Light-Matter Interactions at the Atomic Level


By Robert M. Hazen, Ph.D., George Mason University

The Bohr atom model states that electrons can jump from one level to another, and, in the process, energy must be absorbed or emitted as photons. What happens when light-matter interactions take place by this process? How does it assist in understanding the natural world?

Illustration of Quantum leap with electrons orbiting around concentric circle.
Quantum leap—electrons in an atom can only jump directly from one energy level to another. (Image: chemistrygod/Shutterstock)

The Bohr atom was embraced by physicists because it was extremely successful in modeling the behavior of atoms, especially things such as light-matter interactions and the way an atom interacted with light.

For example, when hydrogen is heated up, this emission of the line spectra, or the specific wavelengths, is very significant, and the Bohr atom model successfully explains it.

It was for all of this work on the atom that Niels Bohr won the 1922 Nobel Prize in physics, one of the many honors he received.

Learn more about the quantum world.

Quantum Jump or Quantum Leap

Light-matter interactions can be explained using the Bohr model of the atom. The model postulates that energy and light must come in discrete bundles called quanta.

When the electron moves from one energy level to the other, the smallest possible increment in energy occurs. This increment is called a quantum jump or a quantum leap. The quantum leap is absolutely the smallest increment possible in the universe.

A quantum jump from a higher energy level to a lower energy level releases one quantum of electromagnetic radiation, called a photon. And so light moves out as the electron jumps down.

A single photon is a single wave of electromagnetic radiation that radiates out at 186,000 miles per second. The wavelength of a photon has an energy that corresponds to that difference between one state and the other state.

Learn more about atoms.

Electromagnetic Radiation in Everyday Life

Light Bulb—Cascades of excited electrons are actually dropping down in a glowing light bulb or a flame in a fire. As the object is heated up, the electrons are moved into excited states, and they start dropping down, moving up, dropping down. And every time the electrons drop down, they emit some wavelength of light and produce the light in a light bulb in an incandescent bulb.

Electromagnetic radiation is a continuous spectrum, so not only does one of these light bulbs produce visible light, but it also produces infrared heat. Thus, the incandescent bulb is very hot because of the longer wavelengths of radiation produced.

Black Asphalt—When an atom absorbs a photon, an electron jumps to a higher energy state. So, for example, when black asphalt is bathed by sunlight, the asphalt is soaking up the sun’s energy. That means electrons are moving up into excited states and have much more energy. The atoms get hot and then part of that energy the blacktop absorbs is re-radiated out in longer wavelengths as heat energy, making the blacktop hot.

This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.

Light-Matter Interactions: The Quantum Phenomenon

Image of a mercury vapor lamp.
Mercury vapor lamps, which are examples of quantum effects, have a distinctive bluish-white glow. (Image: Unknown/Public domain)

Fluorescence—Fluorescence occurs when a material is subject to ultraviolet light, which is invisible to the human eye. When materials are subjected to ultraviolet radiation, electrons are bumped up to a higher energy level and then they may drop down back to the ground state in two or three jumps.

The higher energy of ultraviolet radiation represents the first jump, but the individual steps down might be in the visible light wavelength and so the fluorescence phenomenon is seen. If ultraviolet light is shone on something that looks just sort of dull or white, the electrons in these materials are moved into an excited state. They cascade down in two or three increments and one of those increments then involves a visible light. So under fluorescent light these materials appear brightly colored.

Day Glo Colors—Day Glo colors occur with, for example, a blue or violet light, high-energy visible light, or perhaps the very longest wave ultraviolet light in normal lights, or from sunlight. When this light hits these materials, they re-emit very intense photons in one particular wavelength. As a result, Day Glo colors appear unusually bright, and this is a kind of fluorescence phenomenon that is seen every day.

Mercury vapor lamps and sodium vapor lamps—Mercury vapor lamps with their distinctive bluish-white glow are examples of quantum effects. The bright yellow of sodium vapor lamps is another example of quantum leaps.

Learn more about the periodic table of the elements.

Practical Applications of Light-Matter Interactions

The study of light-matter interactions in great detail is called spectroscopy. Spectroscopy is proven to be absolutely essential to all sorts of modern sciences and technologies. Spectroscopy is used in astronomy, biology, chemistry, and environmental science. 

The Bohr model of the atom reveals that the way a material absorbs light is dependent on the orbit of its electrons. So wavelengths of absorbed light and wavelengths of emitted light correspond to quantum jumps.

Spectroscopy provides a direct probe of this atomic-scale interaction because it measures both the wavelength and intensity of electromagnetic radiation.

There are several different kinds of spectra that are found in many objects that play an important role in research. Some of these include emission spectra, flame spectra, absorption spectra, and reflectance spectra.

Spectra of all the different kinds of common objects reveal that humans’ eyes and brains perceive and interpret many different kinds of emission and absorption and reflection phenomena as the colors. 

The natural world is much more complex than people’s eyes and brains can interpret, so it is simplified, and things that people call red, or green, or blue really arise from very complex, different phenomena.

Common Questions about Light-Matter Interactions at the Atomic Level

Q: What is a quantum leap?

When an electron moves from one energy level to the other, the smallest possible increment in energy called a quantum jump or quantum leap occurs. The quantum leap is absolutely the smallest increment possible in the universe.

Q: What is spectroscopy?

Spectroscopy is the in-depth study of light-matter interactions. Scientists study light-matter interactions and by extension, understand the natural world using spectroscopic techniques.

Q: What is a photon?

A photon is one quantum of electromagnetic radiation released when an electron jumps from a higher energy level to a lower energy level.

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