Transcript Lecture5. ppt - UC Davis Physics

Physics 7C SS1, Lecture 5:
Wave and Ray Models
Standing waves
Reflection/Refraction
Optics
1
Agenda
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
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)
2
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
3
What should lecture cover?
(2nd choice)
Review of aspects of the wave model
n Multiple-slit interference
1) Beats
2) Standing Waves
Move forward with the ray model
4) Reflection and refraction
5) Next material: lenses
4
Interference Fundamentals
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= -4, -2, 0, +2… is constructive
= -3, -, + … is destructive
=anything else…is partial
t
2π
T
x
 2π



Unit
circle
Wave 1
Wave 2

Difference

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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
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Fundamental
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Standing Waves
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2nd Harmonic

3rd Harmonic
N = “node”
A = “antinode”
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Standing Waves
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
2nd Harmonic
3rd Harmonic
• What type of
a)
b)
c)
d)
interference occurs
at an antinode?
Constructive
Destructive
Time-dependent
Depends which
antinode
11
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
12
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
13
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)
14
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
15
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
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Spherical wave front
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Planar 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:
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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
Law of Refraction or “Snell’s Law:”
n1sin1=n2sin2
n : “index or refraction”
speed of light in vacuum
speed of light in material
Angle of
Refraction
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Reflected Ray

(a)
Throw a ball at a wall, view from above.
Which is the correct path, assuming a
perfectly elastic collision?
(b)
(c)
(d)
Depends
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Reflected Ray
Law of Reflection:
i=r
Normal
Line
Angle of
Angle of
Incidence Reflection
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Reflected Wavefronts
Law of Reflection:
i=r
Normal
Line
Angle of
Angle of
Incidence Reflection
24
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?
25
Observe on the Optics Board
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What are you seeing?
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Reflection
Refraction
Total Internal Reflection
Convergence
Divergence
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Light at an Interface
(Activity 8.6.3 & FNT 1)
Incoming Ray
Air
n=1
Plastic prism
n = 1.5
On entering the prism (first boundary), which way will
the light bend?
a) Up
b) Down
c) No bend
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Light at an Interface
Incoming Ray
(a)
Which Ray?
(b)
Air
n=1
Plastic prism
n = 1.5
(c)
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Parallel rays are uncommon…
how do lenses work on regular objects? 29
What do we see?
30
Every part of the tree reflects light,
scattering it all directions.
A ray diagram is used to
simplify this picture.
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