Elementary Physics 84 Answers

1. Electrons made to vibrate to and from at a few hundred thousand hertz emit radio waves. What class of waves is emitted from electron vibrations of a few million billion hertz? At that extremely high frequency, ultraviolet light is emitted. 2. What does it mean to say an energy state is discrete? Discrete refers to something that is disconnected and separate. Other energy states do not overlap with it; each energy state has a minimum distance between it and the ones adjacent to it. 3. Which has more potential energy relative to the nucleus: electrons in inner electron shells or electrons in outer electron shells? The potential energy of electrons in outer electron shells is higher. 4. In a neon tube, what occurs immediately after an atom is excited? The excited atom in the neon tube then relaxes and produces light when it returns to its original state. 5. How does the difference in energy between energy levels relate to the energy of the photon that is emitted by a transition between those levels? The energy difference is equal to the energy of the photon that was emitted. 6. How is the energy of a photon related to its vibrational frequency? The relationship is E = hf. 7. Which has the higher frequency: red or blue light? Which has the greater energy per photon: red or blue light? The frequency of blue light is greater. It also has a higher photon energy density. 8. Can a neon atom in a glass tube be excited more than once? Explain. It is possible to repeatedly excite atoms. 9. What do the various colors displayed in the flame of a burning log indicate? Colors are released by a variety of atoms that are stimulated and then relax, generating light. 10. Which puts out the greater percentage of its energy as light: an incandescent lamp or a mercuryvapor lamp? In comparison to an incandescent bulb lamp, a mercury-vapor lamp emits a higher percentage of its energy as visible light. It's a more cost-effective light. The remaining energy is "wasted" as heat or released as invisible electromagnetic radiation frequencies. 11. What is a spectroscope, and what does it accomplish? A spectroscope is a device that separates and measures the frequencies contained in a beam of light. 12. When a gas glows, discrete colors are emitted. When a solid glows, the colors are smudged. Why? Light is emitted by electrons travelling between distinct, well-defined energy levels in isolated atoms in an excited gas. As a consequence, different light frequencies emerge. 13. How is the peak frequency of emitted light related to the temperature of its incandescent source? The absolute temperature T determines the peak frequency. When the absolute temperature is twice, the peak frequency is also doubled. 14. How does an absorption spectrum differ in appearance from an emission spectrum? On a black backdrop, discrete colored streaks of light are exhibited in an emission spectrum. On a background of a continuous spectrum of light, an absorption spectrum features gaps (black lines). 15. What are Fraunhofer lines? Fraunhofer lines are sun-emitted light absorption lines. 16. How can astrophysicists tell whether a star is receding or approaching Earth? This is seen by the Doppler effect. 17. Why is ultraviolet light, but not infrared light, effective in making certain materials fluoresce? Ultraviolet light has a higher energy per photon than visible light. The energy difference between such luminous materials is equivalent to the energy of a uv photon. Infrared light can't activate electrons in such materials since it's too weak. 18. Distinguish between fluorescence and phosphorescence. Fluorescence with a delay between excitation and de-excitation is referred to as phosphorescence. 19. What is a metastable state? A metastable state is a temporary condition of excitement. 20. Why is argon, instead of air, used inside an incandescent bulb? Argon is a non-combustible gas that won't oxidize the heated filament. The filament will last longer as a result of this. tomor 21. Distinguish between the primary and secondary excitation processes that occur in a fluorescent lamp. Electron impact on the atoms is the fundamental source of excitation. Because of photon impact, secondary excitation occurs. 22. How does the lifetime of a typical CFL compare with the lifetime of an incandescent bulb? CFL bulbs can live up to 10 times longer than incandescent bulbs. 23. How does the lifetime of a typical LED compare with the lifetime of an incandescent bulb? In comparison to an incandescent bulb, an LED bulb may live up to 100 times longer. 24. Distinguish between monochromatic light and sunlight. Photons in monochromatic light all have the same frequency (or color). Sunlight comprises a wide range of photons with various frequencies. 