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18-Nov-10

Chapter 29Wave Properties of Light

(Beyond Refraction)

Lecture 34

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Light Wavefronts and Rays

Circular wavefronts(could be electric field wave crests)

Rays - show direction of wave motion.

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Plane Waves• “Plane” waves have wavefronts that are planes (in 3

dimensions) or straight lines (in 2 dimensions). The wavefronts are all parallel.

• Wavefronts far from a small source look like plane waves

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Huygens’ Principle• Throw a rock in a quiet pool, and

waves appear along the surface of the water.

• Huygens proposed that the wavefronts of waves spreading out from a point source can be regarded as the overlapped crests of tiny secondary waves.

• Wavefronts can be seen as made up of smaller wavefronts—this is the basis of Huygens’ principle.

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Huygens’ PrincipleEvery point of a wavefront may be considered the source of secondary wavelets that spread out in all directions with a speed equal to the speed of propagation of the waves.

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Huygens’ Principle & Diffraction• A line of small particles striking a barrier with an opening in

it would cast a sharp shadow.• Plane waves striking the same barrier would cast a “fuzzy”

shadow -- wavefronts would spread into shadow regions• Each point on the wavefront in the opening is a new source

of spherical (circular) wavefronts• As the width of the opening narrows, the spreading of

waves into the shadow region becomes more pronounced.

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Diffraction

Diffraction• Bending of waves by

means other than reflection and refraction

• Property of all kinds of waves: sound, light,...

• Seen around edges of many shadows

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DiffractionWaves diffract after passing through a narrow opening.

Plane waves passing through openings of various sizes. The smaller the opening, the greater the bending of the waves at the edges.

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DiffractionAmount of diffraction depends on wavelength of the wave compared to the size of the obstruction that casts the shadow. Diffraction effects are greater for longer wavelengths.

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Diffractiona. Light casts a sharp shadow with some

fuzziness at its edges when the opening is large compared with the wavelength of the light.

b. When the opening is very narrow, diffraction is more apparent and the shadow is fuzzier.

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DiffractionEffects of diffraction• Puts limitations on focusing images in optical

instruments– object about the same size as wavelength of light,

diffraction blurs– object smaller than wavelength of light, no image

• Example: Use of blue light in “Blue-Ray” disks allows smaller tracks and more information storage than red light used in standard DVDs

• Better radio reception in “shadows” of buildings with long radio waves (AM)

• For dolphins and medical imaging, use of shorter wavelengths gives finer detail—ultrasound

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Superposition and Interference

Superposition of waves

Waves in phase Waves out of phase

Waves partly in phase, partly out of phase

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Superposition and Interference

Interference patterns of overlapping waves from two vibrating sources

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Superposition and Interference

Interference pattern• Caused by interference between a pair of

waves

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Superposition and Interference

Interference pattern (continued)• Constructive interference produces bright

region where waves reinforce each other (waves arriving in phase).

• Destructive interference produces dark region where waves cancel each other (waves arriving half a wavelength out of phase).

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Superposition and Interference

InterferenceExperiment

Detail of InterferencePattern

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The phenomenon of interference occurs for

A. sound waves.B. light waves.C. Both A and B.D. Neither A nor B.

Interference of LightCHECK YOURSELF

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The phenomenon of interference occurs for

A. sound waves.B. light waves.C. Both A and B.D. Neither A nor B.

Explanation:Interference is a property that characterizes waves in general.

Interference of LightCHECK YOUR ANSWER

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Superposition and InterferenceSingle-color thin-film interference

• Reflection from the upper and lower surfaces of a wedge of air between two glass plates

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Superposition and InterferenceInterference colors by reflection from thin films.

• The thin film of gasoline is just the right thickness to result in the destructive interference of blue light.

