OS-4 Diffraction and Interference

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OS-4 Diffraction and Interference

• Chapter 31

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Review from OS 3

• Superposition-The adding of waves
• Constructive interference-Two crests meet or 2
  troughs meet
• Increase in amplitude
• Anti-nodal
• Produces bright bands of light
• Destructive interference-A crest meets a trough
• Decrease amplitude or cancellation (NO amplitude)
• Nodal
• Produces dark bands of light

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Antinodal and Nodal

• Antinodal line-Antinodes line up and form a line
• Constructive interference
• Nodal line-Nodes line up to form a line
• Destructive interference

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Review Cont.

• Frequency-How OFTEN waves pass a point
  (waves/sec)
• Wavelength-The length of one complete crest and
  trough
• As wavelength increases, frequency decreases (and
  vice versa)

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Diffraction

• Diffraction-The bending of waves around an object
  or through an opening
• Occurs for all waves

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Diffraction

• The MOST diffraction takes place when the opening
  or obstacle is small compared to the wavelength
  of the wave (ex. Sound passing through a doorway)
• So a longer wavelengthmore diffraction
• If opening is wide compared to the wavelength,
  the wave will NOT diffract much (ex. Light)

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How to increase diffraction

• Decrease the size of the opening (for single
  slit/opening)
• Decrease the distance between openings (for
  double slit)
• Longer wavelength (Remember as wavelength
  increases, frequency decreases)

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Electromagnetic Waves

• All electromagnetic waves are the same type but
  vary according to wavelength.

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Electromagnetic Spectrum
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Youngs Experiment

• 1801-Thomas Young performed Youngs Experiment
• Greatly strengthened the wave theory of light

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Youngs Experiment

• Young passed light through a pinhole
• The spot on the 2nd screen was larger than the
  original opening (showed diffraction)
• On the 2nd screen he added 2 small openings close
  together
• On the 3rd screen he saw a series of light and
  dark regions (an interference pattern-waves
• overlapping and interfering
• with each other)

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Single Slit vs. Double Slit

• Single slit interference patterns have a wide,
  bright central band and the side bands are
  narrower and not as bright
• Double slit interference patterns-all of the
  bands are equally spaced and equally bright

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Simulations for Youngs experiment

• Colorado site http//www.colorado.edu/physics/2000
  /schroedinger/two-slit2.html
• JAVA http//www.ub.edu/javaoptics/applets/YoungEn.
  html
• Hawaii http//www.phys.hawaii.edu/teb/optics/java
  /slitdiffr/index.html
• Water simulation http//www.acoustics.salford.ac.u
  k/feschools/waves/diffract3.php

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Spectral analysis

• Another use of diffraction to help identify
  elements present in charged gases
• Colorado explained http//www.colorado.edu/physics
  /2000/quantumzone/index.html
• Spectral simulation http//phys.educ.ksu.edu/vqm/h
  tml/emission.html
• Every elements spectral line http//chemistry.bd.p
  su.edu/jircitano/periodic4.html

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Hologram

• Hologram-Interference pattern that is recorded on
  film
• Creates a 3-D image when light reflected off an
  object interferes with incoming light
• Used in forgery prevention, data storage,
  licenses, passports, money

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Hologram

• Holograms are made using laser light. Why?
• Laser light is monochromatic and coherent
• Monochromatic-one color
• Coherent-light having the same frequency,
  wavelength and phase and traveling in the same
  direction

Normal white light is not monochromatic or
coherent!
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Examples of Diffraction

• Sound diffracting http//www.acoustics.salford.ac.
  uk/feschools/waves/diffract.phpdiffraction
• Various effects (scrool down to radio waves)
  http//www.acoustics.salford.ac.uk/feschools/waves
  /diffract2.phpradiotv
• Acoustic room design http//www.acoustics.salford.
  ac.uk/feschools/waves/diffract4.phproomDesign

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MATH

• Example Light having a wavelength of 6.2 X 10-7
  meters is projected on a screen. Three bright
  and three dark bands are observed on a screen 2.0
  m away. The 3 pairs of bands measure 4.8 cm.
  What is the distance between the slits?

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MATH

• Required Problems 11
• Work on all homework!!!