Physics 5.24 - Radiation from atoms and nuclei
Matter can emit electromagnetic radiation, and absorbed radiation can change matter. Follow the energy transfer in both directions and explain why different atomic and nuclear changes involve different frequencies.
Radiation and matter as a system
Start by choosing a system. Here the matter is an atom or its nucleus, and the surroundings include any electromagnetic radiation travelling towards or away from it. An atom and a nucleus are related but not interchangeable: the nucleus is the small central part of an atom.
The word change is deliberately broad in this specification point. It can mean that the atom or nucleus changes its energy state. It does not have to mean that the atom becomes a different element, and this lesson does not require a detailed model of electron or nuclear energy levels.
Electromagnetic radiation
Electromagnetic radiation is the transfer of energy through space by electromagnetic waves. Radio waves, visible light, X-rays and gamma rays are examples at different frequencies.
There are two opposite energy-transfer directions to keep separate:
| Process | Direction of energy transfer | Result |
|---|---|---|
| generation or emission | from the changing atom or nucleus into radiation | radiation travels away |
| absorption | from incoming radiation into the atom or nucleus | the matter can change |
The words generate and absorb therefore describe more than where radiation is located. They identify which way energy is transferred.
Radiation generated by changes in matter
When an atom or nucleus changes to a state with less energy, the energy difference can leave as electromagnetic radiation. The radiation carries energy away, so energy has not been created from nothing.
Changes in atoms and nuclei can generate radiation over a wide frequency range. There is no single atomic radiation frequency: different changes can release different amounts of energy and so generate radiation at different frequencies. For example, changes in atoms can generate visible, ultraviolet or X-ray radiation, while a change in a nucleus can generate gamma radiation. These examples illustrate the range; they are not a claim that every atom or nucleus produces every frequency.
Worked causal example
Consider an atom that has previously gained energy.
- Initial state: the atom has more energy than it has after the change.
- Change: the atom changes to a lower-energy state.
- Energy transfer: energy leaves the atom as electromagnetic radiation.
- Observation: radiation travels away from the atom at a frequency associated with that change.
The same energy account can be used for a nucleus: replace atom with nucleus, while keeping the direction of energy transfer the same.
Radiation absorbed by matter
Now reverse the energy-transfer direction. If radiation is absorbed, energy is transferred from the radiation into matter. That transfer can cause a change in an atom or in a nucleus.
Continue this lesson.
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