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PhysicsLO9

anniebananytree
Phys 101 LO9
Interference
Double-Slit 
The Experiment
  • Thomas Young preformed the double slit experiment in 1801 to help prove the wave nature of light.

  • He set up a source of light that illuminates two closely spaced slits and project an interference pattern on a screen some distance away.

  • If light is a particle, then only two maxima would be projected since only the particles that pass through the two slits would hit the screen. 

  • If light is a wave then then as soon as the rays pass through the slits they would diffract (diffraction is when light passes through a small opening and begins to spread out).
 
The wave nature of light was seen by the interference pattern project on the screen.
  • Where the waves meet crest to crest we would have constructive interference and the projection on the screen would be bright. 
  • Where the waves meet crest to trough there would be destructive interference and the projection on the screen would be dark.
Results
 
Geometry of the double slit experiment
  • As long as the distance from the slits to the screen is greater than the slit spacing (d<<D):
  • The distance from S1 to P is approximately equal to the distance from Q to P.
  • The extra distance the light from S2 travels to reach point P is equal to dsinθ.
  • Constructive interference occurs when dsinθ=mλ 
  • Destructive interference occus when dsinθ= (m + 0.5) λ
  • m is an integer called the spectral order. 
  • The central band projected on the screen is assigned a spectral order of =0. The bands on the sides are labelled m = 1, 2, 3...
If a yellow light with a wavelength of 540 nm shines on a double slit with the slits cut 0.0100 mm apart, what angle you should look away from the central band to see the second order fringe?
Example 1
 
Solution
 
d= 0.0100 mm =1.00e-5 m
λ= 540 nm = 540e-9 m
m=2

Equation: dsinθ=mλ
If a yellow light with a wavelength of 540 nm shines on a double slit with the slits cut 0.0100 mm apart, what angle you should look away from the central band to see the second order fringe?
Light of wavelength 575 nm falls on a double-slit and the third order bright fringe is seen at an angle of 6.5 degrees. What is the separation between the double slits? 

Example 2
 
Solution
 
θ=6.5
λ= 575 nm = 575e-9 m
m=3

Equation: dsinθ=mλ
Light of wavelength 575 nm falls on a double-slit and the third order bright fringe is seen at an angle of 6.5 degree. What is the separation between the double slits?
 
Resources
  • http://www.csun.edu/~rseki/teaching/100b/QMC24.htm
  • http://www.studyphysics.ca/newnotes/20/unit04_light/chp1719_light/lesson58.htm
  • http://www.physicsclassroom.com/class/light/Lesson-3/Young-s-Experiment


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