Download Waves Guided Notes Packet 2

Waves Guided Notes Packet 2
Name:_______________ Class:____
Light
What we call “light” is merely a small fraction of the total ______________________________________.
The electromagnetic spectrum
Consists of transverse waves of varying frequencies
Occur due to oscillation of electric and magnetic fields
Does not require a medium
Travel at the speed of light (c = 3.00 X 108 m/s)
Like all waves, the EM spectrum carries,
what?
Vision
We see when light when visible light enters our eyes and focuses on our retinas. The light may come from a
____________________________ source (something that emits light, such as the sun or a light bulb) or may
bounce (reflect) off an _________________________ object.
We have photoreceptor cells in our retinas that send a signal to our brain whenever light hits them.
•
Rods are sensitive to all wavelengths of visible light
 provide black and white vision
•
Cones are sensitive to only certain wavelengths
 provide color vision
We have three cones types:
blue, green, red.
Our brain interprets color based on the combinations of
cones that are triggered by certain light.
… if only the red cone is triggered, we see
… if red and green are triggered equally, we see
Different materials absorb, reflect, and/or transmit different wavelengths of light.
•
_______________________________ – light energy that has the same natural frequency as the
electrons in the object will be absorbed – the energy will be converted into vibrational motion
of the electrons.
•
_______________________________ – the light ‘bounces off’ the surface of the object
•
______________________________ – the light goes through the object and out the opposite
side
Which type of light hits our eyes? _______________________________________
______________________________ objects transmit most of the light that hits them.
______________________________ objects transmit some of the light that hits them.
______________________________ objects reflect or absorb most of the light that hits them
How would the following objects appear?
1. An object that reflects blue and green light; absorbing other wavelengths.
2. An object that reflects all visible light waves.
3. An object that absorbs all visible light waves.
4. An object that reflects red light but transmits all other wavelengths.
5. An object that transmits most of the light hitting it.
Barrier behavior of waves
Waves of any sort – light, sound, water, etc. – exhibit different behaviors when they encounter a barrier.
•
______________________ – bouncing back of a wave as it encounters a new medium
•
______________________ – the bending of a wave as it transmits through a different medium
•
______________________ – The separation of a wave into its component frequencies
•
______________________ – the spreading of a wave behind an obstruction
Law of Reflection
Refraction
When a wave passes from one medium to another, its velocity changes. The change in speed results in a
change in direction of propagation of the refracted wave.
Frequency is determined by the source so it doesn’t change. Only wavelength changes. Wavelength of the
same wave is smaller in the medium with smaller speed.
Snell’s Law
Greater speed – greater angle
Slower speed – slower angle
Refraction animations
Draw the wavefronts as they pass through the two materials, and answer the questions below.
1) Why is there no bending of light in the first animation?
2) Is the block a slow medium or a fast medium? Justify your response.
Optics
Do Now
Draw a reflected sound wave, labeling the angle of incidence and the angle of reflection.
How do these angles compare?
Draw the wave fronts of a light wave passing through three different media.
The first medium be medium speed
The second should be faster speed
The third should be slow speed
Your picture should show realistic angles of refraction and changes in wave length.
Medium speed
Fast speed
speed
Slow speed
speed
Index of Refraction
The index of refraction (n) is a value that describes the __________________________________________
______________________________________.
n = index of refraction, has no units
c = speed of light in a vacuum, 3.00 X 108 m/s
v = speed of light in a particular medium
Since c is the fastest speed of light, n is always greater than 1.
The larger the index the _______________ the speed in that medium .
The greater the difference in n, the ______________________ the angle
of refraction.
Total Internal Reflection
Critical angle: qc ____________________________________________________________________________________
Application: Fiber Optics
A fiber optic cable is a _____________________________________________________________________________
__________________________________________________________________________________________________.
 Critical angle is very small  almost everything is totally internally reflected.
 The light is guided through the cable by successive internal reflections with almost no loss (a little escapes).
 Even if the light pipe is bent into a complicated shape (tied into knots), light is transmitted practically undiminished to
the other end.
Why use fiber optics?
 ___________________________________________________________________________________________
___________________________________________________________________________________________
 One single optical fiber can transmit several TV programs and tens of thousands of telephone conversations, all
at the same time.
 takes 300 lbs of copper to carry same info as 1 lb of fiber optics
 Downside: expensive
Dispersion
Even though all colors of the visible spectrum travel with the same speed in vacuum, the speed of the colors of the
visible spectrum varies when they pass through a transparent medium like glass and water. That is, the refractive index
of glass is different for different colors.
Different colors (frequencies) are refracted by different amounts! True for different pitches, as well!
Diffraction
When waves pass through a small opening, or pass the edge of a barrier, they always spread out to some extent into the
region that is not directly in the path of the waves.
_______________________________________________________________________ obstruction is called diffraction.
Diffraction effects are greater when the wavelength is big relative to the size of the obstruction.
