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The Wave Nature of Light AP Physics Chapter 24

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The Wave Nature of Light 24.1 Waves Versus Particles

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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. 24.1

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24.1 Waves Versus Particles Particularly useful for analyzing when a wave hits an obstacle For example reflection and refraction Or diffraction – the bending of waves behind obstacles 24.1 Diffraction

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24.1 Waves Versus Particles Ray model of light can not account for diffraction 24.1

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The Wave Nature of Light 24.3 Interference – Young’s Double-Slit Experiment

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24.3 Young’s Double-Slit Experiment Thomas Young (1801) Double Slit Experiment 1.Light from a single source (sun) 2.Sent through a thin slit first (so one point source of light 3. Allowed to pass through two small slits 24.3

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24.3 Young’s Double-Slit Experiment If light is a ray we would expect two slits of light The actual pattern looked like this Caused by constructive and destructive interference 24.3

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24.3 Young’s Double-Slit Experiment The geometry of the path of two rays of light Constructive interference occurs when the path difference, equals a whole number of wavelengths Or 24.3

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24.3 Young’s Double-Slit Experiment For destructive interference the difference in distance has to be by some half wavelength So m is called the order Constructive interference – bright fringe Destructive interference – dark fringe 24.3

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24.3 Young’s Double-Slit Experiment The distance between slits will depend on how far the screen is from the slits 24.3

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S-106 A monochromatic light with a frequency of 5.35x10 14 Hz is shot through a set of double slits that are 0.0034 m apart and the resulting interference pattern is placed on a screen 1.5 m away. What will be the angle of the 3 rd order maxima?

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S-107 A monochromatic light with a frequency of 5.35x10 14 Hz is shot through a set of double slits that are 0.0034 m apart and the resulting interference pattern is placed on a screen 1.5 m away. What is the minimum path difference between two waves that will allow a dark fringe to be produced?

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24.3 Young’s Double-Slit Experiment Brightness fades as distance increases from the 0 th order maxima (central maxima) 24.3 Applet

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24.3 Young’s Double-Slit Experiment If white light is incident on slits, the central maxima is white, but rainbows occur at higher order maxima Because different colors have different wavelengths 24.3

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The Wave Nature of Light 24.4 The Visible Spectrum and Dispersion

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The Visible Spectrum Ranges from 750 nm (red) to 400 nm (violet) Dispersion - the spreading of white light into a spectrum 24.4

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24.4 The Visible Spectrum and Dispersion Prism The index of refraction for each frequency is slightly different, so the angle of refraction differs 24.4

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24.4 The Visible Spectrum and Dispersion Rainbow Caused by refraction and internal reflection 24.4

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The Wave Nature of Light 24.5 Diffraction by a Single Slit of Disk

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If light is a wave, then light from a point source will diffract around a disk and create a constructive interference point in the middle of the shadow A single slit will produce a pattern for the same reason 24.5 Applet

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The Wave Nature of Light 24.6 Diffraction Grating

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Diffraction Grating – a large number of equally spaced parallel slits The maxima are sharper and brighter for a diffraction grating Equations are the same 24.6

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S-109 A rabid raccoon shoots a laser through a diffraction grating. The frequency of the light is 4.11x10 14 Hz. The result is a interference pattern on a wall 2.5 m from the diffraction grating. If the 1 st order minima is located 12.4 cm above the center, how many slits per cm are on the diffraction grating?

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The Wave Nature of Light 24.8 Interference by Thin Films

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Occur when films are just a few wavelengths thick Soap Bubbles Oil 24.8

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24.8 Interference by Thin Films Light strikes a surface Some of the light reflects Some of the light refracts, then reflects off the other boundary If the path difference ABC = n, then constructive interference If the path difference ABC = (n+½), the destructive interference 24.8

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24.8 Interference by Thin Films The wavelength of light in a medium is given as 24.8

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The Wave Nature of Light 24.10 Polarization

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Light is Polarized when it only vibrates in one plane 24.10 Demo

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S-110 Light with a wavelength of 482 nm is incident on a thin layer of oil (n=1.45). The light strikes at an angle of 38 o from the normal. What is the minimum thickness of oil that would cause an interference pattern to form?

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S-112 Light strikes a block of plastic (n=1.4) at an angle of 28 o to the normal. Using the diagram below calculate the angle that it will exit the block. random walrus =42 o

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S-108 Happy Test Day!

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