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If the thin film of gasoline was a bit thinner, the wavelength to be canceled would be

A. shorter than that of blue.B. longer than that of blue.C. white.D. None of the above.

Superposition and InterferenceCHECK YOURSELF

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If the thin film of gasoline was a bit thinner, the wavelength to be canceled would be

A. shorter than that of blue.B. longer than that of blue.C. white.D. None of the above.

Superposition and InterferenceCHECK YOUR ANSWER

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If blue light were canceled from white light by the double reflection of sunlight from gasoline on a wet surface, the resulting color would likely be

A. red.B. yellowC. greenD. violet.

Superposition and InterferenceCHECK YOURSELF

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If violet light were canceled by the double reflection of sunlight from gasoline on a wet surface, the resulting color would likely be

A. red.B. yellow.C. green.D. violet.

Explanation:Yellow is the complementary color of blue.

Superposition and InterferenceCHECK YOUR ANSWER

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If you see the color blue reflected in the interference from gasoline on water, and you lower your head so a greater angle from the normal results, you’ll likely see a color having a wavelength

A. shorter than that of blue.B. longer than that of blue.C. with a white appearance.D. None of the above.

Superposition and InterferenceCHECK YOURSELF

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If you see the color blue reflected in the interference from gasoline on water, and you lower your head so a greater angle from the normal results, you’ll likely see a color having a wavelength

A. shorter than that of blue.B. longer than that of blue.C. with a white appearance.D. None of the above.

Explanation: The path through the gasoline would be longer, and a longer wavelength would be canceled. The result of a long wave being canceled is a shorter wave.

Superposition and InterferenceCHECK YOUR ANSWER

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Superposition and InterferenceInterference colors

• Note the colors in the bubble are subtractive primaries—magentas, yellows, and cyans.

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Diffraction GratingDiffraction grating - device for producing sharp optical

interference patterns• Composed of a large number of close, equally spaced

slits or lines

• Good at dispersing white light into component colors• Can see effect by viewing light reflected from DVD

Polarization of EM Waves• Direction of Electric field determines polarization

Polarization Direction

Polarization Direction

Unpolarized EM Waves• Random polarization, or unpolarized light - many

waves with different electric field directions

Light from thermal sources (incandescent light bulb, candle, sun) and fluorescent light is unpolarized.

Linear Polarization• E has constant direction

• Produced by – Reflection at certain angles– Laser– Unpolarized radiation

through polarizing filter

Er

Er

Vertical polarization

Horizontal polarization

Polarization with Polarizing Filter

• Pass unpolarized wave through polarizing filter(Material having transmission axis & absorption axis)

• E1 passes through; E2 absorbed

E1

E2

absorption axis

transmission axis

• If unpolarized EM wave strikes filter, transmitted wave is polarized:

• For light, use “molecular wires” -- Polaroid

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Two Polarizers CombinedAmount of light passing through depends on

relative alignment of transmission axes

Combination of Two Polarizers

•

Polarization by Reflection

Polaroid sunglasses with vertical transmission axis reduce “glare” from reflected light

Liquid Crystal Display (LCD)

• LCD display with polaroid

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Polarization occurs for waves that are

A. transverseB. longitudinal.C. Both A and B.D. Neither A nor B.

PolarizationCHECK YOURSELF

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Polarization occurs for waves that are

A. transverse.B. longitudinal.C. Both A and B.D. Neither A nor B.

PolarizationCHECK YOUR ANSWER

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Polarization & 3-Dim Viewing• Vision in three dimensions depends primarily on the fact

that both eyes give their impressions simultaneously (or nearly so), each eye viewing the scene from a slightly different angle.

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Polarization & 3-Dim Viewing• If you place the Polaroids in front of the projectors so

that they are at right angles to each other, and you view the polarized image with polarized glasses of the same orientation, each eye will see the proper view, as with the stereoscopic viewer.

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Key Points of Lecture 34Key Points of Lecture 34

Before Friday, read Hewitt Chap. 29 (first half).

Homework #24 due by 11:00 PM Monday Nov. 29

• Huygens’ Principle• Diffraction• Superposition and Interference• Polarization