chapter21

# chapter21 - 21 Alternating Current Circuits and...

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21 Alternating Current Circuits and Electromagnetic Waves Clicker Questions Question O1.01 Description: Relating object color to the wave nature of light. Question A particular object appears to be red. The reason it appears red is: 1. It absorbs only red frequencies as they hit. 2. It reF ects only red frequencies as they hit. 3. It absorbs red frequencies more than other frequencies. 4. It reF ects red frequencies more than other frequencies. 5. It emits red frequencies more than other frequencies. 6. Not enough information. Commentary Purpose: To hone the concept of “color” as a property of an object. Discussion: If an object appears red when you look at it, the light emanating from it and reaching your eye must be predominantly red in frequency. Physically, answers (2), (4), and (5) are all possible. Case (5) would describe a glowing object such as a neon sign or red LED. Most common objects, however, do not emit light, but rather reF ect some of the light striking them. A real red object will reF ect predominantly red frequencies of light, but will not perfectly absorb all other frequencies. Thus, (4) is a better answer than (2). If you interpret “a particular object” as a typical non-luminous object, (4) would be the best answer. However, (6) — not enough information — is defensible, since you were not told whether the object is luminous, which would make (5) accurate. Key Points: The apparent color of an object is determined by the mix of frequencies in the light traveling from it to the observer; the light from a red object will be dominated by light in the red portion of the spectrum. An object may emit light if it is luminous (glowing), but most objects merely reF ect some of the light incident upon them. For Instructors Only Questions that may support productive discussion: “What will you see if you look at a red object through a piece of red-tinted glass? How about through blue-tinted glass? If you shine red light on it? Or blue light? How does an RGB monitor display millions of different colors?” 252

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Alternating Current Circuits and Electromagnetic Waves 253 Question O1.02 Description: Linking optics to real-world experience of color. Question The Sun appears to be yellow when overhead because: 1. The human eye is most sensitive to yellow. 2. Yellow frequencies are transmitted better through the atmosphere. 3. Air does not absorb yellow frequencies. 4. The temperature of the Sun is 6000 K. 5. More than one of the above. 6. None of the above. Commentary Purpose: To probe your understanding of the colors of glowing objects, and to connect physics processes to your everyday experience. Discussion: Hot matter emits photons of many frequencies, but more of some frequencies than others. The higher the temperature of the matter, the more high-frequency photons are emitted. If the temperature high enough, the emitted light will be visible (have enough photons in the visible part of the spectrum), the color will be determined by the mix of photon frequencies. The higher the temperature, the closer to the blue end of the spectrum the radiated light will appear.
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chapter21 - 21 Alternating Current Circuits and...

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