Chapter 25. Electromagnetic Waves

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1 Chapter 25 Electromagnetic Waves

2 EXAM # 3 Nov Chapter 23 Chapter 25 Powerpoint Nov. 4 Problems from previous exams Physics in Perspective (pg )

3 Units of Chapter 25 The Production of Electromagnetic Waves The Propagation of Electromagnetic Waves The Electromagnetic Spectrum Energy and Momentum in Electromagnetic Waves Polarization

4 25-2 The Propagation of Electromagnetic Waves Light from the Andromeda Galaxy, left, takes about 2 million years to reach us. From the most distant galaxies in the Hubble Deep Field image, right, it takes 13 billion years.

5 Propagation of EM Waves Electromagnetic waves are transverse waves Electromagnetic waves travel at the speed of light c Because em waves travel at a speed that is precisely the speed of light, light is an electromagnetic wave 1 o o

6 The Propagation of Electromagnetic Waves The Doppler effect applies to electromagnetic waves as well as to sound waves. The speed of the waves in vacuum does not change, but as the observer and source move with respect to one another, the frequency does change. f is the observed frequency f is the frequency emitted by the source u is the relative speed between the source and the observer The equation is valid only when u is much smaller than c

7 Doppler Effect and EM Waves A Doppler Effect occurs for em waves, but differs from that of sound waves For sound waves, motion relative to a medium is most important For light waves, the medium plays no role since the light waves do not require a medium for propagation The speed of sound depends on its frame of reference The speed of em waves is the same in all coordinate systems that are at rest or moving with a constant velocity with respect to each other

8 Doppler Equation, cont The positive sign is used when the object and source are moving toward each other The negative sign is used when the object and source are moving away from each other Astronomers refer to a red shift when objects are moving away from the earth since the wavelengths are shifted toward the red end of the spectrum

9 The speed of electromagnetic waves is independent of the motion of the source and observer. 1. True 2. False 0% 0% 0% 0%

10 Sound waves require a medium through which to travel 1. True 2. False 0% 0% 0% 0%

11 Which of the following states comparing sound waves to electromagnetic waves are false? 1. They both satisfy the wave equation 2. For both, the wave speed c is related to frequency and wavelength according to c=lf 3. They both travel through a medium 4. The Doppler effect for sound waves is different in electromagnetic waves and in sound waves 0% 0% 0% 0% 0%

12 EXAMPLE 1: Astronomers think that they have detected radiation from a star 15x10 9 light years away. How far is the star in m? d m/ s m 12 11/10/2013

13 EXAMPLE 2: When the Mars rover was deployed on the surface of Mars in July 1997, radio signals took about 12 min to travel from Earth to the rover. How far was Mars from Earth at that time? 13 11/10/2013

14 Conceptual Understanding DO IT HOME: Three electromagnetic waves have electric and magnetic fields pointing in the directions shown in the figure below. For each of the three cases, state whether the waves propagate in the +x, -x, +y, -y, +z, or -z direction /10/2013

15 EXAMPLE 3: A cell phone transmits at a frequency of 1.25 x 108 Hz. What is the wavelength of the electromagnetic wave used by this phone? 15 11/10/2013

16 EXAMPLE 4: A distant star is traveling directly away from Earth with a speed of km/s. By what factor are the wavelengths in this star's spectrum changed? By what factor are the wavelengths in this star's spectrum changed? 16 11/10/2013

17 EXAMPLE 5: (a) How fast would a motorist have to be traveling for a yellow (λ= 590 nm) traffic light to appear green (λ = 550 nm) because of the Doppler shift? (b) Should the motorist be traveling toward or away from the traffic light to see this effect? Explain 17 11/10/2013

18 EXAMPLE 6: Do It at Home Most of the galaxies in the universe are observed to be moving away from Earth. Suppose a particular galaxy emits orange light with a frequency of x Hz. If the galaxy is receding from Earth with a speed of 3025 km/s, what is the frequency of the light when it reaches Earth? (Enter your answer to 4 significant figures.) 18 11/10/2013

19 25-3 The Electromagnetic Spectrum of EM Waves Forms of electromagnetic waves exist that are distinguished by their frequencies and wavelengths c = ƒλ Wavelengths for visible light range from 400 nm to 700 nm There is no sharp division between one kind of em wave and the next

20 The EM Spectrum Note the overlap between types of waves Visible light is a small portion of the spectrum Types are distinguished by frequency or wavelength

21 25-3 The Electromagnetic Spectrum Radio waves are the lowest-frequency electromagnetic waves that we find useful. Radio and television broadcasts f 6 ~ 10 Hz to 10 9 Hz l ~ 300m to 0. 3m Microwaves are used for cooking and also for telecommunications. Radar system f 9 ~ 10 Hz to Hz l ~ 300mm to 0. 3mm

22 25-3 The Electromagnetic Spectrum Infrared waves are felt as heat by humans. Remote controls operate using infrared radiation. Readily absorbed by most materials Vipers see the infrared rays given off by a warm-blooded prey. Used by remote controls TVs to DVD players l ~ 0.3mm to 700nm f ~ 10 Hz to 4.3x10 Hz

23 25-3 The Electromagnetic Spectrum Visible light has a fairly narrow frequency range f 14 ~ 4.3x10 Hz to 7.5x10 14 Hz l ~ 700nm to 400nm Ultraviolet light starts with frequencies just above those of visible light. These rays cause tanning, burning, and skin cancer. Some insects can see in the ultraviolet, and some flowers have special markings that are only visible under ultraviolet light. f 14 ~ 7.5x10 Hz to Hz l ~ 400nm to 3nm X-rays have higher frequencies still. They are used for medical imaging. f 17 ~ 10 Hz to Hz l ~ 3nm to nm

25-3 The Electromagnetic Spectrum Gamma rays have the highest frequencies of all. These rays are extremely energetic, and are produced in nuclear reactions.

