Review - Explain this.

St. Mary's Physics YouTube Channel



E. Wave Nature of Light


2 Competing Theories:



 1. particle

2. wave



Wikimedia - (c) 2002 B. Crowell, GFDL licensed





Behavior of Light Supporting Wave Theory


1. Diffraction – light spreads out into the region behind an obstacle

Laser Diffraction and Interference





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St. Mary's Physics YouTube Channel






2. Interference – Young’s Double Slit - Late 1800's


Pattern created by a single light wave
 passing through 2 small slits!


(146 × 200 pixels, file size: 1 KB, MIME type: image/jpeg)

Prof Dr. Claus Jönsson


Which letters show constructive interference?

destructive interference?


Move your mouse left and right over the simulation below

[ Flash ]




St. Mary's Physics





λ/d = x/L - Flash

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St. Mary's Physics






λ/d = x/L


Monochromatic Coherent
Light Source was used


Which light source is coherent?

June 1997 Regents Test





Double Slit - Wavelength Difference-
Michael Fowler Flashlets




Name that interference / Wavelength shift?




Whole Wavelength Shift




Name that interference / Wavelength shift?





1/2 λ difference 



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St. Mary's Physics




If you draw a line from each slit to the 1st order bright line, you will notice that one line is longer than the other.



The difference between these two lines is 1λ.



Destructive Interference
1/2, 1 1/2, 2 1/2 ...
Wavelength difference




Constructive Interference

causes ...


 Bright Lines


Destructive Interference



Dark Lines



AP B Physics

Finding 2nd, 3rd, 4th
 .... order bright lines


sinq = ml/d

q = angle between slits and line

d = distance between slits

m = 0, 1, 2, .... order bright lines

m = 1/2, 1 1/2, 2 1/2 dark lines

q =  angle to bright line

x/L = ml/d

x = distance from central maxima




AP Physics

Ex) Calculate the distance between two slits in a Double Slit Experiment if the wavelength of the light used is is 5 x 10-7 m and the distance between the central maximum and the first order bright line is 2 x 10-2 m. The distance from the slits to the screen is 4 meters.


λ/d = x/L

(5 x 10-7 m)/d = (2 x 10-2 m)/4 m

Answer d = 1 x 10-4 m

Question: If they give you d, L and x and asked you to find the COLOR of the light used, What would you do? Where would you go to find the color?



1. Solve for λ use Young's Equation

2. Use 3 x 108 m/s = fλ to find the colors frequency

3. Look up the color on page 2 Ref.



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