# Diffraction and Interference

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Diffraction and Interference
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.
Indicate the location of the crests in the waves that are traveling together.

Huygen’s Principle Wave fronts are made up of tinier wave fronts.
Every point on any wave front is a new source for a secondary wave front. Show this on the board. Use large compass if you can. Ask, why can you have a straight line wave front make circular patterns in the secondary wave front? Do demonstration online.

Huygen’s Principle You can explain reflection and refraction using Huygen’s Principle. Show this on the board. Use large compass if you can. Ask, why can you have a straight line wave front make circular patterns in the secondary wave front? Do demonstration online.

Huygen’s Principle 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.

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

Diffraction Demo 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.

Is Diffraction a Good Thing?
Why would we ever want waves to bend past an obstruction?

Is Diffraction a Good Thing?
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.

Is Diffraction a Good Thing?
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.

Interference

Young’s Interference Experiment
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.

Young’s Interference Experiment
Bright fringes = constructive interference Waves arrive at the screen in phase Dark fringes = destructive interference Waves arrive at the screen out of phase

Diffraction Grating 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.

Single-Color Interference from Thin Films
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.

Single-Color Interference from Thin Films
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.

Single-Color Interference from Thin Films
Practical uses would be to test the precision of lenses. Straight/round fringes = perfectly flat/round glass Irregular fringes = irregular surface

Iridescence from Thin Films

Iridescence from Thin Films
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.

Iridescence From Thin Films
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.

Iridescence From Thin Films
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?

Iridescence From Thin Films
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!

Iridescence from Thin Films
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.

Incoherent Light 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.

Coherent Light 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 Laser light is coherent.
“LASER” = Light Amplification by Stimulated Emission of Radiation

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

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

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

The Hologram 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. for a hologram

The Hologram 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.

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