1 physics 7c ss1, lecture 5: wave and ray models standing waves reflection/refraction optics
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Physics 7C SS1, Lecture 5: Wave and Ray Models
Standing waves
Reflection/Refraction
Optics
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Agenda Determine content students want lecture
about Discuss any review material about
interference (including beats and standing waves)
Develop any current material about the ray model (reflection and refraction)
Introduce any new material about the ray model (optics)
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What should lecture cover?(1st choice)
Review of aspects of the wave model
1) Multiple-slit interference
2) Beats
3) Standing Waves
Move forward with the ray model
4) Reflection and refraction
5) Next material: lenses
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What should lecture cover?(2nd choice)
Review of aspects of the wave modeln Multiple-slit interference
1) Beats
2) Standing Waves
Move forward with the ray model
4) Reflection and refraction
5) Next material: lenses
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Interference Fundamentals = -4, -2, 0, +2… is constructive = -3, -, +… is destructive =anything else…is partial
2π 2π Unit circle
Wave 1
Wave 2
Difference€
t
T
€
x
λ
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Interference for differing frequencies: Beats
1 2 3 4 5
What type of interference occurs at each point?
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Interference for differing frequencies: Beats
1 2 3 4 5
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Interference for differing frequencies: Beats
1 2 3 4 5
The carrier frequency
The beat frequency
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Standing Waves Fundamental
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Standing Waves 2nd Harmonic
3rd Harmonic
N = “node”A = “antinode”
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Standing Waves 2nd Harmonic
3rd Harmonic
• What type of interference occurs at an antinode?
a) Constructive
b) Destructive
c) Time-dependentd) Depends which
antinode
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Testing Standing Waves When the wave looks like this:
the frequency is 45Hz. What is the lowest possible frequency?
a) 9 Hz
b) 12.5 Hz
c) 15 Hz
d) 30 Hz
e) 135 Hz
f) Other
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Testing Standing Waves When the wave looks like this:
the frequency is 45 Hz. What is the highest possible frequency?
a) 9 Hz
b) 12.5 Hz
c) 15 Hz
d) 30 Hz
e) 135 Hz
f) Other
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Testing Standing Waves The figure below is for node-node waves. Draw the same
harmonic for antinode-node waves.
If this frequency is 45 Hz. What is the lowest possible frequency?(enter a numeric response rounded to the nearest whole number, no
decimal places)
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Testing Standing Waves The figure below is for node-node waves. Draw the same harmonic for
antinode-node waves.
If this frequency is 45 Hz. What is the lowest possible frequency?(enter a numeric response rounded to the nearest whole number, no
decimal places)
a) 9 Hz
b) 12.5 Hz
c) 15 Hz
d) 30 Hz
e) 135 Hz
f) Other
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Wavefronts and Rays A wavefront represent
points of equal phase (e.g. the crest of the wave).
The ray shows the direction in which the wavefront is moving. Rays are
perpendicular to wavefronts.
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From spherical to planar wavefronts
Planar wave front Spherical wave front
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Identify the representation:1 2
(a) Both wavefront(b) Both ray(c) 1 is wavefront, 2 is ray(d) 1 is ray, 2 is wavefront
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What (typically) happens next?
1 2
(a) There is a reflected wave
(b) There is a transmitted wave
(c) Both reflected and transmitted waves
(d) Neither reflected nor transmitted waves
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Transmitted Ray: Drive a car from a well-paved road into rough
grass. Which way will it go?
(a) (b) (c)
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Transmitted Ray: Transmitted ray is bent, or “refracted”
Angle of Incidence
Angle of Refraction
Law of Refraction or “Snell’s Law:”
n1sin1=n2sin2
n : “index or refraction” speed of light in vacuum speed of light in material
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Reflected Ray Throw a ball at a wall, view from above.
Which is the correct path, assuming a perfectly elastic collision?
(a) (b) (c) (d)Depends
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Reflected Ray
Angle of Incidence
Angle of Reflection
NormalLine
Law of Reflection:
i=r
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Reflected Wavefronts
Angle of Incidence
Angle of Reflection
NormalLine
Law of Reflection:
i=r
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Observe the Water Compass
In which medium does light travel faster?
a) Air
b) Water
c) Same in both
What happens if the angle in the water gets increasingly large?
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Observe on the Optics Board What are you seeing?
Reflection Refraction Total Internal Reflection Convergence Divergence
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Light at an Interface (Activity 8.6.3 & FNT 1)
Incoming Ray
Plastic prismn = 1.5
On entering the prism (first boundary), which way will the light bend?
a) Up b) Down c) No bend
Airn = 1
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Light at an Interface
Incoming Ray
Plastic prismn = 1.5
Which Ray?(a)
(b)
(c)
Airn = 1
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Parallel rays are uncommon…how do lenses work on regular objects?
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What do we see?
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Every part of the tree reflects light, scattering it all directions.
A ray diagram is used to simplify this picture.
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