Emission and Absorption Spectra
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AP Physics 2 › Emission and Absorption Spectra
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?
The electron transitions to a higher allowed energy level because the photon energy matches the level separation.
The electron remains in place, and the line appears because intensity determines which wavelengths can propagate.
The electron transitions to any energy value because atomic levels are continuous at low pressure.
The electron transitions to a lower allowed energy level because absorbing a photon releases energy.
Explanation
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.
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?
It moves to a lower allowed energy level by absorbing a photon and emitting the energy difference.
It moves to a higher allowed energy level by absorbing a photon whose energy matches the level difference.
It stays in the same level, and the missing wavelengths occur because intensity sets the allowed transitions.
It can move to any energy because energy levels are continuous, so absorption removes random wavelengths.
Explanation
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.
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?
Only certain photon energies are emitted because electrons must absorb photons before emitting them at the same energy.
Only certain photon energies are emitted because line spacing is determined by the brightness of the hot gas.
Only certain photon energies are emitted because electrons release photons when transitioning between quantized levels.
Only certain photon energies are emitted because atoms have continuous energy bands but emit intermittently.
Explanation
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.
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?
An electron emitted that photon while absorbing energy and dropping to a lower level.
The photon was removed because the beam intensity sets which wavelengths can propagate.
An electron absorbed that photon to jump to a higher quantized energy level.
An electron absorbed that photon because it can take any energy in a continuous band.
Explanation
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.
A low-pressure hydrogen gas tube is excited by an electric discharge and viewed through a spectroscope, showing discrete bright lines. Which statement best explains the discrete lines?
They occur because electrons emit photons only when dropping between allowed energy levels, giving specific energies.
They occur because the spacing of lines is set by the light intensity from the discharge, not energy levels.
They occur because electron energies in the atom are continuous, producing all wavelengths but only some are detected.
They occur because electrons must emit energy when absorbing photons, so only certain colors appear.
Explanation
This question tests understanding of emission and absorption spectra. When electrons in hydrogen atoms are excited by the electric discharge, they jump to higher energy levels. These excited electrons then spontaneously drop back down to lower energy levels, emitting photons with energies exactly equal to the difference between the initial and final energy levels. Since atoms have quantized (discrete) energy levels rather than continuous ones, only specific energy transitions are possible, producing photons of specific wavelengths that appear as discrete bright lines. Choice A incorrectly suggests energy levels are continuous, which would produce a continuous spectrum rather than discrete lines. The key strategy is to remember that discrete spectral lines always indicate quantized energy levels with specific allowed transitions.
An atom absorbs a photon and an electron moves to a higher energy level. Which statement best describes the absorbed photon’s energy?
It equals the energy difference between the initial and final quantized levels.
It is determined by the brightness of the light source, not by level spacing.
It equals the average energy of all possible electron energies in the atom.
It is released by the electron during absorption as it falls to a lower level.
Explanation
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.
White light passes through cool sodium vapor and a dark yellow line appears in the transmitted spectrum. The absorption line occurs because
line position is determined by how bright the source is, not by energy levels.
electrons absorb only photons matching allowed energy-level gaps, promoting them to higher levels.
electrons emit photons at all energies, removing yellow light from the beam.
electrons have continuous energies, so they absorb a narrow band by resonance.
Explanation
This question tests understanding of emission and absorption spectra. When white light passes through cool sodium vapor, sodium atoms absorb photons whose energies exactly match the energy differences between quantized levels. These absorbed photons promote electrons from lower to higher energy levels, removing those specific wavelengths from the transmitted light and creating dark absorption lines. Choice B incorrectly suggests electrons emit photons during absorption, which reverses the actual process where absorption involves electrons gaining energy and moving up. The fundamental principle is that absorption lines occur when photons matching allowed energy transitions are removed from continuous light.
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
rates of absorbing photons while emitting energy, which selects element-specific wavelengths.
continuous energy-level distributions, producing different interference patterns.
light intensities, which set the allowed wavelengths emitted by each gas.
quantized energy-level spacings, so the photon energies from transitions differ by element.
Explanation
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.
White light passes through cool sodium vapor, and dark lines appear in the transmitted spectrum. The absorption line occurs because the atoms
absorb photons whose energies match allowed electron transitions between quantized energy levels.
absorb photons of any energy, but only certain wavelengths are removed by collisions.
emit photons while absorbing them, canceling most colors except a few wavelengths.
absorb more strongly at wavelengths where the light source intensity is lowest.
Explanation
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.
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?
The electrons emit energy when they absorb photons, producing only a few allowed colors.
The atoms have continuous energy levels, so the spectrum should appear as separated bands instead of a rainbow.
The spacing of the lines is determined mainly by the intensity of the discharge current in the tube.
The atoms emit photons only when electrons drop between specific energy levels, so only certain photon energies occur.
Explanation
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.