24.7 The Spectrum of EM Waves

1. 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

2. The Spectrum of EM Waves

3. 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

4. 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)

5. More About Visible Light • Different wavelengths correspond to different colors • The range is from red (l ~7 x 10-7 m) to violet (l ~4 x 10-7 m)

6. Visible Light – Specific Wavelengths and Colors

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. Intensity of a Polarized Beam • The intensity of the polarized beam transmitted through the second polarizer (the analyzer) varies as • I = Io cos2θ • 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 • q = 0 or 180o • The intensity is zero when the transmission axes are perpendicular to each other

18. 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

19. 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

20. Stimulated Emission of an atom

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. One of Laser Applications • Laser trapping • Optical tweezers • Laser cooling

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