Interference and DiffractionPhysicsMrs. Coyle

Lights NatureWave nature (electromagnetic wave)

Particle nature (bundles of energy called photons)

Past- Separate Theories of Either Wave or Particle NatureCorpuscular theory of Newton (1670) Light corpuscles have mass and travel at extremely high speeds in straight lines

Huygens (1680) Wavelets-each point on a wavefront acts as a source for the next wavefront

Why was it difficult to prove the wave part of the nature of light?

Proofs of Wave NatureThomas Young's Double Slit Experiment (1807) bright (constructive) and dark (destructive) fringes seen on screen Thin Film Interference Patterns

Poisson/Arago Spot (1820) Diffraction fringes seen within and around a small obstacle or through a narrow opening

Proof of Particle Nature:The Photoelectric EffectAlbert Einstein 1905Light energy is quantizedPhoton is a quantum or packet of energy

The Photoelectric EffectHeinrich Hertz first observed the photoelectric effect in 1887 Einstein explained it in 1905 and won the Nobel prize for this.

Thomas Youngs Double Slit Interference ExperimentShowed an interference patternMeasured the wavelength of the light

Two Waves Interfering

Youngs Double SlitInterference Pattern

http://galileo.phys.virginia.edu/classes/USEM/SciImg/home_files/introduction_files/doubleslit.jpg

Interference of Waves FromTwo SourcesSimulation Ripple Tank http://www3.interscience.wiley.com:8100/legacy/college/halliday/0471320005/simulations6e/index.htm?newwindow=true

Interference

Youngs Double Slit Interferencehttp://galileo.phys.virginia.edu/classes/109N/more_stuff/flashlets/youngexpt4.htm

For Constructive Interference: The waves must arrive to the point of study in phase. So their path difference must be integral multiples of the wavelength:DL= nl n=0,1,2,3,

For destructive interference:, the waves must arrive to the point of study out of phase. So the path difference must be an odd multiple of l/2:

DL= n l m=1/2,3/2,5/2,.

Typical QuestionWhere is the first location of constructive or destructive interference?

Fo Constructive Interference of Waves from Two Sourcesx=Ltanqsinq= DL/dDL=nlFor small angles: Lsinq~Ltanq dsinq=nl

nl = dx L

dqLxqn=0,1,2,3,

Double Slit Interferencedsinq=nl

nl = dx LConstructive (brights) n=0,1,2,3,..Destructive (darks) n=1/2, 3/2, 5/2,..

Note:To find maximum # of fringes set q to 90o for n.

QuestionHow does x change with wavelength?

How does x change with slit distance?

ProblemTwo slits are 0.05 m apart. A laser ofwavelength 633nm is incident to the slits. A screen is placed 2m from the slits.

a) Calculate the position of the first and second bright fringe.

b) What is the maximum number of destructive interference spots there can be on either side of the central maximum?

Diffraction Gratinghttp://des.memphis.edu/lurbano/vpython/matter_interactions/spectrum/spectrum_02.jpg

Diffraction GratingLarge number of equally spaced parallel slits.Equations are same as for double slit interference but first calculate the d (slit separation) from the grating density, N. d=1/N , N slits per unit lengthdsinq=nlnl = dx LConstructive (brights) n=0,1,2,3,..Destructive (darks) n=1/2, 3/2, 5/2,..

ProblemA neon laser of wavelength 633nm is pointedat a diffraction grating of 3000lines/cm. Find the angle where the first bright occurs.(Hint: slit separation d is inverse of grating density)

DiffractionWave bends as it passes an obstacle.

Diffraction through a Narrow Slit Each part of the slit acts as a point source that interferes with the others. (Based on Huygens Principle)

Pattern of Diffraction of Light through a Narrow Slit Lwx

Intensity of the Diffraction Interference PatternsSimulation Interference of Light

http://www3.interscience.wiley.com:8100/legacy/college/halliday/0471320005/simulations6e/index.htm?newwindow=true

Diffraction from Narrow Slit wsinq=nl l= nw y L w: is the width of the slit Destructive (dark fringes): m=0,1,2,3,.

QuestionsHow does x change with the width?

How does x change with the wavelength

Diffraction around a Penny and Poison Spot

Example of Diffraction

**