Emission and Absorption Spectra - AP Physics 2

1. A student compares two emission spectra from different elements and finds different sets of discrete lines. Which statement best explains the difference?
   Answer: They differ because each element has different quantized energy levels, leading to different allowed transitions and photon energies. This question tests understanding of emission and absorption spectra. Different elements have different numbers of protons in their nuclei and different electron configurations, resulting in unique sets of quantized energy levels for each element. When electrons transition between these element-specific energy levels, they emit photons with energies (and wavelengths) unique to that element. This creates distinct emission line patterns that serve as spectroscopic fingerprints for element identification. The discrete nature of these lines confirms that energy levels are quantized, not continuous. Choice B incorrectly suggests energy levels are continuous and emissions are random, which would make element identification impossible. The fundamental principle is that each element's unique atomic structure produces a unique spectral signature through its specific allowed energy transitions.
2. A student observes dark absorption lines after light passes through a cool gas. Which statement best describes what happens to an electron in an atom during absorption?
   Answer: It moves to a higher allowed energy level by absorbing a photon whose energy matches the level difference. This question tests understanding of emission and absorption spectra. When light passes through a cool gas, atoms can absorb photons whose energies exactly match the energy difference between two allowed energy levels. During absorption, an electron gains energy equal to the photon's energy and transitions from a lower energy level to a higher energy level. This removes specific wavelengths from the transmitted light, creating dark absorption lines. The electron cannot move to arbitrary energies because atomic energy levels are quantized, not continuous. Choice B incorrectly states the electron moves to a lower level during absorption, which would violate energy conservation. The strategy is to remember that absorption always involves electrons moving up in energy by gaining photon energy.
3. An atom absorbs a photon and later emits a photon of the same energy, producing an absorption line in transmission. Which statement best explains the initial absorption step?
   Answer: The electron transitions to a higher allowed energy level because the photon energy matches the level separation. This question tests understanding of emission and absorption spectra. During the absorption step, an atom absorbs a photon whose energy exactly matches the energy difference between two allowed energy levels. This absorbed energy promotes an electron from a lower energy level to a higher energy level, conserving energy in the process. The electron cannot transition to arbitrary energies because atomic energy levels are quantized. Later, the electron may spontaneously drop back down, emitting a photon of the same energy, though possibly in a different direction. Choice B incorrectly claims absorption causes the electron to move to a lower level, which would violate energy conservation. The key insight is that absorption always involves electrons gaining energy and moving to higher allowed levels.
4. A gas discharge produces an emission spectrum with only a few colored lines. Which statement best explains why only those wavelengths appear?
   Answer: Photons are emitted when electrons drop between discrete atomic energy levels, giving specific energy differences. This question tests understanding of emission and absorption spectra. In a gas discharge, electrons are excited to higher energy levels through collisions or electrical energy. When these electrons spontaneously drop back to lower energy levels, they emit photons with energies exactly equal to the difference between the initial and final energy levels. Since atoms have discrete, quantized energy levels, only specific energy differences are possible, resulting in emission at specific wavelengths only. Choice C incorrectly suggests electrons emit photons while being excited, which reverses the actual process. The fundamental principle is that spectral lines correspond to specific energy differences between quantized levels.
5. A gas absorbs photons of energy $E$ and later emits photons of the same energy $E$ in random directions. Which statement best links this to energy levels?
   Answer: A photon of energy $E$ matches a specific energy-level gap, so excitation and de-excitation involve the same gap. This question tests understanding of emission and absorption spectra. When a photon of energy E is absorbed, it must match a specific energy gap between quantized levels, exciting an electron from a lower to higher level. When the electron later drops back down, it releases a photon with the same energy E, corresponding to the same energy gap. This process explains resonance fluorescence where absorbed and emitted photons have identical energies. Choice C incorrectly states that electrons emit energy while absorbing photons, which is physically impossible and contradicts energy conservation. The strategy is: absorption and emission involve the same energy-level transitions, so photon energies must match.
6. White light passes through cool sodium vapor, and dark lines appear in the transmitted spectrum. The absorption line occurs because the atoms
   Answer: absorb photons whose energies match allowed electron transitions between quantized energy levels. This question tests understanding of emission and absorption spectra. When white light passes through cool sodium vapor, atoms absorb photons whose energies exactly match the energy differences between their quantized electron energy levels. This absorption removes specific wavelengths from the continuous spectrum, creating dark lines at those wavelengths. The absorbed energy promotes electrons from lower to higher energy levels. Choice B incorrectly suggests atoms can absorb photons of any energy, which would create a continuous absorption rather than discrete lines. Remember that absorption lines occur at wavelengths corresponding to allowed electron transitions between quantized energy levels.
7. Light from a hot, dense source passes through a cooler gas and produces a continuous spectrum with dark lines. Which statement best explains the dark lines?
