Physics Beyond 2000

1. Physics Beyond 2000 Chapter 11 Electromagnetic Waves

2. - What are electromagnetic waves? • EM waves are energy emitted resulting from acceleration of electric charges.

3. EM Waves • They can travel through vacuum. • In vacuum, their speed = 3 × 108 ms-1 • c = f.λ • An EM wave consists of electric and magnetic fields, oscillating in phase and at right angles to each other. http://www.geo.mtu.edu/rs/back/spectrum/

4. Electromagnetic spectrum • The range of the wavelength of EM waves is enormous. • 10-14 m – 1 km • The electromagnetic spectrum is named according to the range of the wavelength and the method of production.

5. oscillating electric field direction of propagation of radio wave oscillating a.c. Radio Waves • Production: • Apply an a.c. voltage of high frequency to a pair of metal rods (dipole). • If the rods are vertical, the radio wave is also said to be vertically polarized. transmitter

6. Radio Waves • Receiver: • The receiver dipole is parallel to the direction of polarization. In this case, it is vertical. transmitter receiver oscillating electric field direction of propagation of radio wave oscillating a.c.

7. Spectrum of Radio Waves

8. Microwaves • Microwave is polarized along the length of the dipole. transmitter receiver oscillating electric field direction of propagation of microwave oscillating a.c.

9. Microwaves • Vertical metal rod can absorb the energy of the microwave. metal rod transmitter receiver oscillating electric field direction of propagation of microwave no response oscillating a.c.

10. Microwaves • Horizontal metal rod cannot absorb the energy of the microwave. metal rod transmitter receiver oscillating electric field direction of propagation of microwave oscillating a.c.

11. image of transmitter metal plate P transmitter Interference of Microwaves • At P, the wave from the transmitter meets the reflected wave. Interference occurs.

12. image of transmitter metal plate P transmitter Interference of Microwaves • We may consider it as an interference from two coherent sources, the transmitter and its image.

13. image of transmitter metal plate P transmitter Interference of Microwaves • The two sources are in anti-phase because there is a phase change of  on reflection.

14. image of transmitter metal plate P transmitter Interference of Microwaves • If the path difference at P = n., there is destructive interference.

15. image of transmitter metal plate P transmitter Interference of Microwaves • If the path difference at P = , there is constructive interference.

16. Microwave Cooking • One possible frequency of microwave is 2.45 GHz which is equal to the natural frequency of water molecules. • Microwave can set water molecules into oscillation. The water molecules absorb the energy from microwave. http://www.gallawa.com/microtech/howcook.html

17. Microwave in satellite communications • Reading the following http://www.s-t.au.ac.th/~supoet/satel.htm#1

18. Infrared Radiation • Self-reading.

19. Ultraviolet Radiation • Self-reading.

20. Visible light • Self-reading.

21. Colored Video Pictures • Self-reading.

22. incident light atom oscillating E-field Scattering of Light • Light energy is absorbed by an atom or molecule. • The atom (molecule) re-emits the light energy in all direction. • The intensity of light in initial direction is reduced.

23. scattered light atom Scattering of Light • Light energy is absorbed by an atom or molecule. • The atom (molecule) re-emits the light energy in all directions. • The intensity of light in initial direction is reduced. axis along which the atom oscillating

24. scattered light atom Scattering of Light • Note that there is not any scattered light along the direction of oscillation of the atom. • The scattered light is maximum at right angle to the axis. axis along which the atom oscillating strongest strongest

25. Why is the sky blue at noon and red at sunrise and sunset? • Why is the sky blue in daytime? • http://physics.about.com/science/physics/library/weekly/aa051600a.htm • Why is the sky red in sunset/sunrise? • http://physics.about.com/science/physics/library/weekly/aa052300a.htm

26. Why is the sky blue at noon and red at sunrise and sunset? • At noon, we see the most scattered light. • Note that the natural frequency of air molecules is in the ultraviolet region. Blue light is easily scattered by air molecules. white light from the sun Blue light is most scattered Red light is least scattered

27. Why is the sky blue at noon and red at sunrise and sunset? • At sunset, we see the least scattered light. • Red light is least scattered. white light from the sun Red light is least scattered Blue light is most scattered

28. Polarization of light • Light is transverse wave so it exhibits polarization. • Unpolarized light: the electric field is not confined to oscillate in a plane. • Plane-polarized light: the electric field at every point oscillates in the same fixed plane. • Plane of polarization: the plane in which the electric field of a plane polarized light oscillates.

