What is the physical process by which a mirror reflects light rays?
Peter N. Saeta, an assistant professor of physics at Harvey Mudd College, responds:
"The oscillating electrical field in the incoming light wave produces a force on the charges inside the mirror. Most of the charges are either too heavy (as is true for the nuclei of the atoms) or too tightly bound (as is true for most of the electrons) to vibrate significantly in response to this field. The comparatively loosely held bonding electrons, along with the free electrons present in metals, can move in response to these electrical forces, however. These electrons oscillate at the same frequency as the incident light, which gives rise to the reflected wave.
"Because there are a great many electrons in the mirror, all vibrating at the frequency of the incident light, reflection from the mirror is really a group effort. All the electrons dance to the same music, whose rhythm is provided by the incident light wave. This coordination causes the reflected wave to make the same angle with respect to the mirror's surface as does the incident beam.
"A typical mirror consists of a piece of glass that has been coated with a layer of metal. Glass by itself reflects a little of the light, but the metal layer greatly boosts the reflectivity. If the metal were perfectly conducting, it would reflect all of the light, but the conductivity of real metals is less than perfect. This imperfection leads to some absorption of light in the metal. A polished silver surface, for example, reflects about 93 percent of the incident visible light, which is very good as metals go. Interestingly, if the metal layer is very thin--only a few hundred atoms thick--then much of the light leaks through the metal and comes out the back. If you get the thickness of a metal layer right, you can make a beam splitter that divides an incident beam of light into two equal parts, with just a little bit of the light lost to the metal film itself.
"As good as the reflectivity of a silver mirror is, you can do much better with dielectric mirrors. These reflectors consist of alternating layers of two transparent materials that have different indices of refraction. Dielectric mirrors can have reflectivities of 99.999 percent or better at the wavelength for which they are designed. In these mirrors, essentially all of the incident light reflects, and virtually none is absorbed in the mirror or transmitted through it."
Answer posted on October 21, 1999
How exactly does light transform into heat--for instance, when sunlight warms up a brick wall? I understand that electrons in the atoms in the wall absorb the light, but how does that absorbed sunlight turn into thermal energy?
It turns out that, as often happens, that there is more than one way to explain the same basic phenomenon.
Tom Zepf of the physics department at Creighton University in Omaha, Neb., notes that "Sunlight heats a material such as water or a brick primarily because the long wavelength, or infrared, portion of the sun's radiation resonates wellwith molecules in the material, thereby setting them into motion. So the energy transfer that causes the temperature of the substance to rise takes place at the molecular rather than the electronic level."
Scott M. Auerbach, a theoretical chemist at the University of Massachusetts at Amherst offers a more detailed answer:
"Light from the sun excites electrons in the atoms which constitute the brick wall. How does that electronic energy get converted to heat, you ask. The key is 'radiationless transitions.' Here's how it works: the atoms of the brick are perpetually vibrating. Some of those atoms vibrate sufficiently vigorously that their vibrational energy is roughly equal to the electronic energy (photons) absorbed from the sun--in essence, they are in resonance with the solar energy. Those atoms then make a quantum transition from 'electronically excited' to 'vibrationally excited,' meaning that the energy causes the whole atom to move. We feel that motion as "heat." The atoms which make the jump to vibrational excitation soon collide into neighboring atoms, dissipating their vibrational energy throughout the entire brick, making the brick hot throughout.
Answer posted on October 21, 1999
