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Island Physics

unit 15 - interference and diffraction

Image from Google Earth of wave diffraction in Castle Harbour, Bermuda.  Note that there are two good examples, one at the bottom left that is through a tiny gap between Charles and Rushy Islands that has produced almost perfectly circular waves, the other right in Castle Roads itself that has less curvature as it is a wider gap.   This image was spotted by a student, Peter Cooper, a few years ago when he was writing a report about a project on Nonsuch Island.
Home >> AP Physics II >> Interference and Diffraction
Optics >>

15.1 - Superposition of Waves (recap)

Objectives:
  • To understand what happens when two waves meet each other, both in phase and out of phase.
When two waves meet, if the crests and troughs coincide we get CONSTRUCTIVE interference.  If they are completely out of phase, the crest of one cancels out the trough of the other and we have DESTRUCTIVE interference.  This assumes that the amplitude and the wavelengths of the wave are the same - i.e. that they are monochromatic.
Picture

15.2 - Interference

Objectives:
  • To understand the concept of interference from two sources, such as a double slit.
  • To be able to calculate the wavelength from a simple interference pattern (lab work).
  • To know some applications of this effect.
The position of the MAXIMA (bright fringes) is given by:
\[​n\lambda=d\sin{\theta}\]
Picture
Picture
Picture
Picture
Wave Interference
Click to Run
Brand new updated PhET sim.  Now runs on iPads.
Sound
Click to Run
One of the oldie-but-goodies.  

15.3 - Diffraction

Objectives:
  • ​To understand the concept of diffraction of a wave as it passes through a single slit.
  • To be able to recognise a diffraction pattern.
  • To be able to calculate either the wavelength or the slit gap from a diffraction pattern (lab work).
When a wave meets a barrier or a slit we see changes as it goes around the edges.  Typically we consider gaps.  If the gap is much larger than the wavelength, the wave passes through with very little distortion except at the edges.  If the gap is approximately the same size as the wavelength, we get almost circular waves formed.  This is because the wave in the gap acts like a point source.
Picture
Screenshot of a PhET Simulation of diffraction where the gap is larger than the wavelength - spreading is only seen at the edges.
Picture
Screenshot of a PhET Simulation of diffraction where the gap is approximately equal to the wavelength.  The wave coming from the gap almost appears if it was a circular wave produced at the gap.
Picture
Picture

15.4 - Interference and Diffraction Together

Objectives:
  • To be able to draw or recognise the pattern formed from a double-slit experiment where both the separation and the gap width have an effect on the wave. (lab work).
Picture
Picture
Rayleigh's Criterion (see PPT below)

15.5 - Thin Films

Objectives:
  • Understand that the extra path length due to transmission and reflection of light through a thin film can cause interference patterns.
  • Be able to calculate the thickness required to either cause constructive or destructive interference.
  • To be able to explain the cause of the effect.

PPT
Michelson-Morley Experiment

Other Resources

CK-12 Interactive:  What happens when a wave meets a slit.
<< Magnetic Fields
Optics >>
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    • Gravitation and Orbits >
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      • Orbits
      • Circular Motion Animation
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      • PE and KE
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      • Work-Energy Theorem
    • Linear Momentum >
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      • Conservation of Momentum
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      • 2-D Collisions
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      • Surface Currents
      • Vertical Motion
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