Modern Optics Lab
Lab 6 Part 1: Interference Experiments
Topics
 Young’s double slit experiment with laser light: Determine the
wavelength of the laser light.
 The double slit experiment with white light: Determine the coherence
length of white light.
Modern Optics Lab
Lab 6 Part 1: Interference Experiments
Double Slit Diffraction
Wave fronts of light
Dark
Screen
Double slit
Wave going through this slit travels a bit
further to get to this particular place on
the screen.
Waves from the two slits are out of
phase by half a wavelength in that direction
Waves annihilate each other in that direction
(“destructive interference”).
Darkness on that place on the screen.
Modern Optics Lab
Lab 6 Part 1: Interference Experiments
Dark
The light exits the slits in all directions
simultaneously.
A pattern of bright and dark
regions appears.
The waves going through both slits travel
the same distance to the screen.
Waves from the two slits in phase.
Waves add together to twice the
amplitude (“constructive interference”).
 Bright spot in center.
Bright
Modern Optics Lab
Lab 6 Part 1: Interference Experiments


d

Condition for intensity maximum:
m = 0, 1, 2 , 3…
y
Modern Optics Lab
Lab 6 Part 1: Interference Experiments

y

a

Modern Optics Lab
Lab 6 Part 1: Interference Experiments



a
y

Condition for intensity maximum:
m = 0, 1, 2 , 3…

 sin 
a
 m
  m  a sin   sin  
a
Modern Optics Lab
Lab 6 Part 1: Interference Experiments
Interference and Coherence Length
The path length difference must be less than the coherence length of the
light. Otherwise no definite phase relationship exists and the interference
pattern is not visible.
 No problem for laser light (coherence length of order or meters).
 White light has much shorter coherence length  Interference pattern
only visible for small m.
Modern Optics Lab
Lab 6 Part 1: Interference Experiments
V I.A. Young’s Double Slit Experiment with Laser Light
Double Slit
Screen
Large Screen
(to keep laser
light within
your area)
Laser
 Vary slit separation and observe changes.
 Measure interference maxima spacing and determine the wavelength of the laser
from it.
Modern Optics Lab
Lab 6 Part 1: Interference Experiments
V I.B Young’s Double Slit Experiment with White Light
Double Slit
Diffuser
Incandescent
Light
 The double slit interference pattern (if there) will appear when looking against the
direction of light propagation through the diffuser.
 Try different slit widths.
 Estimate coherence length of white light.
Modern Optics Lab
Lab 6 Part 1: Interference Experiments
V I.B Young’s Double Slit Experiment with White Light
Photometer
aperture
Double Slit
Incandescent
Light
 Use the photometer aperture to only let light through one slit.
 Observe and explain.
Diffuser