 # The Wave Nature of Light

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The Wave Nature of Light
AP Physics Chapter 24

The Wave Nature of Light
24.1 Waves Versus Particles

24.1 Waves Versus Particles
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

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 Diffraction 24.1

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

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

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

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

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

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

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

S-106 A monochromatic light with a frequency of 5.35x1014 Hz is shot through a set of double slits that are m apart and the resulting interference pattern is placed on a screen 1.5 m away. What will be the angle of the 3rd order maxima?

S-107 A monochromatic light with a frequency of 5.35x1014 Hz is shot through a set of double slits that are 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?

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

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

The Wave Nature of Light
24.4 The Visible Spectrum and Dispersion

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

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

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

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

24.5 Diffraction by a Single Slit of Disk
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 Applet 24.5

The Wave Nature of Light
24.6 Diffraction Grating

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

S-109 A rabid raccoon shoots a laser through a diffraction grating. The frequency of the light is 4.11x1014 Hz. The result is a interference pattern on a wall 2.5 m from the diffraction grating. If the 1st order minima is located 12.4 cm above the center, how many slits per cm are on the diffraction grating?

The Wave Nature of Light
24.8 Interference by Thin Films

24.8 Interference by Thin Films
Occur when films are just a few wavelengths thick Soap Bubbles Oil 24.8

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

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

The Wave Nature of Light
24.10 Polarization

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

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 38o from the normal. What is the minimum thickness of oil that would cause an interference pattern to form?

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

S-108 Happy Test Day!