__________________________________________________________ = noticeable diffraction
Diffraction explains why we can hear but not see around corners and other obstructions...
Wavelengths of audible sound: 16.7 mm – 16.7m
Wavelengths of visible light:
400 nm – 700 nm
Audible ___________________________________________are around the same size, or even bigger, than most
obstructions (open doorway, side of a building, etc.), so they ___________________________________.
Visible _____________________________ are much much smaller than these obstructions, and so they ____________
_____________________________________________________________.
Why do dolphins, bats, SONAR, and doctors use ultrasound for localizing objects, instead of lower frequency sounds?
Interference
Can two different objects occupy the exact same location at the exact same time? Of course not … but two waves can!
When waves meet, they ____________________________ (add together), but then continue traveling unaltered, as if
they had never had contact.
The superimposition of waves can create complex wave patterns.
Constructive interference
Occurs when ________________________________________________________________________
at a given point in space.
Causes an ____________________________and thus energy) at that point.
Causes bright spots / loud sounds / increase in vibration
Destructive interference
Occurs when the ______________________________________________________________________
at a given point in space.
Causes a ________________________________(and thus energy) at that point.
Causes a decrease (or absence!) of light, sound, vibration
Constructive interference  waves are in phase
Destructive interference  waves out of phase
Principle of Superposition
When two waves interfere, the resulting displacement of the medium at any location is the _____________
______________________________________________________________________ at that same location
Position
Blue
Displacement
Red
Total
A
0
0
0
B
3
1
4
Displacement
C
D
E
F
G
Example: To find the resulting wave pattern for the two interfering waves shown above, simply add the
displacements of each wave at each point.
Is this an example of constructive or destructive interference?
You Do: Principle of Superposition
Draw the interference pattern for the two waves shown on the graph below. (Do at least to point J)
Which points have constructive
interference?
Which points have destructive interference?
Constructive or Destructive?
Two boys, each 10 m apart, are splashing, creating identical waves of wavelength 2 m that travel towards each
other. Their little sister is playing in between them, located 4 m away from one boy, 6 from the other.
Will the waves by the sister be big (due to
constructive interference) or small (due to
destructive interference)?
How can we tell?
We have to determine whether the waves are _______________________________, or not. To do this, we
can compare _______________________________ two waves. If the difference in path length equals
______________________(_____________________ 2λ, 3λ, 4λ, etc.) then the waves will be in phase.
X and Y are coherent sources of 2cm waves.
At each of the following points, determine whether they interfere constructively or destructively.
A–
B–
C --
The wavelength of a transverse wave is 4cm. At some point on the wave the displacement is -4cm. At the
same instant, at another point 50cm away in the direction of propagation of the wave, the displacement is
A. 0cm
B. 2cm
C. 4cm
D. -4cm
We have to find out what ________________ of the wave is 50 cm
away.
_______________________________
The other point is _____________________ /
______________________________-
Real life examples of interference
Noise-canceling headphones create sound waves 180o
out of phase with ambient noise to cancel out the sound
Instruments are tuned with a tuning fork by listening for
_____________ – fluctuation in the loudness of the sound
– when both the instrument and the tuning fork are played
together. If the instrument is out of tune, then the
wavelengths of the two notes will not match and will
fluctuate between constructive and destructive interference,
creating the beats. _____________________________
___________________________________________.
Standing Waves
Wave Review
What is the definition of a wave? _____________________________________________________________________
All waves ‘travel’ or carry energy from one location to another until the
wave encounters __________________________________________
_________________________________________________________.
What happens then?
Depending on the barrier, some of the wave energy may ____________
through. But some of the wave energy will be ____________________.
Waves on a String
A great example of wave reflection and the interesting patterns that result from it are waves on a string/slinky. Because
the string has a short length, the wave quickly runs into a barrier and reflects backward.
Slinky Demo
What would happen if I increased the rate of the pulses?
The original and reflected waves would ______________________, creating
odd patterns.
Are the original and reflected waves still traveling through the medium?
____________________________________________________________.
Example: Rope attached
to a fixed point. A wave
pulse travels to the end
and reflects back.
Standing Waves
If we vibrate the string at _____________________________________________________, we can create standing
waves. Standing waves have points – called ___________________ – that appear to ‘stand still’.
Standing waves result from the _____________________________________________________________________
________________________________________________________________________________________________ –
such as we get with reflected waves (of certain frequencies) along a string.
Which points ‘stand still’ (e.g. have zero displacement?
Which points have maximum displacement?
Harmonics
By adjusting the __________________________ of disturbance, standing waves of different wavelengths can be
generated.
The production of standing waves is how tones are generated in most instruments (strings, winds, brass, etc.)!
Harmonic Math
What wavelengths can be produced? __________________________________________________
Note that the fundamental frequency has _______
_____________________ (one loop) across the
string.
In other words, it has
1 * ½ wave lengths.
As we go up harmonics, we add ½ wavelengths.