General Physics 2 Light as a Wave 1

The Nature of Light

• When studying geometric optics, we used a ray model to describe the behavior of light.

• A wave model of light is necessary to describe phenomena such as:• interference• diffraction

• A particle model of light is necessary to describe phenomena observed in modern physics, for example, the interaction between light and atoms. We’ll get back to this later...

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Wave Nature of Light

• Christian Huygens (1629-1695)• contemporary of Newton • developed wave theory of light

• Huygen’s Principle• Every point on a wave front can

be considered as a source of tiny wavelets that spread out in the forward direction at the speed of the wave itself.

• The new wave front is the envelope of all the wavelets - tangent to all of them

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Huygen’s Principle

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Diffraction

• Huygen’s Principle is useful for understanding diffraction - the bending of waves behind obstacles into the shadow region

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Interference

• Thomas Young (1773-1829)• definitively (at least

temporarily) demonstrates wave nature of light

• Young’s Double-Slit Experiment• coherent light passes through 2

slits, S1 and S2

• light from S1 and S2 then interferes and pattern of dark and light spots is observed on the screen

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Interference

• Constructive interference occurs when• d sin = m , m = 0,1,2,...• m = order

• Destructive interference occurs when• d sin = (m + 1/2) , m = 0,1,2,...

• Source must be coherent • waves at S1 and S2 are in-phase

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what you see on the screen:

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Think-Pair-Share

• Monochromatic light falling on two slits 0.016 mm apart produces the fifth-order fringe at an 8.8 degree angle. What is the wavelength of the light used?

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Conceptual Question

• What happens to the interference pattern if the wavelength of light is increased from 500 nm to 700 nm?

• What happens instead if the wavelength stays at 500 nm but the slits are moved farther apart?

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Pair Problem

• Light of wavelength 680 nm falls on two slits and produces an interference pattern in which the fourth-order fringe is 38 mm from the central fringe on a screen 2.0 m away. What is the separation of the two slits?(Hint: tan = for small angles, and angles must be in radians!)

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Diffraction by a Disk

• Diffracted light interferes constructively at center of shadow• requires a point source of monochromatic light (e.g. laser)

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Diffraction by a Single Slit

• D sin = m • m = 1, 2, 3, ...

position of minima

for m=1, theta gives 1/2 width of central maximum

Motivation for making large diameter telescopes

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Diffraction Grating

• a large number of equally spaced parallel slits

• same relation as double-slit• d sin = m • m = 0, 1, 2, ...

• produces sharper and narrower interference patterns that double slit

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Diffraction Grating (L)

• double slit versus diffraction grating

• for multi-wavelength light

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Emission Tubes

• Look at several emission tubes using diffraction gratings & sketch spectrum

• Foundation of spectroscopy, a technique used in numerous scientific applications

Element

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Interference by Thin Films

Produces rings of constructive/destructive

interference

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Thin-Film Interference: Examples

Soap bubblesOil on water

Beetles

Butterflies

Group Problems

• Q2B.6, Q2S.9

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