley experiment
a. There is no ether that pervades all space. EM radiation (light) doesn’t need it
anyway, since it is the warping of space by E and M fields that in a sense “wave.”
3. 1900—experiments slowly started showing light to be a particle:
a. Max Planck—the Quantum Theory
i.
Blackbody Ration- Theoretical device that absorbs ALL the EM radiation
that falls on it.
ii.
Blackbody radiation is proven to give off “packages” or “chunks” of light
1. Waves cannot do this, only matter can
2. Maxwell called these “packages” “quanta” (packet in german)
iii.
Beginning of dual nature of light theory
b. Albert Einstein
i.
Wrote the Photoelectric Effect
ii.
Shone a monochromatic (1 color) light on a metal plate
1. Using just the right color he could knock electrons off the surface
2. This is the basis of solar cells
iii.
Calls “quanta” “photons”
1. Bundles of light energy
iv.
Main pioneer of dual nature of might
1. Explicitly mentions in aper
2. Takes the best from Maxwell and Planck's theories
4. Quantum Theory of Light
a. Light can never act as a particle and a wave at the same time
b. Frequency, for light, is directly proportional to the energy
i.
Amplitude is not energy but the probability of a wave
1. It the likelihood of finding a phone in a location
c. All EM waves travel at the speed of light
d. We still don't completely understand light, however
5. 1920’s—Louis de Broglie
a. Proposed by symmetry of nature
i.
Based on Planck and Einstein on waves, and Niels Bohr work on
understanding electrons
b. Says that all matter has associated waves (not just light)
i.
Is proven true!
1. We can get electrons to destructively interfere
2. The more massive an object is the smaller its waves
a. So hard to get larger object to destructively interfere
c. We now know everything in our universe has a particle and wave