   Answer: They occur because the cool gas absorbs photons that match energy-level differences of its atoms. This question tests understanding of emission and absorption spectra. When continuous light from a hot, dense source passes through a cooler gas, atoms in the cool gas absorb photons whose energies match the energy differences between their quantized energy levels. This selective absorption removes specific wavelengths from the continuous spectrum, creating dark lines at those wavelengths. The absorbed photons excite electrons to higher energy levels, and while these electrons eventually drop back down and re-emit photons, the re-emission occurs in random directions, so most don't reach the original detector. Choice B incorrectly suggests the gas emits photons that cancel incoming ones, misunderstanding the absorption mechanism. The key concept is that absorption spectra reveal the same quantized energy levels as emission spectra, just observed differently.
8. In an absorption spectrum, a photon is missing at wavelength $40$ nm after passing through a gas. Which statement best explains what happened in the gas?
   Answer: An electron absorbed that photon to jump to a higher quantized energy level. This question tests understanding of emission and absorption spectra. When a photon at 540 nm is missing from the spectrum after passing through gas, it means an atom absorbed that photon. The absorption occurred because the photon's energy exactly matched the energy difference needed to promote an electron from a lower to a higher quantized energy level. Choice B incorrectly describes emission (photon release) rather than absorption and suggests electrons drop to lower levels during absorption, which is backwards. The key principle is that missing photons in absorption spectra indicate upward electron transitions between quantized levels.
9. A student observes an emission spectrum with discrete lines from a hot, low-density gas. Which statement best explains why the spectrum is not continuous?
   Answer: Electrons occupy quantized energy levels, so photon energies correspond to specific transitions between levels. This question tests understanding of emission and absorption spectra. Hot gas atoms have electrons in various excited states due to thermal collisions. These electrons occupy specific quantized energy levels, and when they drop to lower levels, they emit photons with energies equal to the differences between those levels. Since only certain energy differences are allowed, only discrete wavelengths appear, creating a line spectrum rather than a continuum. Choice D incorrectly claims electrons emit photons when absorbing energy, confusing the emission process with absorption. The fundamental principle is: quantized energy levels produce discrete spectral lines, while continuous energy distributions produce continuous spectra.
10. A gas of atoms is heated and produces an emission spectrum with a few bright lines. Which statement best explains why only certain photon energies are emitted?
    Answer: Only certain photon energies are emitted because electrons release photons when transitioning between quantized levels. This question tests understanding of emission and absorption spectra. In a hot gas, electrons are excited to higher energy levels through thermal collisions. These excited electrons spontaneously transition back to lower energy levels, emitting photons whose energies equal the difference between the initial and final quantized energy levels. Since atoms have discrete energy levels rather than continuous bands, only specific energy differences (and thus specific photon energies) are possible. This produces the characteristic bright emission lines at specific wavelengths. Choice C incorrectly suggests electrons must first absorb photons before emitting them, confusing emission with fluorescence. The key principle is that emission spectra reveal the quantized nature of atomic energy levels through discrete photon energies.
11. A low-pressure hydrogen tube emits light that, through a spectroscope, shows several narrow colored lines rather than a rainbow. Which statement best explains the discrete lines?
    Answer: The atoms emit photons only when electrons drop between specific energy levels, so only certain photon energies occur. This question tests understanding of emission and absorption spectra. In hydrogen atoms, electrons occupy specific quantized energy levels, not continuous ones. When electrons drop from higher to lower energy levels, they emit photons with energies exactly equal to the difference between those levels. Since only certain energy differences are possible, only specific photon energies (and thus specific colors) appear as discrete lines. Choice B incorrectly claims continuous energy levels, which would produce a continuous spectrum, not discrete lines. The key strategy is: discrete spectral lines always indicate quantized energy levels with specific allowed transitions.
12. A student compares two gases and finds each has a different set of discrete emission lines. The best explanation is that different atoms have different
    Answer: quantized energy-level spacings, so the photon energies from transitions differ by element. This question tests understanding of emission and absorption spectra. Different elements have different nuclear charges and electron configurations, resulting in unique sets of quantized energy levels with different spacings. When electrons transition between these levels, they emit photons with energies equal to the level differences, which vary by element. This creates a unique spectral fingerprint for each element, allowing identification through spectroscopy. Choice D incorrectly suggests the process involves absorbing photons while emitting energy, which confuses the emission mechanism. Remember that each element's unique energy-level structure produces its characteristic spectral lines.
13. In an emission spectrum, one line corresponds to a photon energy of $3.0\ \text{eV}$. Which statement best describes the atomic process producing this line?
    Answer: An electron drops from a higher to a lower allowed energy level, releasing a photon of $3.0\ \text{eV}$. This question tests understanding of emission and absorption spectra. An emission line at 3.0 eV indicates that an electron has dropped from a higher to a lower quantized energy level, with the energy difference being exactly 3.0 eV. During this downward transition, the electron releases a photon carrying away this energy difference. This is the fundamental process behind all emission spectra. Choice B incorrectly states that electrons rise to higher levels while emitting photons, which violates energy conservation since emission releases energy. The strategy is: emission always involves electron transitions from higher to lower energy levels, releasing photons.