25. Distinguish between coherent light and sunlight. Monochromatic photons (of the same frequency) in phase with each other make up coherent light. Sunlight is made up of a wide range of photons with varying phases. 26. How does the avalanche of photons in a laser beam differ from the hordes of photons emitted by an incandescent lamp? Coherent photons are emitted by lasers. They are in phase with one other and have the same frequency. An incandescent lamp's light is made up of a spectrum of photons with random phase variations between them. 28. Borrow a diffraction grating from your physics instructor. The common kind looks like a photographic slide, and light passing through it or reflecting from it is diffracted into its component colors by thousands of finely ruled lines. Look through the grating at the light from a sodium-vapor street lamp. If it’s a low-pressure lamp, you’ll see the nice yellow spectral “line” that dominates sodium light (actually, it’s two closely spaced lines). If the street lamp is round, you’ll see circles instead of lines; if you look through a slit cut in cardboard or some similar material, you’ll see lines. What happens with the now common high-pressure sodium lamps is more interesting. Because of the collisions of excited atoms, you’ll see a smeared-out spectrum that is nearly continuous, almost like that of an incandescent lamp. Right at the yellow location, where you’d expect to see the sodium line, is a dark area. This is the sodium absorption band. It is due to the cooler sodium, which surrounds the high-pressure emission region. The sodium emission and absorption spectra given in the issue will be visible to you. 30. Why is a gamma-ray photon more energetic than an X-ray photon? It has a higher frequency. 32. Green light is emitted when electrons in a substance make a particular energy-level transition. If blue light were instead emitted from the same substance, would it correspond to a greater or lesser change of energy in the atom? It would correspond to a greater change of energy. 34. If we double the frequency of light, we double the energy of each of its photons. If we instead double the wavelength of light, what happens to the photon energy? We will cut the photon energy in half. 36. An investigator wishes spectral lines in a spectrum to be thin crescents. What change in the spectroscope will accomplish this? In the spectroscope, the investigator should employ a crescent-shaped slit. 38. If we use a prism or a diffraction grating to compare the red light from a common neon tube and the red light from a helium–neon laser, what striking difference do we see? The neon tube's red light is not monochromatic, and its emission spectrum comprises many lines, the majority of which are reddish. Use your preferred search engine to look for "neon emission spectrum." A helium-neon laser's light is monochromatic (one hue) and has only one line in its emission spectrum. Use your preferred search engine to look for "HeNe laser emission spectrum." 40. How might the Fraunhofer lines in the spectrum of sunlight that are due to absorption in the Sun’s atmosphere be distinguished from those due to absorption by gases in Earth’s atmosphere? You can look at the spectra of other stars or use a space-based observatory. 42. What difference does an astronomer see between the emission spectrum of an element in a receding star and a spectrum of the same element in the lab? (Hint: This relates to frequency measurements for a moving wave source.) A Doppler shift is visible in the light from the fading star. 44. Which has the greatest energy: a photon of infrared light, of visible light, or of ultraviolet light? The photon of ultraviolet light has the most energy. 46. Low-pressure sodium-vapor lamps emit line spectra with well-defined wavelengths, but highpressure sodium vapor lamps emit light whose lines are more spread out. Relate this to the continuous smear of wavelengths emitted by solids. The atomic energy levels in a high-pressure gas overlap in the same manner that they do in a solid or liquid. As a consequence, frequencies are dispersed and the emitted light spectrum is smoother. 48. How can a hydrogen atom, which has only one electron, have so many spectral lines? When hydrogen is stimulated, a single electron can exist in a variety of energy states. Each single transition between two of these energy levels produces a photon with a particular frequency, and there are a lot of them. to 50. If atoms of a substance absorb ultraviolet light and emit red light, what becomes of the “missing” energy The energy may be emitted as a series of lower-energy visible photons, or it could be emitted in the invisible spectrum (e.g., infrared light). Another explanation for the lost energy is that it may cause the material to heat up. 52. Your friend reasons that if ultraviolet light can activate the process of fluorescence, infrared light ought to also. Your friend looks to you for approval or disapproval of this idea. What is your position? With concerns, you concur. 