Electromagnetic Radiation
Quantum and Nuclear | Light, Sound and Waves

Absorption ends the journey

Physics Narrative for 14-16 Supporting Physics Teaching

Three possibilities as a beam strikes a material: reflection, absorption and transmission

When a beam strikes a block of material, there are three possibilities: reflection, absorption and transmission. All three may happen; you usually have to idealise to get only one. Which processes actually happen depends on the block: the material it is made from, and the surface treatment (and perhaps the internal structure as well); the frequency of the radiation striking the block; and the angle at which the beam strikes the block.

How complex structures within the material interact with particular frequencies makes for a fascinating study, accounting for iridescence on butterfly wings, among other things, but here we suggest building up a comparatively simple model, assuming that the block of material is solid and homogeneous. For beams that strike the surface head-on, this leaves only interactions at the surface (reflections): depending on how smooth the surface is and on the frequency of the radiation; and interactions in the body of the material, which determine the absorption. The mechanisms behind both are complex, and are still, in many cases, not completely understood. However, at the phenomenal but quantitative level, the patterns of more or less complete reflection and constant fractional absorption are well understood, and both of these are explored in this topic.

Detectors are cunningly chosen absorbers

Vibrations travel out from a source (some need a particle-based medium and some do not) and after a journey these radiations get absorbed.

A description of absorption is, at root, based in thinking about energy. You should develop an energy description. To recap, one store of energy at the source is emptied and another store at the absorber is filled. For light, and other electromagnetic radiations, these two processes are linked by the heating by radiation pathway (Sound has a somewhat different mechanism: speakers shunt air to-and-fro, and the air, somewhat later, shunts our ears to-and-fro. That suggest a mechanical working pathway as the most helpful way to think about the power radiated in sounding). That is, radiating is a form of remote working: we can shift energy without having to shift matter, or without having to be adjacent to the location associated with the store. The Sun can warm you on a summer's day without having to be next to you – it's about 500 light-seconds away. The nuclear store of the Sun is emptied, and the thermal store of your cheek is filled. That's not the only thermal store: there are many that are filled by the radiations from the Sun. So although we wrote source-medium-detector and used it as the basis for the SPT: Light and SPT: Sound topics, a more encompassing description would be source-radiation-absorbers. The one-to-many alteration is one change, recognising that heating by radiation is dissipative, spreading the energy from one store to many. Then there is the more subtle change from absorber to detector.

Detectors are a subset of absorbers. The radiation, or at least a fraction of it, must come to a sticky end in order to be detected. But that is not enough. Both singing in the shower and singing into a microphone result in the radiations being absorbed. In the case of the microphone, there is something a bit special designed in: a designed-for change, exploiting the interaction of the vibrations with certain parts of the microphone. All detectors are like this, only in some cases, particularly ears and eyes, the design has evolutionary rather than intentional origins. But all detectors, whatever the design origins, are devices, switching from one pathway to another, and indeed many are transducers, switching to the electrical working pathway.

Many detectors need to select not only one physical vibration but also only a small range of frequencies. That is, they need to be differential absorbers. Why some materials absorb only certain frequencies is a topic that we return to in the next episode. For now, just note that we need to choose the material carefully, and perhaps do some engineering of the material to make a device that can function as a detector.

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