29. Polarization of Light • Plane polarized light: • Unpolarised light: electric vector electric vector

30. Polarization by Absorption • An array of parallel conducting wires. • It can absorb electric field of microwave oscillating in a plane parallel to its conducting wires. Plane-polarized microwave Conducting wires are vertical No microwave E-field is vertial.

31. Polarization by Absorption • An array of parallel conducting wires. • It cannot absorb electric field of microwave oscillating in a plane perdpndicular to its conducting wires. Plane-polarized microwave Conducting wires are vertical E-field is horizontal Plane-polarized microwave

32. Polarization by Absorption • An array of parallel conducting wires. • It can be a polarizer of microwaves Unpolarized microwave Conducting wires are vertical Plane-polarized microwave

33. Polarization by Absorption • Polaroid is a plastic sheet consisting of long chains of molecules parallel to one another. • It can absorb electric field of light oscillating in a plane parallel to its chains of molecules. Chains of molecules are vertical Plane-polarized light No light E-field is vertical

34. Polarization by Absorption • Polaroid is a plastic sheet consisting of long chains of molecules parallel to one another. • It cannot absorb electric field of light oscillating in a plane perpendicular to its chains of molecules. Chains of molecules are vertical Plane-polarized light Plane-polarized light E-field is horizontal

35. Polarization by Absorption • Polaroid is a plastic sheet consisting of long chains of molecules parallel to one another. • It can be a polarizer of light. Chains of molecules are vertical Unpolarized light Plane-polarized light

36. plane-polarized incident light No reflected light air glass plane-polarized refracted light Polarization by Reflection Assume that the direction of the reflected light and that of the refracted light are perpendicular.

37. plane-polarized incident light No reflected light air glass plane-polarized refracted light Polarization by Reflection • The electric field sets the • electrons in the glass to • oscillate at right angles to • the refracted ray. • The intensity perpendicular • to the axis of oscillation is • strongest  The refracted ray is bright. • The intensity parallel to • the axis of oscillation is zero  no reflected ray.

38. Polarization by Reflection Assume that the direction of the reflected light and that of the refracted light are perpendicular. The plane of polarization is parallel to the surface of medium. Unpolarized incident light Polarized reflected light air glass Unpolarized refracted light

39. reflected ray is completely polarized incident ray p p air medium r refracted ray The Brewster’s Angle • p = Brewster’s angle • r = Angle of refraction.

40. incident ray reflected ray p p r refracted ray The Brewster’s Angle • n = tan pwhere n is the refractive index of the medium. air medium Prove it!

41. Example 1 • The Brewster’s angle for glass is about 56.3o.

42. Polarization by Scattering • When light energy is absorbed by an atom, the atom re-radiates the light. incident ray atom The atom absorbs the wave energy. The atom re-radiates the wave energy.

43. Polarization by Scattering vertically polarized light no scattered light vertically polarized light water mixed with milk vertically polarized light vertically polarized light no scattered light

44. Polarization by Scattering horizontally polarized light no scattered light horizontally polarized light water mixed with milk horizontally polarized light no scattered light horizontally polarized light

45. Polarization by Scattering horizontally polarized light vertically polarized light unpolarized light water mixed with milk unpolarized light vertically polarized light horizontally polarized light

46. Polaroid Sunglasses • Why are the polaroid sunglasses designed to absorb horizontally polarized light? Study p.233 of the textbook.

47. Interference of Light • Light is a kind of wave. • Interference is a wave property.

48. Interference of Light Conditions for an observable interference pattern of light: • Coherent sources : two sources emit light of the same frequency and maintain a constant phase difference. • The light waves are of same frequency and almost equal amplitude. • The separation of the two sources is of the same order as the wavelength. • The path difference must be not too large.

49. Interference of Light • Young’s double-slit experiment http://surendranath.tripod.com/DblSlt/DblSltApp.html http://members.tripod.com/~vsg/interfer.htm • The incident ray is split into two coherent sources • S1 and S2 by the double-slit. • S1 and S2 are in phase. • The screen is far away from the slit. D>> a. • The angles are very small.

50. Young’s double-slit experiment Suppose that there is a maximum at point P. A constructive interference occurs at P. P  S1 a central line S2 D screen