24 25-3 The Electromagnetic Spectrum Gamma rays have the highest frequencies of all. These rays are extremely energetic, and are produced in nuclear reactions. They are destructive to living cells and are therefore used to destroy cancer cells and to sterilize food. f 20 ~ 10 Hz and higher l ~ 0.003nm and smaller

25 EXAMPLE 7: How many red wavelengths (λ = 705 nm) tall are you? 25 11/10/2013

26 EXAMPLE 8: A cell phone transmits at a frequency of 1.75 x 10 8 Hz. What is the wavelength of the electromagnetic wave used by this phone? 26 11/10/2013

27 EXAMPLE 9: (a) Which color of light has the higher frequency, red or violet? (b) Calculate the frequency or blue light with a wavelength of 470 nm, and red light with a wavelength of 680 nm /10/2013

28 EXAMPLE 10: (a) Which color of light has the higher frequency, red or violet? (b) Calculate the frequency or blue light with a wavelength of 470 nm, and red light with a wavelength of 680 nm /10/2013

29 25-4 Energy and Momentum in EM Waves Energy carried by em waves is shared equally by the electric and magnetic fields E 0 1 o B

30 25-4 Energy of EM Waves The ratio of the electric field to the magnetic field is equal to the speed of light c Electromagnetic waves carry energy as they travel through space, and this energy can be transferred to objects placed in their path E B

31 25-4 Energy and Momentum in Electromagnetic Waves Intensity in terms of electric and magnetic fields, An electromagnetic wave also carries momentum:

32 EXAMPLE 11: The maximum magnitude of the electric field in an electromagnetic wave is V/m. What is the maximum magnitude of the magnetic field in this wave? 32 11/10/2013

33 EXAMPLE 12: What is the maximum value of the electric field in an electromagnetic wave whose maximum intensity is 5.00 W/m /10/2013

34 25-5 Polarization The polarization of an electromagnetic wave refers to the direction of its electric field.

35 EXAMPLE 13: Vertically polarized light with an intensity of 0.55 W/m 2 passes through a polarizer whose transmission axis is at an angle of 65.0 with the vertical. What is the intensity of the transmitted light? /10/2013

36 EXAMPLE 13: A person riding in a boat observes that the sunlight reflected by the water is polarized parallel to the surface of the water. The person is wearing polarized sunglasses with the polarization axis vertical. If the wearer leans at an angle of 21.5 to the vertical, what fraction of the reflected light intensity will pass through the sunglasses? 36 11/10/2013

37 25-5 Polarization Polarized light has its electric fields all in the same direction. Unpolarized light has its electric fields in random directions.

38 A beam of unpolarized light can be polarized by passing it through a polarizer, which allows only a particular component of the electric field to pass through. Here is a mechanical analog: 25-5 Polarization

39 25-5 Polarization A polarizer will transmit the component of light in the polarization direction:

40 25-5 Polarization Since the intensity of light is proportional to the square of the field, the intensity of the transmitted beam is given by the Law of Malus: The light exiting from a polarizer is polarized in the direction of the polarizer.

41 25-5 Polarization A polarizer and an analyzer can be combined; the final intensity is:

42 25-5 Polarization This means that sunlight will be polarized, depending on the angle our line of sight makes with the direction to the Sun.

43 25-5 Polarization Polarization can also occur when light reflects from a smooth surface:

44 Summary of Chapter 25 Electromagnetic waves can travel through a vacuum; their speed in a vacuum is always the same: Doppler effect: Electromagnetic waves can have any frequency.

45 Summary of Chapter 25 The entire range of frequencies is called the electromagnetic spectrum. Named portions of the spectrum, from the lowest frequencies to the highest, are radio waves; microwaves; infrared; visible light; ultraviolet; X-rays; and gamma rays. Relationship of frequency and wavelength: Energy density of an electromagnetic wave:

46 Summary of Chapter 25 The polarization of a beam of light is the direction of its electric field. A polarizer transmits only light whose electric field has a component along the polarizer s axis. An initially polarized beam of light encountering a polarizer at an angle θ has transmitted intensity:

Which type of electromagnetic wave has a wavelength longer than that of yellow light? A. Infrared radiation C. X-rays B. Gamma Rays D.

Which type of electromagnetic wave has a wavelength longer than that of yellow light? A. Infrared radiation C. X-rays B. Gamma Rays D. UV Rays Science Starter! 10.14-15.13! THE UNIVERSE AND ELECTROMAGNETIC