Week 24 PHY Learn - Waves - Diffraction (2)

1.

Recording Notice
This lesson is
being recorded

2.

King’s Interhigh Logo

3.

Learn 1: Diffraction
Waves – Diffraction

4.

Objectives
Understand what is meant by diffraction…
…and use Huygens’s construction to explain what happens to a
wave when it meets a slit or an obstacle.

5.

Page Reference
UK pages 160-162, 196-197
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International pages 116-8, 126-7

6.

Starter
• What is reflection?
Wave direction change when returning from a boundary.
• What is refraction?
Wave direction change when crossing a boundary.
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7.

Diffraction
Diffraction:
wave direction change
when passing a gap or
edge
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8.

SIM: Diffraction
Change slit width and observe effect
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8

9.

Ripple Tank
Diffraction is greatest when gap matches wavelength.
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10.

Wavelength calculation
What is the wavelength of these waves?
Speed of sound 340 ms -1
Sound 850 Hz
Light 7.5 × 10 14 Hz
Sound λ
= 340 / 850
Light λ
= 3 × 10 8 / 7.5 × 10 14 = 400 nm
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10
= 0.4 m

11.

Hearing round corners
Why can you hear though a 0.9 m
doorway but can’t see round the
doorway?
• The gap is near the size of sound
waves at 0.4 m.
• It is much larger that light waves at
400 nm.
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12.

Huygens’s Principle
Christiaan Huygens
1629_ -_1695
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12
Wave phenomena result from
perpetual points sources

13.

Front propagation
• Point sources cancel laterally
• They combine in the forward direction
• This reforms the wave front
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14.

Huygens Diffraction
• No neighbour at edge
• No laterally cancelling
• Can expand left and right
• D-shaped front forms
Refraction via Huygens Principle
5:16 min
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15.

Plenary
What is diffraction?
Spreading out at an edge or gap as a result of sideways point
propagation being blocked.
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16.

Learn 2: Diffraction Grating
Waves – Diffraction

17.

Objectives
Be able to use the below for a diffraction grating:
nλ = dsinθ
CORE PRACTICAL 8 (INT 6): Determine the wavelength of light
from a laser or other light source using a diffraction grating.
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18.

Starter
• When does constructive interference
occur?
• When a peak meets a peak.
• When does destructive interference
occur?
• When a peak meets a trough.
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18

19.

Gratings
Gratings are made up of many microscopic slits.
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20.

Grating Diffraction
• Shining a light source at a grating creates a light/dark pattern on a screen.
• Let’s examine why this happens.
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20

21.

SIM: Interference Pattern
• Replicate the pattern.
• Investigate the effect of frequency and slit width.
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22.

Reminder of interference patterns
interference pattern
constructive interference
destructive interference
phase difference (zero)
phase difference (½λ)
path difference (1½λ)
path difference (1λ)
path length (10λ)
SLIDE:
22
diffraction
path length (11λ)

23.

One more wavelength
Path difference of one leads to phase difference of one.
Exactly 1 more λ
24 λ
23 λ
So arrive in phase
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23

24.

Corresponding angles
The two angles are ‘locked’ together.
ϴ
ϴ
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24

25.

Trigonometry of one more wavelength
Opp
nλ
d
Hyp
Adj
ϴ
• The triangle created links
angle to wavelength.
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25
sin (ϴ)
=
Opp / Hyp
sin (ϴ)
=
nλ / d
d sin (ϴ)
=
nλ

26.

n = number of wavelengths
d
d sin (ϴ)
=
nλ
d sin (ϴ)
=
n λ
d sin (ϴ)
=
n
ϴ
• More, smaller, whole wavelengths
can fit in the space.
SLIDE:
26
λ

27.

Relationship between θ and n
• As angle increases so, step by step,
does path difference in multiples of
wavelength.
ϴ
ϴ
d sin(ϴ) = nλ
ϴ) = nλ
d sin(
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27

28.

Core Practical: Measuring λ
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28
Diffraction Grating Experiment 1:58 min

29.

