Version 001 HW 15 TJC Hewitt Conceptual Fundamantals sizemore (Phys fall-tjc-jts) 1. proportional to the frequency.

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1 Version 001 HW 15 TJC Hewitt Conceptual Fundamantals sizemore (Phys fall-tjc-jts) 1 This print-out should have 28 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. Concept points Greenlightisemittedwhenelectronsinasubstance make a particular energy-level transition. If blue light were instead emitted from the same substance, would it correspond to a greater or smaller change of energy in an atom? 1. The same energy change 2. Greater energy change correct 3. More information is needed. 4. Smaller energy change Higher-frequency blue light is more energetic; it corresponds to a greater change of energy in the atom. Concept points 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? 1. No change 2. Halved correct 3. Quadrupled 4. Doubled c = fλ Doubling the wavelength of light halves its frequency because the wavelength is inversely proportional to the frequency. ( ) 1 2 f (2λ) = fλ = c Lightofhalfthefrequency hashalftheenergy per photon (E f). Concept points Why doesn t a neon sign finally run out of excited atoms and produce dimmer and dimmer light? 1. There are numerous excited atoms in a neon sign; it takes a very long time for them to be exhausted. 2. None of these 3. Exhausted atoms will be re-excited by the energy source and emit light again. correct 4. The brightness of light is not determined by the number of excited atoms but by the energy difference between the excited atoms and the unexcited atoms. A neon tube does not run out of excited atoms because its atoms are re-excited over and over by the energy source, which, for neon signs, is usually electricity. Concept points Which has the greatest energy? 1. They have the same energy. 2. A photon of infrared light 3. A photon of ultraviolet light correct 4. A photon of visible light E f The higher-frequency ultraviolet photon has more energy than a photon in the visible part

2 Version 001 HW 15 TJC Hewitt Conceptual Fundamantals sizemore (Phys fall-tjc-jts) 2 of the spectrum, which in turn has more energy than a photon in the infrared part of the spectrum. Concept points Your friend reasons that if ultraviolet light can activate the process of fluorescence, infrared light should also. Your friend looks to you for approval or disapproval of this idea. What is your position? 1. Your friend could be wrong; the energy of the ultraviolet photon is higher than the energy of infrared photon, so the fluorescence activated by the ultraviolet light may not necessarily be activated by the infrared light. correct 2. It depends; we need to know the intensity and the duration of the irradiation that will be used to activate the fluorescence. 3. None of these 4. Your friend is correct; if the intensity of the infrared light is strong enough, fluorescence will be easily activated. Fluorescence is the process in which highfrequency (high energy) ultraviolet radiation converts to low-frequency (lower energy) visible radiation with some energy left over, possibly appearing as heat. If your friend is suggesting that low-energy infrared radiation can be converted to higher-energy visible light, that is clearly a violation of conservation of energy. If your friend is suggesting that infrared radiation can cause the fluorescence of still lower-frequency infrared radiation, which is not seen as light, then your friend s reasoning is well founded. Concept points We know that a lamp filament at 2500 K radiates white light. Does the lamp filament also radiate energy when it is at room temperature? 1. Yes; the filament has a smaller heat capacity than its environment. 2. No; the filament will have the same temperature as its environment. 3. Yes; the filament willbe at a temperature that is greater than absolute zero. correct 4. No; the filament will be dark at room temperature. A large filament or any object emits radiation at all temperatures greater than absolute zero. The peak freqency of this radiation is proportional to the absolute temperature of the object. At room temperature this frequency is in the far infrared part of the spectrum and therefore cannot normally be seen. When the temperature of the filament is increased, more of the radiation moves into the visible part of the spectrum and we see light. keywords: Concept (part 1 of 3) 10.0 points Consider just four of the energy levels in a certain atom, as shown in the diagram below. n = 4 n = 3 n = 2 n = 1 How many spectral lines will result from all possible transitions among these levels? Which transition corresponds to the highestfrequency light emitted? Which transition corresponds to the lowest-frequency? 1. three; level 4 to level 3 transition; level 2 to level 1 transition.