14. A gas discharge emits three bright lines; increasing the current makes them brighter but at the same wavelengths. Which statement best explains why wavelengths stay fixed?
    Answer: The atoms' quantized energy-level differences set photon energies, independent of intensity. This question tests understanding of emission and absorption spectra. The wavelengths of spectral lines are determined by the fixed energy differences between quantized atomic energy levels, which are intrinsic properties of the atom. Increasing the discharge current excites more atoms, making the lines brighter, but doesn't change the energy level spacing or the wavelengths emitted. Choice A incorrectly suggests that higher current changes electron energies continuously, contradicting the quantized nature of atomic energy levels. Remember that spectral line positions depend only on atomic structure, while intensity depends on the number of excited atoms.
15. Light from a star passes through a cooler outer gas layer, producing dark lines in the observed spectrum. Which statement best explains the dark lines?
    Answer: Atoms in the cooler gas absorb photons that match allowed transitions to higher quantized energy levels. This question tests understanding of emission and absorption spectra. When starlight passes through a cooler gas layer, atoms in that gas absorb photons whose energies match the differences between their quantized energy levels. These absorbed photons excite electrons to higher levels, removing specific wavelengths from the continuous stellar spectrum and creating dark absorption lines. Choice A incorrectly suggests the cool gas emits light at those wavelengths, which would create bright lines, not dark ones. The principle is that absorption occurs when photons matching allowed atomic transitions are removed from incident light.
16. A hydrogen atom emits a photon when an electron transitions from a higher to a lower energy level. Which statement best explains why only certain photon energies occur?
    Answer: Electrons can occupy only specific quantized levels, so emitted photons match allowed energy differences. This question tests understanding of emission and absorption spectra. Hydrogen atoms have electrons in quantized energy levels, meaning electrons can only exist at specific, discrete energies. When an electron transitions from a higher to a lower level, it emits a photon whose energy exactly equals the difference between those levels, producing only certain photon energies. Choice C incorrectly states that electrons emit photons when absorbing energy, which confuses emission with absorption processes. The key concept is that quantized energy levels create discrete energy differences, resulting in specific photon energies during transitions.
17. White light passes through cool sodium vapor and the transmitted spectrum contains two dark lines. Which statement best explains the absorption lines?
    Answer: Atoms absorb only photons that match energy-level gaps, exciting electrons to higher allowed energy levels. This question tests understanding of emission and absorption spectra. When white light passes through cool sodium vapor, sodium atoms absorb only those photons whose energies exactly match the energy differences between their quantized levels. These absorbed photons excite electrons from lower to higher energy levels, removing those specific wavelengths from the transmitted light and creating dark absorption lines. The two dark lines correspond to two specific allowed transitions in sodium atoms. Choice B incorrectly suggests atoms absorb all energies but re-emit only two, which violates energy conservation and quantization. The strategy is: absorption lines occur at the same wavelengths as emission lines because they involve the same energy-level transitions.
18. An atom absorbs a photon and an electron moves to a higher energy level. Which statement best describes the absorbed photon's energy?
    Answer: It equals the energy difference between the initial and final quantized levels. This question tests understanding of emission and absorption spectra. When an atom absorbs a photon, the photon's energy must exactly match the energy difference between the electron's initial level and a higher allowed level. This energy conservation ensures that the electron gains precisely the right amount of energy to make the quantum jump. Choice D incorrectly suggests energy is released during absorption and that electrons fall to lower levels, which describes emission rather than absorption. The fundamental principle is that absorbed photon energy equals the energy gap between quantized levels in upward transitions.
19. In an emission tube, electrons in atoms are excited to higher energy levels by collisions. The observed bright spectral lines are produced when electrons
    Answer: return to lower quantized energy levels and emit photons with energies equal to level differences. This question tests understanding of emission and absorption spectra. In an emission tube, collisions excite electrons to higher quantized energy levels. These excited electrons are unstable and spontaneously return to lower energy levels, emitting photons with energies exactly equal to the difference between the initial and final energy levels. Since atoms have discrete energy levels, only specific energy differences are possible, producing bright lines at specific wavelengths. Choice B incorrectly suggests electrons move to higher levels while emitting, which violates energy conservation. Remember that emission occurs when electrons transition from higher to lower quantized energy levels.
20. Two different gases produce different sets of emission lines under identical conditions. Which statement best explains why the line patterns differ?
    Answer: Different gases have different quantized energy-level spacings, so their allowed transition photon energies differ. This question tests understanding of emission and absorption spectra. Each element has a unique set of quantized energy levels determined by its nuclear charge and electron configuration. Different gases therefore have different energy-level spacings, leading to different sets of allowed transitions and thus different emission line patterns. This uniqueness allows spectroscopy to identify elements. Choice D incorrectly claims all gases have the same energy levels, which would make spectroscopic identification impossible. The key concept is: each element's unique energy-level structure produces a characteristic spectral fingerprint.

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