54. Why are fabrics that fluoresce when exposed to ultraviolet light so bright in sunlight? They make use of the fluorescence principle. 56. Some doors have spring-and-damper combinations so they close slowly when released. How is this similar to phosphorescence? C B A 580 part six light In each situation, there is a lag in time. 58. To keep chickens warm in a chicken coop, why would a CFL be a poor choice compared with an incandescent bulb? A CFL generates very little heat. 60. What color results when a red and a green LED shine together? Yellow light is created by combining red and green light. 62. The forerunner to the laser involved microwaves rather than visible light. What does the word maser mean? MASER = Microwave Amplification by Stimulated Emission of Radiation 64. How do the avalanches of photons in a laser beam differ from the hordes of photons emitted by an incandescent lamp? The acronym MASER stands for Microwave Amplification by Stimulated Emission of Radiation. 66. In the operation of a helium–neon laser, why is it important that the metastable state in the helium atom closely match the energy level of a more-difÏcult-to-come-by metastable state in neon? In laser operation, the aim is for an excited helium atom to collide with a de-excited neon atom, boosting/exciting the latter into a higher-energy metastable state. This selective transfer of excitation energy would not occur if the two energies were not near enough. 68. A laser cannot produce more energy than is put into it. A laser can, however, produce pulses of light that have more power output than the power input required to run the laser. Explain. There is no technology that can produce more energy than it gets. Power, on the other hand, is a different story. 70. We know that an incandescent lamp filament at 2500 K radiates white light. Does the lamp filament also radiate energy when it is at room temperature? Radiation is emitted by objects that are warmer than absolute zero. The peak frequency of this radiation is related to the absolute temperature of the item; the hotter the object, the higher the peak frequency. A lamp filament radiates predominantly in the infrared part of the electromagnetic spectrum at ambient temperature. Thermal imaging cameras (which are widely used to locate heat leaks in a home's walls, ceilings, doors, and windows) can catch this radiation, which is invisible to the naked eye. The filament releases increasing radiation at all frequencies as it heats up, and the peak of that radiation shifts towards the visible spectrum. 72. Since every object has some temperature, every object radiates energy. Why, then, can’t we see objects in the dark? Infrared radiation has no effect on human vision. 74. If we continue heating a piece of initially room-temperature metal in a dark room, it will begin to glow visibly. What will be its first visible color, and why? Red 76. Sketch (a) shows a radiation curve of an incandescent solid and its spectral pattern as produced with a spectroscope. Sketch (b) shows the “radiation curve” of an excited gas and its emission spectral pattern. Sketch (c) shows the curve produced when a cool gas is between an incandescent source and the viewer; the corresponding spectral pattern is left as an exercise for you to construct. Sketch (d) shows the spectral pattern of an incandescent source as seen through a piece of green glass; you are to sketch in the corresponding radiation curve. Light is lacking at the locations of the black lines in Pattern c, which is an absorption spectrum. This is also depicted in Figure 30.9's bottom graphic. Pattern d resembles an incandescent source's radiation curve (Figure 30.7), but only the green component is permitted to light through the green glass. 78. Consider just four of the energy levels in a certain atom, as shown in the diagram below. How many spectral lines will result from all possible transitions among these levels? Which transition corresponds to the highest-frequency light emitted? To the lowest frequency light emitted Light is lacking at the locations of the black lines in Pattern c, which is an absorption spectrum. This is also depicted in Figure 30.9's bottom graphic. Pattern d resembles an incandescent source's radiation curve (Figure 30.7), but only the green component is permitted to light through the green glass. 80. For the transitions described in the preceding exercise, is there any relationship among the wavelengths of the emitted photons? There is a link between the wavelengths, yes. 82. A laboratory laser has a power of only 0.8 mW, 8 3 1024 W. Why does it seem more powerful than light from a 100-W lamp? The laser's energy is focused in a narrow region. Although the power is low, the intensity (power per unit area) is enormous.