 # Chapter 24 Wave Nature of Light: © 2006, B.J. Lieb

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Chapter 24 Wave Nature of Light: © 2006, B.J. Lieb
Some figures electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey Giancoli, PHYSICS,6/E © 2004. Ch 24

Huygens’ Principle Wave motion is described by some rather complex equations but Huygens’ principle provides a simple way to predict wave propagation. Every point on a wave is a source of tiny wavelets that spread out in the forward direction at the speed of the wave itself. The new wave is the envelope of all the wavelets. Ch 24

Diffraction Diffraction occurs when a wave encounters an obstacle. Applying Huygens principle results in a bending of the wave behind the obstacle. Note that if light were a stream of particles, there would be no light in the shadow of the obstacle. Note that a small opening (usually it’s a slit) results in a nearly spherical wave. Ch 24

Interference We will study several situations where a wave is split in half, travels over different paths and then is reunited. Destructive Interference results when the amplitudes of the two waves add to give a smaller amplitude. Constructive Interference results when the amplitudes of the two waves add to give a larger amplitude. Ch 24

Young’s Double-Slit Interference
Waves passing through two slits will diffract and spread out. The two waves will then interfere with each other when they reach the screen. Ch 24

Young’s Double-Slit Interference
In the figure below, the path taken by the lower wave is greater by “d sin ” The waves interfere constructively when the path difference is “m ” where m is an integer. Condition for Constructive interference Ch 24

Double Slit Pattern The double slit interference pattern is a series of bright lines with the m = 0 line the brightest. The value of m is often called the order of the interference fringe. Ch 24

Small Angle Approximation
In working these problems it is often convenient to use the small angle approximation which is true when    7o Ch 24

Example 24-1: If 520-nm and 640-nm light passes through two slits 0
Example 24-1: If 520-nm and 640-nm light passes through two slits 0.50 nm apart, calculate the angle of the second order fringes for these two wavelengths. How far apart are the second-order order fringes for these two wavelengths on a screen 1.5m away? For small angles Ch 24

Diffraction Grating A diffraction grating has many slits. As additional slits are added, the interference maxima become sharper and narrower as shown below (a) for two slits and (b) for six slits. If “d” is the distance between adjacent slits, the equation for the grating is the same as for double slits. Ch 24

Using a Diffraction Grating to Produce an Atomic Spectrum
If the light striking a diffraction grating is not monochromatic (single color) then the light is spread out into its component wavelengths. The resulting pattern is called a spectrum. We can the determine the wavelength from Ch 24

Types of Spectrum Continuous Spectrum: includes all wavelengths
Line Spectrum: contains only certain discrete wavelengths characteristic of the atom. Only emitted by gases. Absorption Spectrum: continuous spectrum with dark lines characteristic of the atoms absorbing the light. Example is the solar absorption spectrum shown below. Note that double dark lines correspond to the sodium spectrum. Ch 24

Example 24-2: What is the highest spectral order that can be seen if a grating with 6000 lines per cm is illuminated with white light? White light spans the visible range The limit of the spectra is θ = 90º and in order to see the full spectra, you can figure m for each λ So only the second order of red would be seen and so m = 2 Ch 24

Dispersion of a Prism The index of refraction n depends slightly on wavelength with n being highest for short wavelengths. This allows a prism to produce a spectrum with red being deviated the least. Ch 24

Rainbows Rainbows are produced by dispersion in spherical water droplets. Red light is bent the least, so it is seen from droplets higher in the sky than violet light. This separation produces a spectrum of the sunlight. Ch 24

Single Slit Diffraction
This picture shows the diffraction pattern caused by monochromatic light on a narrow slit The single-slit pattern is produced by interference of light from different parts of the slit as shown below. Ch 24

Single Slit Diffraction II
This equation gives the angular position  of the minima of the intensity curve. D is the width of the slit. Note that if the width of the slit is made narrower, the above pattern gets wider. Ch 24

Thin Film Interference
Light reflected off of the air/oil interface interferes with light reflected off of the oil/water interface. The situation can be complicated by a phase shift that can occur during reflection. Ch 24

Phase Shift of Reflected Ray
n2 > n n2 < n1 180o phase shift no phase shift There is no single equation to solve these problems. For constructive interference, the path difference ( usually 2 t where t is the thickness) must equal m  if there is no phase shift If there is a phase shift then for constructive interference 2 t = (m+1/2) Ch 24

Example 24-4: Solar cells are often coated with a transparent thin film such as silicon monoxide SiO, n = 1.45) to minimize losses due to reflection. A silicon solar cell (n = 3.50) is coated with silicon monoxide for this purpose. Determine the minimum thickness of film that will produce the least reflection for light of wavelength 500nm. Note that there is a phase change at each reflection. In order to cancel. The path difference (2 t ) must equal half of the wavelength in SiO. Ch 24

Polarization We pictured light as a wave with transverse electric fields (E) perpendicular to transverse magnetic fields (B). Most light is a jumble of photons with different orientations of E fields (always with B field perpendicular to E). This is unpolarized light. With polarized light, all of the E fields have the same orientation. Ch 24

Polaroid Materials Plane-polarized light can be made using special materials with oriented long molecules. These molecules act like slits that pass light with E fields in the slit direction but eliminate light with perpendicular E fields. The action of polaroid materials is illustrated below Ch 24

Intensity Ch 24

First Polarizer Ch 24

Intensity Note: The first polarizer reduces the intensity by 1/2 , so the intensity after a pair of polarizers is Ch 24

Example 24-5 Unpolarized light passes through two polaroids
Example Unpolarized light passes through two polaroids. The axis of the first polaroid is vertical and the second is at 450 to the vertical. What percent of the light intensity is transmitted through the two polaroids. The first polaroid eliminates half of the light Answer = 25% A third polaroid is added at 900 to the vertical. What percent is transmitted? (Note that none would be transmitted without the polaroid at 45º.) Light emerging from the second polaroid is polarized at 45º to the vertical and this is 450 to the third polaroid. Answer = 12.5% Ch 24

Polarization by Reflection
Reflected light is partially polarized. It is 100 % plane polarized when the angle between the reflect and refracted ray is 90o. For light from air on a medium of index of refraction n, reflected light is 100 % plane polarized when: Polarized with E  page Ch 24