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Chapter 24

The Wave Nature of Light

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24.1 Waves Versus Particles; HuygensPrinciple and Diffraction

Huygens principle:

Every point on a wavefront acts as a pointsource; the wavefrontas it develops is

tangent to their envelope

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Examples of lights wave nature

Light can exhibit behavior specific to bothwaves and particles (later in course)

Need to know examples of each

Interference Youngs Double Slit ExperimentThe Visible Spectrum and Dispersion

Diffraction by a Single Slit

Diffraction by a Grating of slits

Interference by Thin Films

Refraction next unit 3

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24.1 Waves Versus Particles; HuygensPrinciple and Diffraction

Huygens principle is consistent with diffraction:

Diffraction occurs with water waves but not withstreams of particles.

We will examine 3 types of diffraction through

double slit

single slit

diffraction grating 4

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single slit diffraction

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24.3 Interference Youngs Double-SlitExperiment

If light is a wave, interference effects will be seen,where one part of wavefront can interact with another part.

The interference pattern that resulted was proof of wave

not particle nature of light

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If light is a wave,there should be aninterference patternwhich is a set of alternating brightand dark lines onthe screen.

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Formation of an interference patternThe interference occursbecause each point on the

screen is not the samedistance from both slits.Depending on the pathlength difference, the wavecan interfere constructively(bright spot) or destructively (dark spot).

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24.3 Interference Youngs Double-SlitExperiment

We can use geometry to find the conditions for constructive and destructive interference:

(24-2a)

(24-2b)

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sin tan

sin

x

L

xd d m

L

U U

U P

} !

! !

A ngle U is a very small angle so:

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m is the number of the order of the higher fringesfrom the bright central maximum

Since the position of the maxima (except the centralone) depends on wavelength, the first- and higher-order

fringes contain a spectrum of colors.11

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monochromatic light

white light

Youngs experiment

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24.4 The Visible Spectrum and Dispersion

Wavelengths of visible light: 400 nm to 750 nm

Shorter wavelengths are ultraviolet; longer areinfrared

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24.4 The Visible Spectrum and Dispersion

The various wavelengths of visible light refractdifferent amounts another example of the wavenature of light

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24.4 The Visible Spectrum and Dispersion

Actual rainbows are created by dispersion in tinydrops of water.

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I set My bow in the cloud, and it shall be for a sign of a covenant between Meand the earth. Genesis 9:13

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24.5 Diffraction by a Single Slit or Disk

Light will also diffract through a single slit or around an obstacle.

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24.5 Diffraction by a Single Slit or Disk

The resulting pattern of light and dark stripes iscalled a diffraction pattern.

P attern has a wide, very bright

central maximum with weakhigher order fringes.

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1 8single slit diffraction

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The minima of the single-slit diffraction patternoccur when

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cant use sin U = tan U because U is large

use tan U = x/L to get width of maxima

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24.6 Diffraction Grating

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diffraction gratings

sin 1, 2,3..

1/

m d m

d N lines cm N

P U! !

! !

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24.6 Diffraction Grating

The maxima of the diffraction pattern aredefined by

(24-4)

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Interference pattern from diffraction gratings

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23diffraction with laser

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diffraction with white light