homework
Foot
interference-introAB.pdf
Interference
Chapter 28: Walker
1
2
The Wave Nature of Light
• Effects due to the wave nature of light: o Interference o Diffraction
• Examples: o Ripples on a pond o Acoustic design o Reading CDs/DVDs o Pretty colors on soap bubbles o Irridescent butterflies o Etc.
3
Water Waves
• Try at home: put water in a shallow pan o Tap with two fingers o Make waves and slits with blocks of wood
4
Interference
• Constructive and destructive interference and the principle of superposition o Constructive: waves add o Destructive: waves cancel
• Examples of coherent waves: o 2 fingers tapping on water in unison (water) o Water waves through 2 narrow slits (water) o 2 loudspeakers driven by same amp (sound) o Shining laser through 2 narrow slits (light) o Easier to observe in everyday life for water than for
light because of size of wavelength
5
Begin with Water Waves Consider circular waves in a tank:
______________ = the distance from crest to crest (or trough to trough) ________ = the distance from crest to trough
crest
trough
Look along the dotted line.
What is the shape of the water on the dotted line?
crest trough crest
trough
x
crest
6
(1/2 λ) Wavelength (1 λ)
x
x
crest
trough
y
height of water above equilibrium
level
Equilibrium level
λ
7
Again, • (1 λ) = the distance from crest to crest
(or trough to trough) • (1/2 λ) = the distance from crest to trough
x
y
Crest and crest Crest and trough
Now think about the water level when wavefronts interfere
8
x
y
x
Wave 1
Wave 2
+
x
superposition
y2 y1
y1+y2
y1 y2
y1+y2
Total Constructive Interference 9
x
Wave 2
x
y Wave 1
superposition
y2
y1
y1+y2 = 0
+
Total Destructive Interference
Shifted λ/2
10
Animation: Sources in phase
Increasing source separation
• _____ when crest meets crest (constructive interference) • _____ when trough meets trough (constructive) • _____ (_____) when crest meets trough à line of nodes (destructive interference)
11
High Low Zero flat
Lab: Interference
• What patterns do you notice?
12
