Physics 113
Lecture 8
Quiz 2 mean 12-13 Too Low, Will make easier
Quizzes will be curved if average best-of-5 below 80
Last Time:
Standing Waves
pressure node = displacement antinode
pressure antinode = displacement node
fn 
nv
n  1,3,5, 
4L
nv
fn 
n  1,2,3, 
2L
L
Wave Nature of Light
Huygen’s (1579-1625) – Wave
Newton (1642-1727) - Particle
Maxwell’s Revolution (1864)
Electromagnetism →transverse electromagnetic wave
Faraday’s law E  B Maxwell B  E
t
c
1
 0 0
=3.0x108 m/s
t
E=cB
Permittivity Permeability
→ Wave Nature Not obvious since  small
Changing E induces Change B induces Changing E induces Changing B …
Oscillating Charge  Change E Field (Break-Off Electromagnetic Wave)
Electromagnetic Spectrum
No limit on 
remember: f=c
Red – 600 nm
Violet – 400 nm
1 nm = 10-9 m
Since light is a wave on get constructive and destructive interference
L2  L1  n (constructive)  (n  12 ) (destructive)n  0,1,2,3,
Young’s Double Slit Experiment 1803
Just like speakers – send light through slits
(view on distant screen)
From diagram:
sin  
m
d
(m  12 )
sin  
d
constructive
destructive
where m=…-3,-2,-1,0,1,2,3,…
View on a distant screen
y

L
For small  use (radians):
y  L tan 
sin   tan   
Example (red light Young’s Experiment): light coherent (in phase)
Find bright fringes
L=15 m so use small angle approx.
d=1mm=1x10-3 m
=664 nm =664x10-9 m
m
sin    
d
plug in numbers:
m=0 y=0.0 cm
m=1 y=1.0 cm
m=2 y=2.0 cm
Lm
y  L tan   L  y 
d
Diffraction Single Slit
Just like sound light diffracts
Dark Fringe (destructive interference):
sin   m

W
Note: no m=0, the center is a light fringe
m=1,2,3,,,,
Example (red light Single Slit): light coherent (in phase)
measure distance on screen between central bright
max to first minimum y=10cm
L=15 m so use small angle approx.
W=0.1mm=1x10-4 m
=?
y  L tan   L so =y/L=0.1/15=0.00667 rads
sin    
m    W  1.0 x104 (0.00667)  667 x109 m
d
• So this red light has wavelength 667 nm!
• Using either single or double slit we can measure
wavelength of light
10 cm
Qualitative: Young’s Experiment (Double slit)
sin  
m
d
as d decreases  increases -> larger fringe separation
as  increases  increases -> larger fringe separation
Qualitative: Single slit
sin   m

W
as W decreases  increases -> larger central fringe
as  increases  increases -> larger central fringe