Light waves are electromagnetic radiation, with a very wide range of wavelengths. Visible light makes up a very small portion of the electromagnetic spectrum. Image source. Light waves can travel in vacuum, which means they do not rely on the motion of particles in a medium, like mechanical waves.
The electric and magnetic components of the wave are generally perpendicular, and peak at the same time. The strength of the electromagnetic field oscillates as the wave propagates. A charged particle will oscillate in response to a passing electromagnetic wave.
Light is the signal passing between charged particles. The light produced by stimulated emission is said to be coherent as it is similar in every way to the photon that caused it. Lasers produce coherent electromagnetic radiation by stimulated emission. Electrons with extremely high kinetic energy, such as those in particle accelerators, will produce high energy photons when their path is altered. This alteration is accomplished by a strong magnetic field.
All free electrons will emit light in this manner, but synchrotron radiation has special practical implications. Synchrotron radiation is currently the best technology available for producing directional x-ray radiation at precise frequencies.
Certain types of radioactive decay can involve the release of high energy photons. One such type of decay is a nuclear isomerization.
In an isomerization, a nucleus rearranges itself to a more stable configuration and emits a gamma ray. While it is only theorized to occur, proton decay will also emit extremely high energy photons. Light incident on a metal plate may cause electrons to break loose from the plate surface Fig. This interaction between light and electrons is called the photoelectric effect.
The photoelectric effect provided the first conclusive evidence that beams of light was made of quantized particles. The energy required to eject an electron from the surface of the metal is usually on the same order of magnitude as the ionization energy. As metals generally have ionization energies of several electron-volts, the photoelectric effect is generally observed using visible light or light of even higher energy. At the time this phenomenon was studied, light was thought to travel in waves.
Contrary to what the wave model of light predicted, an increase in the intensity of light resulted in an increase in current, not an increase in the kinetic energy of the emitted electron. Einstein later explained this difference by showing that light was comprised of quantized packets of energy called photons. His work on the photoelectric effect earned him the Nobel Prize. The photoelectric effect has many practical applications, as current may be generated from a light source.
Generally, the photoelectric effect is used as a component in switches that respond to light. Some examples are nightlights and photomultipliers. Usually the current is so small that it must be amplified in order to be an effective switch. The energy of a photon is a discrete quantity determined by its frequency. This result can be determined experimentally by studying the photoelectric effect. The kinetic energy of an emitted electron varies directly with the frequency of the incident light.
If the experimental values of these energies are fitted to a line, the slope of that line is Planck's constant. The principle of conservation of energy dictates that the energy of a photon must all go somewhere. The results from a photoelectric experiment are shown in Figure 2. The solid lines represent the actual observed kinetic energies of released electrons. The dotted red line shows how a linear result can be obtained by tracing back to the y axis. Electrons cannot actually have negative kinetic energies.
Whereas the double slit experiment initially indicated that a beam of light was a wave, more advanced experiments confirm the electron as a particle with wavelike properties. The diffraction of a beam of light though a double slit is observed to diffract producing constructive and destructive interference. Modern technology allows the emission and detection of single photons. In an experiment conducted by Philippe Grangier, a single photon is passed through a double slit.
Unpolarized light can also undergo polarization by reflection off of nonmetallic surfaces. The extent to which polarization occurs is dependent upon the angle at which the light approaches the surface and upon the material that the surface is made of. Metallic surfaces reflect light with a variety of vibrational directions; such reflected light is unpolarized.
However, nonmetallic surfaces such as asphalt roadways, snowfields and water reflect light such that there is a large concentration of vibrations in a plane parallel to the reflecting surface. A person viewing objects by means of light reflected off of nonmetallic surfaces will often perceive a glare if the extent of polarization is large.
Fishermen are familiar with this glare since it prevents them from seeing fish that lie below the water. Light reflected off a lake is partially polarized in a direction parallel to the water's surface. Fishermen know that the use of glare-reducing sunglasses with the proper polarization axis allows for the blocking of this partially polarized light.
By blocking the plane-polarized light, the glare is reduced and the fisherman can more easily see fish located under the water. Polarization can also occur by the refraction of light. Refraction occurs when a beam of light passes from one material into another material. At the surface of the two materials, the path of the beam changes its direction. The refracted beam acquires some degree of polarization.
Most often, the polarization occurs in a plane perpendicular to the surface. The polarization of refracted light is often demonstrated in a Physics class using a unique crystal that serves as a double-refracting crystal.
Iceland Spar, a rather rare form of the mineral calcite, refracts incident light into two different paths. The light is split into two beams upon entering the crystal. Subsequently, if an object is viewed by looking through an Iceland Spar crystal, two images will be seen. The two images are the result of the double refraction of light. Both refracted light beams are polarized - one in a direction parallel to the surface and the other in a direction perpendicular to the surface.
Since these two refracted rays are polarized with a perpendicular orientation, a polarizing filter can be used to completely block one of the images. If the polarization axis of the filter is aligned perpendicular to the plane of polarized light, the light is completely blocked by the filter; meanwhile the second image is as bright as can be. And if the filter is then turned degrees in either direction, the second image reappears and the first image disappears.
Now that's pretty neat observation that could never be observed if light did not exhibit any wavelike behavior. Polarization also occurs when light is scattered while traveling through a medium.
When light strikes the atoms of a material, it will often set the electrons of those atoms into vibration. The vibrating electrons then produce their own electromagnetic wave that is radiated outward in all directions. This newly generated wave strikes neighboring atoms, forcing their electrons into vibrations at the same original frequency. These vibrating electrons produce another electromagnetic wave that is once more radiated outward in all directions.
This absorption and reemission of light waves causes the light to be scattered about the medium. This process of scattering contributes to the blueness of our skies, a topic to be discussed later. This scattered light is partially polarized. Polarization by scattering is observed as light passes through our atmosphere. The scattered light often produces a glare in the skies. Photographers know that this partial polarization of scattered light leads to photographs characterized by a washed-out sky.
Young reasoned that if light were made of particles as Newton suggested, only two bright bands of light would be projected on the white surface. The bright and dark bands demonstrated that the slits were causing light waves to interfere with each other. Sometimes this interference is constructive, and the light waves add together to create a bright patch. Sometimes the intereference is destructive and results in the light waves cancelling each other out creating dark patches on the screen.
Electromagnetic waves, including visible light, are made up of oscillating electric and magnetic fields as shown.
0コメント