Emission spectrum of oxygen. When an electrical discharge is passed through a substance, its atoms and molecules absorb energy, which is reemitted as EM radiation. The discrete nature of these emissions implies that the energy states of the atoms and molecules are quantized. Such atomic spectra were used as analytical tools for many decades before it was understood why they are quantized. (credit: Teravolt, Wikimedia Commons)
It was a major puzzle that atomic spectra are quantized. Some of the best minds of 19th-century science failed to explain why this might be. Not until the second decade of the 20th century did an answer based on quantum mechanics begin to emerge. Again a macroscopic or classical body of gas was involved in the studies, but the effect, as we shall see, is due to individual atoms and molecules.
Phet explorations: models of the hydrogen atom
How did scientists figure out the structure of atoms without looking at them? Try out different models by shooting light at the atom. Check how the prediction of the model matches the experimental results.
The visible spectrum of sunlight shows a range of colors from red to violet. This spectrum has numerous dark lines spread throughout it. Noting that the surface of the Sun is much cooler than the interior, so that the surface is comparable to a cool gas through which light passes, which of the following statements correctly explains the dark lines?
The cooler, denser surface material scatters certain wavelengths of light, forming dark lines.
The atoms at the surface absorb certain wavelengths of light, causing the dark lines at those wavelengths.
The atoms in the Sun’s interior emit light of specific wavelength, so that parts of the spectrum are dark.
The atoms at the surface are excited by the high interior temperatures, so that the dark lines are merely wavelengths at which those atoms don’t emit energy.
A log in a fireplace burns for nearly an hour, at which point it consists mostly of small, hot embers. These embers glow a bright orange and whitish-yellow color. Describe the characteristics of the energy of this system, both in terms of energy transfer and the quantum behavior of blackbodies.
The first indication that energy is sometimes quantized came from blackbody radiation, which is the emission of EM radiation by an object with an emissivity of 1.
Planck recognized that the energy levels of the emitting atoms and molecules were quantized, with only the allowed values of
where
is any non-negative integer (0, 1, 2, 3, …).
is Planck’s constant, whose value is
Thus, the oscillatory absorption and emission energies of atoms and molecules in a blackbody could increase or decrease only in steps of size
where
is the frequency of the oscillatory nature of the absorption and emission of EM radiation.
Another indication of energy levels being quantized in atoms and molecules comes from the lines in atomic spectra, which are the EM emissions of individual atoms and molecules.
Conceptual questions
Give an example of a physical entity that is quantized. State specifically what the entity is and what the limits are on its values.
If Planck’s constant were large, say
times greater than it is, we would observe macroscopic entities to be quantized. Describe the motions of a child’s swing under such circumstances.
A LiBr molecule oscillates with a frequency of
(a) What is the difference in energy in eV between allowed oscillator states? (b) What is the approximate value of
for a state having an energy of 1.0 eV?
A physicist is watching a 15-kg orangutan at a zoo swing lazily in a tire at the end of a rope. He (the physicist) notices that each oscillation takes 3.00 s and hypothesizes that the energy is quantized. (a) What is the difference in energy in joules between allowed oscillator states? (b) What is the value of
for a state where the energy is 5.00 J? (c) Can the quantization be observed?
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