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Transcript
Chapter 24 Electromagnetic Waves Chapter 24 ELECTROMAGNETIC WAVES PREVIEW Electromagnetic waves are produced and propagated through space by vibrating electric and magnetic fields. Light is the visible portion of the electromagnetic spectrum, which includes many other electromagnetic waves such as radio waves, ultraviolet light, and x – rays. All electromagnetic waves travel at 3 x 108 m/s in a vacuum. Visible light can be dispersed into its component colors, with the longest wavelength being red, and the shortest wavelength violet. All of the terms we apply to any waves can be applied to light and all other electromagnetic waves, such as wavelength, polarization, diffraction, and interference. The content contained in sections 1, 2, 3, and 6 of chapter 24 of the textbook is included on the AP Physics B exam. QUICK REFERENCE Important Terms electromagnetic spectrum the range of electromagnetic waves from low frequency to high frequency, or colors when white light is passed through a prism electromagnetic wave a wave which is produced by vibrating charges and propagates itself through space by the mutual generation of changing electric and magnetic fields polarized light light in which the electric fields are all in the same plane ultraviolet electromagnetic waves of frequencies higher than those of violet light speed of light in a vacuum, 3 x 108 m/s Equations and Symbols c f where c = speed of light = 3 x 108 m/s f = frequency of light λ = wavelength of light 276 Chapter 24 Electromagnetic Waves Ten Homework Problems Chapter 24 Problems 2, 8, 13, 14, 15, 16, 17, 19, 31, 43 DISCUSSION OF SELECTED SECTIONS 24.1 - 24.2 The Nature of Electromagnetic Waves and The Electromagnetic Spectrum As we briefly discussed in the last chapter, an electromagnetic wave is a vibration of electric and magnetic fields that move through space at an extremely high speed. The electric and magnetic fields in an electromagnetic wave vibrate perpendicular to each other. The electromagnetic wave spectrum, listed from lowest frequency to highest frequency, include radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays. If the visible colors of light are listed from long wavelength (low frequency) to short wavelength (high frequency), they would follow the order red, orange, yellow, green, blue, and violet (ROYGBV). Low freq Long λ High freq Short λ microwave IR UV Radio AM, FM, TV X - ray Gamma ray VISIBLE ROYGBV 24.3 The Speed of Light All electromagnetic waves are a result of the same phenomena, and although they have different wavelengths and frequencies, they all travel through a vacuum at exactly the same speed: 3 x 108 m/s, or about 670 million miles per hour. This speed is often referred to as the speed of light, although light is just one example of an electromagnetic wave. More accurately, this speed is the speed of any electromagnetic wave in a vacuum. In any case, the speed of an electromagnetic wave is given the symbol c, from the Latin word celeritas, meaning “swift”. According to Einstein’s theory of special relativity, the speed of an electromagnetic wave in a vacuum is constant for all observers regardless of any motion between the source of the electromagnetic wave and the observer. Electromagnetic waves travel at slower speeds in more dense media, such as water or glass. 277 Chapter 24 Electromagnetic Waves The speed, frequency, and wavelength of any electromagnetic wave traveleing through a vacuum is given by the wave equation: c f 24.5 The Doppler Effect and Electromagnetic Waves Even though the speed of an electromagnetic wave is the same for all observers, all observers will not measure the same value for the frequency of an electromagnetic wave, just as all listeners will not measure the same pitch from a moving sound source. The Doppler effect applies to any wave in which there is relative motion between a source and an observer. For visible light, for example, if a light source such as a star is moving toward you, you would measure its frequency a little higher than if the source were at rest, that is, the electromagnetic spectrum would be shifted toward the high frequency end (blue). Similarly, the frequency of a light source moving away from you would appear shifted to the low frequency end of the spectrum (red). Astronomers use the blueshift and red-shift of light to measure the speeds of stars. 24.6 Polarization Light is a transverse vibration of electric and a magnetic fields. If you could watch light waves coming toward you, you would see that they actually vibrate in many directions. We say the light is polarized when it is forced to vibrate in only one plane: Light can be polarized by passing it through a polarizing lens or filter. Many sunglasses are actually polarizing filters which will only pass certain vibrations of light. 278 Chapter 24 Electromagnetic Waves CHAPTER 24 REVIEW QUESTIONS For each of the multiple choice questions below, choose the best answer. 1. Which of the following best describes the vibration of an electromagnetic wave? (A) Electric and magnetic fields vibrate perpendicular to one another. (B) Electric and magnetic fields vibrate parallel to one another. (C) The electric field is vibrating and the magnetic field is stationary. (D) The magnetic field is vibrating and the electric field is stationary. (E) Both the electric field and magnetic field are stationary. 4. Violet light has a frequency of 7.5 x 1014 Hz. The wavelength of violet light is most nearly (A) 2.25 x 1023 m (B) 2.5 x 106 m (C) 4 x 10-7 m (D) 7.5 x 10-7 m (E) 2.25 x 107 m 5. The frequency of light emitted by a certain start is known to be 7 x 1014 Hz. An astronomer measures the frequency of the light from the star to be 6 x 1014 Hz. The star is (A) moving toward the astronomer (B) moving away from the astronomer (C) stationary (D) expanding (E) orbiting Earth 2. Which of the following lists the electromagnetic waves from longest wavelength to shortest wavelength? (A) x-ray, ultraviolet, visible light (B) visible light, ultraviolet, x-ray (C) ultraviolet, x-ray, visible light (D) ultraviolet, visible light, x-ray (E) x-ray, visible light, ultraviolet 3. In a vacuum, all electromagnetic waves have the same (A) amplitude (B) frequency (C) wavelength (D) intensity (E) speed 6. We say that light is polarized when it (A) vibrates in all planes (B) moves away from the observer (C) does not vibrate (D) vibrates in only one plane (E) moves toward an observer 279 Chapter 24 Electromagnetic Waves ANSWERS AND EXPLANATIONS TO CHAPTER 24 REVIEW QUESTIONS Multiple Choice 1. A The electric field in an electromagnetic wave vibrates perpendicular to the vibrating magnetic field. 2. B Visible light has the longest wavelength and lowest frequency of the three, and x-rays have the shortest wavelength and the highest frequency of the three. 3. E All electromagnetic waves travel at 3 x 108 m/s in a vacuum. 4. C c 3x108 m / s 4 x10 7 m 14 f 7.5x10 Hz 5. B Since the frequency the astronomer measures is lower than the known frequency, the light is red-shifted and the star must be moving away. 6. D All of the light passed through a polarizing filter is blocked except for that vibrating in one plane. 280