3 Version 001 HW 15 TJC Hewitt Conceptual Fundamantals sizemore (Phys fall-tjc-jts) 3 2. three; level 2 to level 1 transition; level 4 to level 3 transition. 3. three; level 4 to level 1 transition; level 4 to level 3 transition. 4. six; level 4 to level 1 transition; level 4 to level 3 transition. correct Six transitions are possible, as shown. n = 4 n = 3 n = 2 two frequencies is equal to the frequency of light emitted in the transition from quantum level 4 to the ground state, quantum level 1. Because energies are additive, so are the frequencies. However, the wavelength relationship is not that simple: wavelength is inversely proportional to the frequency, so the reciprocals of the wavelengths are additive: 1 λ λ 3 1 = 1 λ (part 3 of 3) 10.0 points Suppose the four energy levels were somehow evenly spaced. How many spectral lines would result? 1. four 2. three correct n = 1 The highest-frequency transition is from quantum level 4 to level 1. The lowestfrequency transition is from quantum level 4 to level (part 2 of 3) 10.0 points An electron de-excites from the fourth quantum level tothe thirdand then directlyto the ground state. Two photons are emitted. How does the sum of their frequencies compare to the frequency of the single photon that would be emitted by de-excitation from the fourth level directly to the ground state? 1. The sum is equal to the frequency of the single photon. correct 2. None of these 3. The sum is smaller than the frequency of the single photon. 4. The sum is larger than the frequency of the single photon. As shown in the diagram, the sum of the 3. five 4. six Only three. n = 4 n = 3 n = 2 n = 1 The transition from 4 to 3 would involve the same difference in energy and be indistinguishable from the transition from 3 to 2, or from 2 to ground. Likewise, the transition from 4 to 2 would have the same change in energy as a transition from 3 to ground. Conceptual 22 Q points A hydrogen atom and a uranium atom are moving at the same speed. Which one has the longer wavelength?

4 Version 001 HW 15 TJC Hewitt Conceptual Fundamantals sizemore (Phys fall-tjc-jts) 4 1. hydrogen correct 2. uranium 3. same for both According to the De Broigle relation, the object with smaller momentum has the longer wavelength. Since hydrogen is much less massive than uranium, it has smaller momentum and a longer wavelength. Conceptual 22 Q points Which light photon is more effective at inducing the photoelectric effect than visible light photons? 1. Visible light has more energy per photon than ultraviolet light; a visible light photon is more likely able to give an electron enough energy to escape a metal surface. 2. Ultraviolet light has more energy per photon than visible light; an ultraviolet photon is more likely able to give an electron enough energy to escape a metal surface. correct Conceptual 22 Q points Your friend, John and Jean, are both driving from Chicago to Des Moines. You know that Jean is on the road, and you know when she left Chicago. On the other hand, you know that John is on the road, but you have no idea when he left. Wave Function 2 1 Chicago Which one is for John? Des Moines correct You have no idea where John is, so it is equally likely that he could be anywhere. Thus, John s wave function has the same value everywhere. Conceptual 22 Q points If you threw baseballs through a large two-slit apparatus, would you produce a diffraction pattern? 1. Yes 2. No correct There would be no diffraction pattern because the wavelengths of baseballs are far too small to produce a noticeable interference pattern. Simply stated, baseballs are not quantum objects! Conceptual 22 Q (part 1 of 2) 10.0 points An electron and a proton are traveling at the same speed. Which one has more momentum? 1. Proton correct 2. Electron The electron has less momentum. 015 (part 2 of 2) 10.0 points Which one has a longer wavelength? 1. Proton 2. Electron correct Since the electron has less momentum, so it has a longer wavelength. Conceptual 21 Q05

5 Version 001 HW 15 TJC Hewitt Conceptual Fundamantals sizemore (Phys fall-tjc-jts) points What can be said about the Rutherford model ofanatombased onnewton slawsofmotion, the laws of thermodynamics, and the nature of electromagnetic radiation? 1. All of these correct 2. None of these 3. The electrons are accelerating, so they would be giving off energy. 4. Rutherford model of the atom could not work 5. Continuous source of energy must be supplied to the atom. Newton s law tell us that an object in circular motion is accelerating. In the Rutherford atom, the electrons are accelerating, so they would be giving off energy. Conservation of energy (first law of thermodynamics) demands that a continuous source of energy must be supplied to the atom in order to do this. Conceptual 21 Q (part 1 of 2) 10.0 points Advertisers often describe improvements in their products as a quantum leap. Is this an appropriate use of the term? I) A quantum leap is a jump with no stops in between. II) A quantum leap is an appropriate analog if the product make a significant improvement. III) A quantum leap is not an appropriate analog if the product make a significant improvement. 1. III only correct 2. I only 3. I and III only 4. II only 5. I and II only A quantum leap is a jump with no stops in between. Quantum leap is an appropriate analog if the product makes a significant improvement. 018 (part 2 of 2) 10.0 points How big is a quantum leap? 1. very small correct 2. very big 3. smaller than an atom 4. Cannot be determined Actual quantum leaps or electrons in atoms are very small indeed! Conceptual 21 Q points A 100-watt bulb becomes warm and glows brightlyenough tolight asmall room. Onthe other hand, a 100-watt laser can cut holes in steel and would not be effective at lighting a small room. What is true about the light coming from these two sources that accounts for these differences? 1. Both of these correct 2. The light bulb illuminates a room. 3. The laser produces beam of light. 4. More information is needed. 5. Neither of these In a light bulb, the photons come out in all directions, so it will illuminate a room. On the other hand, in a laser, the photons are traveling in the same direction, and they are all in phase, producing a very bright beam of

