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# Diffraction and Interference - PowerPoint PPT Presentation

Diffraction and Interference. Diffraction Huygen’s Principle Diffraction Lab. Light. Has wave properties. Can diffract. Can constructively or destructively interfere. Wave Fronts. Lines that are perpendicular to the motion of the wave.

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Presentation Transcript

### Diffraction and Interference

Diffraction

Huygen’s Principle

Diffraction Lab

Has wave properties.

Can diffract.

Can constructively or destructively interfere.

• Lines that are perpendicular to the motion of the wave.

• Indicate the location of the crests in the waves that are traveling together.

• Wave fronts are made up of tinier wave fronts.

• Every point on any wave front is a new source for a secondary wave front.

• You can explain reflection and refraction using Huygen’s Principle.

http://www.microscopy.fsu.edu/primer/java/reflection/huygens/

As the straight waves passed through a narrow hole, they spread out in a circular pattern.

Giving proof to the fact that every point on a wave front is a new source for a new set of wavelets.

http://www.ngsir.netfirms.com/englishhtm/Diffraction.htm

Any bending of a wave around an obstacle or edges of an opening by means other than reflection or refraction.

Diffraction

• The amount of diffraction (bending) depends on the size of the wavelength compared with the size of the obstruction.

• The longer the wavelength is compared to the obstruction, the greater the diffraction.

Why would we ever want waves to bend past an obstruction?

Long AM radio waves can diffract around hills and buildings and can be received better in more places than short waves that don’t diffract as much.

Diffraction is bad when we want to see very small objects with microscopes.

If the size of the small object is the same as the wavelength of light, the image will be blurred by diffraction.

1801, Thomas Young discovered that when light of a single color (monochromatic) was directed through two closely spaced pinholes, fringes of brightness and darkness were produced on a screen.

• Bright fringes = constructive interference

• Waves arrive at the screen in phase

• Dark fringes = destructive interference

• Waves arrive at the screen out of phase

• A series of closely spaced parallel slits or grooves that are used to separate colors of light by interference.

• Different colors have different wavelengths and diffract at different rates.

• So they constructively interfere at different places.

• Interference fringes can be produced by the reflection of light from two surfaces that are very close together.

• If you shine a single-color (monochromatic) light onto stacked (with an air wedge) plates of glass, you’ll see dark and bright bands.

• The reason for the dark/bright bands is that reflected light from the top plate interferes destructively/constructively with light reflected from the bottom plate.

• Practical uses would be to test the precision of lenses.

• Straight/round fringes = perfectly flat/round glass

• Irregular fringes = irregular surface

Iridescence:The phenomenon whereby interference of light waves of mixed frequencies reflected from the top and bottom of thin films produces a spectrum of colors.

• A thin film, such as a soap bubble or oil on water, has two closely spaced surfaces.

• Light that reflects from one surface may cancel light of a certain frequency that reflects from the other surface.

http://webphysics.davidson.edu/physlet_resources/bu_semester2/c26_thinfilm.html

• If the film is illuminated with white light and the light that reflects to your eye has blue cancelled due to the reflected light from the other surface, what color will you see?

• If the film is illuminated with white light and the light that reflects to your eye has blue cancelled due to the reflected light from the other surface, what color will you see?

• The complementary color, yellow!

Same principles as Single-Color Interference

The shapes of the fringes for both are made by the differences in thickness of the materials.

Except we are using light of mixed frequencies and our fringes are made of different colors.

• Light emitted by a common lamp is incoherent. It has many phases of vibration as well as many frequencies.

• Incoherent light spreads out after a short distance and loses intensity.

• A beam of light that has the same frequency, wavelength, phase, and direction is called coherent.

• There is no interference of waves within the beam and the beam will not spread out and diffuse.

• Laser light is coherent.

• “LASER” = Light Amplification by Stimulated Emission of Radiation

In a laser, a light wave emitted from one atom stimulates the emission of light from a neighboring atom so that the crests of each wave coincide. Thus a coherent beam.

• The three-dimensional version of a photograph produced by interference patterns of laser beams.

• The interference of the laser beams produces fringe patterns on the photographic plate that record the depth of the surface of an object.

TheHologram

• The fringe pattern of a hologram diffracts light to produce wave fronts identical to the wave fronts given by the object.

TheHologram

So you see the 3-D image due to the way the hologram diffracts light and the way this diffracted light constructively and destructively interfere. In this way, holograms are like diffraction gratings.

• Every part of the hologram receives and records light from the entire object, so you can cut a hologram in half and still be able to view the whole image.

• You can magnify the image of a hologram by looking at it with light that has a longer wavelength than which it was made.