Core Practical: Measuring λ
Adj = 1.00 m
ϴ = Tan-1 (Opp / Adj) = 6.5°
200 lines / mm
0.001 m / 200 = 5.0 × 10-6
Opp = 0.114 m
n=1
n=2
λ = d sin(ϴ) / n
λ = 5.0 × 10-6 × sin(6.5) / 2
= 5.66 × 10-7 m
= 566 nm
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30.

Plenary
What to the terms in nλ = d sin θ stand for?
• n = number of maxima
• λ = wavelength
• d = grating separation
• θ = Angle of maxima
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31.

Learn 3: Transmission
Waves – Diffraction

32.

Objectives
Understand that waves can be transmitted and reflected at an
interface between media.
Understand how a pulse-echo technique can provide
information about the position of an object…
…and how the amount of information obtained may be limited
by the wavelength of the radiation or by the duration of pulses.
Understand how diffraction experiments provide evidence for
the wave nature of electrons. Be able to use the de Broglie
equation: λ = h / p
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33.

Starter
What do waves always transfer?
What do waves sometimes transfer?
What do waves never transfer?
• Energy always
• Information sometimes
• Matter never
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33

34.

Pulse-echo
reflection (echo)
transmission
absorption
• Waves partly reflect back off boundaries in a set time.
• Some are transmitted and reflect off further boundaries.
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34

35.

Ultrasound A Scan (1 dimension)
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36.

Ultrasound scanning
• The scanners sends a pulse of sound.
• It times how long the pulse takes to
return.
• The scanner ‘knows’ the speed of
sound in the human body.
• It used distance = speed × time to
find distance.
• Many reflection build to form an
image.
SLIDE:
36
soft tissues
safely imaged

37.

High Frequency
• The pulse needs to be short in
duration.
• Otherwise the outgoing and
incoming signal will get confused.
• The pulse needs to be short in
length so the wavelengths that it is
composed of also need to be
short.
• Therefore the frequency needs to
be high; 1-20 MHz
SLIDE:
37

38.

How Ultrasound works
How Ultrasound works 1:40 min
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38
What is a 4D ultrasound scan 2.46 min

39.

Sound Navigation and ranging (SONAR)
• 8.3i
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40.

Recap
• Electrons are definitely definitely
particles; small spherical point-like
bits of matter.
• Light is definitely definitely a wave;
a long wobbly fluctuation.
• Waves are absolutely not point-like.
• Electrons cannot interfere
constructively or destructively.
• All super clear, logical with no
nagging doubts or loose ends?
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40

41.

Electron diffraction
• Just to absolutely sure lets fire some electron at a gap.
• Let’s see if they do anything wave-like, such as
diffracting and interfering.
electron gun
graphite lattice
screen
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41

42.

Electron diffraction
Electron diffraction tube 1.48 min
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42

43.

What is an electron?
probability
density
function
• At least light is definitely still a wave though.
• Right?
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43

44.

Planck’s constant
• Electromagnetic radiation is made of photons.
• Photons are little packets of energy.
• The amount is determined by the frequency.
• The constant is h, found by Max Planck.
• Energy of one photon = Planck’s constant × Frequency
• E = hf
Max Planck
1858 - 1947
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44
2.98 × 10 -19 J = 6.63 × 10 -34 Js × 4.5 × 10 14 Hz [red light]

45.

de Broglie Equation
E
=
mv2
E
=
hf
v
=
fλ
p
=
mv
mv2
=
hf
mv2
λ
λ
=
=
=
hv
λ
hv
mv2
h
mv
Louis de Broglie
1892 – 1987
• Since waves have particle like properties…
• Maybe the reverse is true…
• Particles have wave like properties
• Since particles cannot reach c, v was used
λ
SLIDE:
45
=
h
p

46.

Plenary (Explore foreshadow)
How do animal perceive the world differently to us?
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46
Electrosensory perception 1.48 min

47.

Lesson complete!
See you next lesson