6 Version 001 HW 15 TJC Hewitt Conceptual Fundamantals sizemore (Phys fall-tjc-jts) 6 light. Conceptual 21 Q (part 1 of 2) 10.0 points Space probes often carry compact spectrometers among their scientific hardware. What kind of spectroscopy might scientists use to determine the surface composition of the cold, outer planets that orbit the Sun? 1. Each atom of molecule gives off a set of photons when it is excited. correct 2. All of these 3. The photons emit light energy when they change the energy levels. 4. Excited photons changes chemical property. 5. None of these Each different atom of molecule gives off a charactereistic set of photons when it is excited. 021 (part 2 of 2) 10.0 points How might they use spectroscopy to determine the atmospheric composition of these planets? 1. By analyzing the spectrum of the atmosphere of the planets. 2. All of these correct 3. None of these 4. By analyzing the spectrum of the surface of the planets. 5. By analyzing the molecules that compose the planets. 6. By analyzing the atoms that composed the planets. By analyzing the spectrum (frequencies of emitted photons) of the surface and atmosphere of the planets, the atoms and molecules that compose the planets can be determined. Conceptual 21 Q points Suppose a particular atom has only two allowable electron orbits. How many different wavelength photons (spectral lines) would result from all electron transitions in this atom? 1. two 2. three 3. four 4. one correct 5. None of these There would only be one photon wavelength emitted. The only allowable downward transition is from the second level to the first. Conceptual 21 Q points When you shine invisible ultraviolet light (black light) on certain objects, they glow with brilliant colors. Explain this behavior in terms of the Bohr atom. 1. None of these 2. UV light excites an electron in the atom to a higher energy level. 3. All of these correct 4. When electron moves down to a lower energy level it stops at several intermediate levels. 5. A photon of visible light is emitted for each jump to the lower level.

7 Version 001 HW 15 TJC Hewitt Conceptual Fundamantals sizemore (Phys fall-tjc-jts) 7 The UV light excites an electron in the atom to a higher energy level. When the electron moves back down to a lower energy level it does so by stopping at several intermediate levels along the way. For each jump down, a photon of visible light is emitted. Conceptual 21 Q points The leaves of a tree are bright green. What does a leaf s absorption spectrum look like? 1. The leaf would reflect all colors except green. 2. None of these 3. The leaf would absorb sunlight. 4. The leaf would absorb all colors except green. correct 5. The leaf would reflect sunlight. The leaf would absorb all colors except green. Conceptual points If you were told that fluorine is an extremely reactive element (that is, it combines readily with other elements), what other element(s) would be extremely reactive? 1.Allelements inthesamecolumncorrect 2. All elements on the same row 3. All elements after fluorine 4. None of these 5. All elements before fluorine If fluorine were extremely reactive, we would expect other elements in the same column of the periodic table to be reactive also. This is because they all have the same number of electrons in their outer shell. These elements would be H, Cl, Br, etc. Conceptual 21 Q points Consider objects that appear red. Which is not true? 1. The red coal will be peaked in the infrared, with some overlap into the visible spectrum. 2. The red sweater will have a peak in the red, but not quite as sharp as the laser. 3. The red marker will be sharply peaked in red as compared to other spectrum. correct 4. The red laser will be sharply peaked in the red, with no other colors emitted. 5. None of these The red laser will be sharply peaked in the red, with no other colors emitted. The red coal will be peaked in the infrared, with some overlap into the visible spectrum. The red sweater will have a peak in the red, but not quite as sharp as the laser. Conceptual 21 Q points In his famous experiment, Rutherford fired alphaparticlesatathingoldfilm. Mostofthe alpha particles went through the firm and a very few bounced back. Suppose instead that about one-half the alpha particles bounced back and one-half went through. How would this have changed his conclusion about the structure of the atom? 1. The nucleus is about 1 2 atom. correct 2. The atom is mostly empty space. the size of the

8 Version 001 HW 15 TJC Hewitt Conceptual Fundamantals sizemore (Phys fall-tjc-jts) 8 3. None of these 4. The nucleus covers the entire atom. 5. The atom has no nucleus. Because most of the alpha particles passed through the gold, he concluded that the atom is mostly empty space. The ones that bounced backmusthavebouncedoffaverysmalldense nucleus. If about 1 of the alpha particles 2 bounced back, he would have concluded that the nucleus of the atoms was about 1 the size 2 of the atom. Conceptual 21 Q points In the process of fluorescence, an atom absorbs a photon of ultraviolet light and emits two or more photons of visible light. Is the reverse process possible? (That is, is it possible for an atom to absorb a photon of visible light and emit photons of ultraviolet light?) 1. None of these 2. More information is needed. 3. Yes; ultraviolet photons have less energy than visible light. 4. Yes; the energy is equal; 5. No; ultraviolet photons have more energy than the visible light. correct Conservation of energy forbids this. The ultraviolet photons have more energy than the visible. The energy that comes out of the atom cannot be more than the energy that goes in.