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24.7 The Spectrum of EM Waves According to wavelength or frequency,

the EM waves can be distinguished into various types.

There is no sharp boundary between one kind of EM wave and the next

All types of the EM radiations are produced by the same phenomenon – accelerating charges

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The Spectrum of EM Waves

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Long-wavelength EM Waves Radio Waves

Wavelengths of more than 104 m to about 0.1 m Generated by accelerating electrons in conducting

wire, such as electronic devices in LC circuit Used in radio and television communication

systems Microwaves (short-wavelength radio waves)

Wavelengths from about 0.3 m to 1 mm Well suited for radar systems Microwave ovens are an application

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Infrared Waves and Visible light Infrared waves

Wavelengths of about 10-3 m to 7 x 10-7 m Produced by objects and molecules at room

temperatures and readily absorbed by most materials

Vibrational sprectra of molecules, Remote control

Visible light Wavelength of about 7 x 10-7 m to 4 x 10-7 m Detected by the human eye Most sensitive at about 5.5 x 10-7 m (yellow-

green)

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More About Visible Light Different

wavelengths correspond to different colors

The range is from red ( ~7 x 10-7 m) to violet ( ~4 x 10-7 m)

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Visible Light – Specific Wavelengths and Colors

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Ultraviolet light and X-rays Ultraviolet (UV) light

Wavelength covers about 4 x 10-7 m to 6 x10-10 m Sun is an important source of UV light Most UV light from the Sun is absorbed in the stratosphere

by ozone (O3) X-rays

Wavelengths of about 10-8 m to 10-12 m Most common source is acceleration of high-energy

electrons bombarding a metal target Used as a diagnostic tool in medicine Wavelengths are compared to the separation distances of

atoms in solids Studying crystal and protein structures

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Gamma rays Gamma rays

Wavelengths of about 10-10m to 10-14 m Emitted by radioactive nuclei and cosmic rays Highly penetrating and cause serious damage

when absorbed by living tissue Looking at objects in different portions of the

spectrum can produce different information

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Wavelengths and Information These are images of

the Crab Nebula They are (clockwise

from upper left) taken with x-rays visible light radio waves infrared waves

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24.8 Polarization of Light Waves

The E and B vectors associated with an EM wave are perpendicular to each other and to the direction of wave propagation

Polarization is a property that specifies the directions of the E and B fields associated with an EM wave

The direction of polarization is defined to be the direction in which the electric field is vibrating

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Unpolarized Light

All directions of vibration from a wave source are possible

The resultant EM wave is a superposition of waves vibrating in many different directions

This is an example of the unpolarized wave

The arrows show a few possible directions of the waves in the beam

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Linearly Polarization of Light A wave is said to be linearly

polarized if the resultant electric field vibrates in the same direction at all times at a particular point

The plane formed by the electric field and the direction of propagation is called the plane of polarization of the wave

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Methods of Polarization It is possible to obtain a linearly

polarized beam from an unpolarized beam by removing all waves from the beam expect those whose electric field vectors oscillate in a single plane

The most common processes for accomplishing polarization of the beam is called selective absorption

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Polaroid In 1938, E. H. Land discovered a material,

with long-chain hydrocarbons, that polarizes light through selective absorption He called the material Polaroid Valence electrons can conduct along the

hydrocarbon chain The molecules readily absorb light whose electric

field vector is parallel to their lengths and allow light through whose electric field vector is perpendicular to their lengths

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Polarizer It is common to refer to the direction

perpendicular to the molecular chains as the transmission axis

In an ideal polarizer, All light with the electric field parallel to the

transmission axis is transmitted All light with the electric field perpendicular

to the transmission axis is absorbed

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Polarization by Selective Absorption

Uses a material that transmits waves whose electric field vectors in the plane parallel to a certain direction and absorbs waves whose electric field vectors are perpendicular to that direction

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Intensity of a Polarized Beam The intensity of the polarized beam

transmitted through the second polarizer (the analyzer) varies as I = Io cos2 θ

Io is the intensity of the beam incident on the analyzer This is known as Malus’ Law

The intensity of the transmitted beam is a maximum when the transmission axes are parallel = 0 or 180o

The intensity is zero when the transmission axes are perpendicular to each other

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Intensity of Polarized Light, Examples

On the left, the transmission axes are aligned and maximum intensity occurs

In the middle, the axes are at 45o to each other and less intensity occurs

On the right, the transmission axes are perpendicular and the light intensity is a minimum

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24.9 Properties of Laser Light The light is coherent

The rays maintain a fixed phase relationship with one another

There is no destructive interference The light is monochromatic

It has a very small range of wavelengths The light has a small angle of divergence

The beam spreads out very little, even over long distances

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Stimulated Emission of an atom

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Stimulated Emission Stimulated emission is required for laser

action to occur When an atom is in an excited state, an

incident photon can stimulate the electron to fall to the ground state and emit a photon

The first photon is not absorbed, so now there are two photons with the same energy traveling in the same direction

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Stimulated Emission, The two photons (incident and emitted)

are in phase They can both stimulate other atoms to

emit photons in a chain of similar processes

The many photons produced are the source of the coherent light in the laser

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Necessary Conditions for Stimulated Emission For the stimulated emission to occur,

we must have a buildup of photons in the system

The system must be in a state of population inversion More atoms must be in excited states than

in the ground state This insures there is more emission of

photons by excited atoms than absorption by ground state atoms

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More on Conditions The excited state of the system must be

a metastable state Its lifetime must be long compared to the

usually short lifetimes of excited states, which is typically 10-8 s

The energy of the metastable state is indicated by E*

In this case, the stimulated emission is likely to occur before the spontaneous emission

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Final Condition The emitted photons must be confined

in a space They must stay in the system long enough

to stimulate further emissions In a laser, this is achieved by using mirrors

at the ends of the system One end is generally reflecting and the

other end is slightly transparent to allow the beam to escape

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Schematic of a Laser Design

The tube contains atoms, which is the active medium An external energy source is needed to “pump” the

atoms to excited states The mirrors confine the photons to the tube

Mirror 2 is slightly transparent

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Energy Levels of a Ne atom in a He-Ne Laser

Collisions between atoms in the chamber raise the Ne atoms to the excited state E3*

Stimulated emission occurs when the Ne atoms make the transition to the E2 state

The result is the production of coherent light at 632.8 nm

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One of Laser Applications Laser trapping Optical tweezers Laser cooling

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Exercises of Chapter 24 1, 2, 7, 14, 19, 27, 30, 38, 45, 59, 62, 67