Physics 7C SS1Lecture 4

Hitting an Interface

Standing Waves

Review 2D Interference

2

Reaching the end of a medium

Observe reflection on the wave machine…

“Incident”QuickTime™ and a

TIFF (Uncompressed) decompressorare needed to see this picture.

3

Hitting an Interface

Inverted Upright

What happens if we send a continuous wave down the machine, it reflects, and the incoming & reflected waves interfere?

4

Superposition of 2 traveling harmonic waves

The period and wavelength are exactly the same. One wave travels to the right, one to the left.

5

Standing Waves

Fundamental

6

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 a node?

a) Constructive

b) Destructive

c) Time-dependent

d) Depends which node

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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

9

Hitting an Interface

Reflection and Transmission

“Incident”

or

“Reflected” “Transmitted”

“Reflected”

“Transmitted”

10

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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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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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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Observe the Water Compass

In which medium does light travel faster?a) Air

b) Water

c) Same in both

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Laser Interference

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

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Review: 2D Interference

Why is the center spot bright?a) Same frequency, same phase constant, x=0

b) Same frequency, same phase constant, x=c) Same frequency, phase constant and x are

both different such that =0

d) Same frequency, phase constant and x are both different such that =

e) None of the above

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Review: 2D Interference

Why is the 2nd dark spot dark?a) Same frequency, same phase constant, x=0

b) Same frequency, same phase constant, x=c) Same frequency, phase constant and x are

both different such that =0

d) Same frequency, phase constant and x are both different such that =

e) None of the above

x=3/2, in this case.)

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What is x?

a

b

d

c

e

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Review: 2D Interference

Why calculate d sin ?• d sin estimates the pathlength difference (x). That is, how

much further does the wave travel from slit 1 compared to from slit 2.

• When the wave splits through the slits, frequency and don’t change, so only pathlength matters!

• Constructive interference occurs if one wave travels a whole number of wavelengths further than the other

• if x=n=0 or 2, 4, 6, etc.• Destructive interference occurs if one wave travels a half

number of wavelengths further than the other • if x=(n/2)(for n odd)= or 3, 5, 7, etc.

• …so d sin = determines type: constructive, destructive, or partial