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Modern Optics LabLab 6 Part 1: Interference Experiments
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.
Topics
Modern Optics LabLab 6 Part 1: Interference Experiments
Double Slit Diffraction
Wave fronts of light
Double slit
Screen
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 directionWaves annihilate each other in that direction
(“destructive interference”).Darkness on that place on the screen.
Dark
Modern Optics LabLab 6 Part 1: Interference Experiments
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.
Dark
Bright
Dark
Dark
Dark
Bright
Bright
Dark
Bright
The light exits the slits in all directionssimultaneously.
A pattern of bright and dark regions appears.
Modern Optics LabLab 6 Part 1: Interference Experiments
Condition for intensity maximum:
m = 0, 1, 2 , 3…
d
y
Modern Optics LabLab 6 Part 1: Interference Experiments
a
y
Modern Optics LabLab 6 Part 1: Interference Experiments
Condition for intensity maximum:
m = 0, 1, 2 , 3…
a
y
a
mam
a
sinsinsin
Modern Optics LabLab 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 LabLab 6 Part 1: Interference Experiments
V I.A. Young’s Double Slit Experiment with Laser Light
Laser
Double SlitScreen
Vary slit separation and observe changes. Measure interference maxima spacing and determine the wavelength of the laser
from it.
Large Screen(to keep laser light within your area)
Modern Optics LabLab 6 Part 1: Interference Experiments
V I.B Young’s Double Slit Experiment with White Light
Incandescent Light
Double Slit Diffuser
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 LabLab 6 Part 1: Interference Experiments
V I.B Young’s Double Slit Experiment with White Light
Incandescent Light
Double Slit Diffuser
Use the photometer aperture to only let light through one slit.
Observe and explain.
Photometer aperture
