Radiation absorption and emission

Electromagnetic radiation includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. These are all the same general kind of wave, but they have different frequencies and wavelengths. Higher-frequency radiation transfers more energy in each photon, or packet of electromagnetic radiation.

Changes involving electrons

Electrons in an atom occupy allowed energy levels. If an electron absorbs a photon with the required energy, it can move to a higher energy level. This is called excitation. When it later moves to a lower energy level, the atom emits the energy difference as a photon.

The frequency of the absorbed or emitted radiation depends on the size of the energy difference. Because different atoms have different arrangements of energy levels, their changes can generate or absorb different frequencies. Electron changes commonly involve visible or ultraviolet radiation and can also involve X-rays.

Absorption and emission are therefore opposite energy transfers:

• absorption: radiation gives energy to the atom;
• emission: the atom loses energy as radiation.

An atom does not simply absorb every frequency that reaches it. A photon must transfer an amount of energy that matches an allowed change.

Changes involving nuclei

The nucleus can also exist in different energy states. A nucleus may absorb electromagnetic radiation and gain energy, or it may lose energy by emitting radiation. Radiation emitted because of a nuclear energy change can be gamma radiation.

Gamma rays belong to the electromagnetic spectrum, just like visible light and radio waves. They are not material particles such as alpha or beta particles. Gamma rays have very high frequencies and are usually associated with changes in the nucleus rather than movements of electrons between atomic energy levels.

Taken together, changes in atoms and nuclei can generate and absorb radiation across a wide frequency range. The particular part of the spectrum involved depends on the energy change taking place.

Fluorescence as an observable example

Fluorescence demonstrates that absorbed and emitted radiation need not have the same frequency. Under a black light lamp, ultraviolet radiation is absorbed by fluorescent substances such as quinine in tonic water. Some of the absorbed energy is transferred within the substance, and visible blue light is then emitted. The tonic water therefore appears to glow.

This observation connects an invisible part of the electromagnetic spectrum with a visible one: ultraviolet radiation supplies the energy, while visible radiation carries some of that energy away. The lamp should not be viewed directly, and the demonstration should use a suitable school ultraviolet source.