Topic 12. Reflections, part 3. Metals and dielectrics
The simple model of a metal is a regular, rigid arrangement of positively charged ions and a cloud of electrons that are relatively free to move inside the material. The free electrons respond to applied electric fields. Since the electrons are approximately free particles, they arrange themselves on the surface of the metal so as to cancel the electric field in the interior. For example, an external positive charge attracts electrons to the surface of the metal. This attraction continues to draw more electrons to the surface until the repulsion between these electrons at the surface exactly cancels the attraction of the external positive charge.
Since the electrons in a metal are free particles and the electric field does not penetrate into the interior of the material, very little of the incident energy is absorbed, and most metals are good reflectors. Ionized gases, which are also made up of positive ions and relatively free electrons behave very much like metals in this regard, and tend to be good reflectors as well. Atoms near the top of the atmosphere are ionized by ultraviolet radiation from the sun, and this layer of ionization is a good reflector of radio waves.
Most liquids do not contain free electrons, but they may contain positive and negatively charged ions, which can respond to an electric field. However, the ions are usually not as free as electrons are in a metal, and most liquids are more like dielectrics (see below) even if they have lots of charged ions.
The approximation of completely free electrons and passive, fixed ions that is used to model a metal breaks down for two reasons as the incident frequency is raised. The electrons are not really free particles and cannot respond infinitely rapidly to changes in the electric field. This means that they become unable to follow the field as its rate of change increases. In addition, the positively charged ions will interact with the incident field as well, especially when specific frequencies (which are characteristic of the particular ion are incident). Thus metals tend to be reflectors at low frequencies, but absorbers as the frequency is raised. Some metals (gold, for example), absorb the blue light at the high-frequency end of the visible spectrum and appear yellow as a result. This effect is even more pronounced in copper, which absorbs most of the visible spectrum. Since it reflects only the lowest-frequency portion of the visible spectrum, it appears reddish-orange as a result.
The same thing is true for ionized gases – they tend to change from reflectors to absorbers as the frequency of the incident radiation is raised. If the density of the gas is not too great, there may not be enough atoms to absorb all of the incident radiation, and the gas may become approximately transparent as a result. Thus the ionosphere reflects radio waves but transmits signals in the visible portion of the spectrum. It becomes strongly absorbing for ultraviolet frequencies (above the violet end of the visible spectrum) because these frequencies are strongly absorbed by oxygen and other atmospheric gases.
The simple model of a dielectric is a substance which may have the same regular structure as a metal, but with very few free electrons. The details of the absorption and reflection depend on the details of the material, and it is usually not possible to give a general rule as it was with metals. However, dielectrics generally transmit and reflect the incident energy with coefficients that are specific to the material and are usually functions of the incident frequency. This dependence on frequency is usually much more complicated than the relatively simple variation that characterizes metals and ionized gases.