Quantum mechanics provides a theoretical framework for the development of the structure of matter, starting with the same structure of the atom. Much of quantum mechanics developed in parallel with the latter, although however, is much wider, and actually serves to describe any phenomenon in the atomic spatial scale.
As with the atomic structure quantum mechanics is developed from scattered observations, initially related to the light and its interaction with matter.
black body radiation and the ultraviolet catastrophe
late nineteenth century, Maxwell's electromagnetism had shown that visible light was electromagnetic waves. Moreover, there were other waves of the same type, whose only difference was the frequency of oscillation. All these waves meet a relationship between the frequency ν, and wavelength λ, so
where c is the speed of wave propagation, in this case, c = 2997 × 10
8 m / s. Speaking of frequencies, or discuss wavelength is equivalent, although the change in one of them involves a change in the opposite direction from the other: an increase in frequency is equivalent to a decrease in wavelength, and vice versa.
When a body is heated, it emits radiation. The spectrum of this radiation, ie the amount of radiation emitted by each particular frequency may depend on the temperature and type of the issuing body: when heated an iron bar, it changes color from red to red, yellow and white. This light is emitted by the hot iron itself.
However, there is a body type no matter which size, shape or material but depends only on the specific temperature at which it is. Such a body absorbs all radiation that hits it, hence it is called black body. An example is a black body cavity, with a small hole where the radiation enters. Inside the body, the radiation bounces off the walls without ever leaving the hole through which he entered. However, the fact of being at a temperature, it emits its own radiation hole. This is the black body radiation, which sought to characterize the physical end nineteenth century.
Although the black body more important that we know is the sun: a ball of gas at high temperature, which generates a spectrum of light equivalent to a black body at 5500 K.
underlying assumptions scientists known as thermodynamics and statistical mechanics: a set of molecules is impossible to describe through the path and interactions between each of them (in a container is about 10
23 molecules!), so they resorted to calculate the average value and distribution of their speeds in order to calculate measurable quantities such as temperature and pressure. Within
as a black body cavity, there are many waves, so
Rayleigh (1842-1919) and
Jeans (1877-1946) followed the same pattern: the total energy is distributed equally in all possible wave . The result explained reasonably well the low frequency region (infrared), but nevertheless predicted an increase in the amount of higher frequency radiation, which would involve the issuance of an infinite amount of energy of any object. This result is called the "ultraviolet catastrophe"
Max Planck (1858-1947) in 1900 revised the way you do the calculation. Energy equipartition assumed by Rayleigh and Jeans assumed that each frequency corresponds to the same amount of energy. Planck reviewed this concept to infer that each wave corresponds to an amount of energy proportional to its frequency (E = hν)
According to the development of Rayleigh and Jeans, it could drive an infinite number of waves in a cavity, as will always be some whose wavelength is short enough to get into it, and the equipartition of energy, ensuring that there is always enough energy to excite, however minimal,
no limit on the number of waves possible within the activity ca. In addition, to enter more easily into the cavity wavelengths shorter than longer, accumulate more of them, that is, the more energy accumulates in the ultraviolet, or high frequency: the ultraviolet catastrophe
In contrast, Planck's hypothesis , higher frequency waves (= shorter wavelength), we need a greater amount of energy, so there comes a time when there is enough energy to excite waves at high frequencies. Ie
a limited number of waves can be excited in the cavity . The constant of proportionality between energy and frequency, proved to be very small: h = 6.62 × 10 -34
[J • s], and now called Planck's constant
.
The implication of the result is deeper than just the explanation of black body spectrum. The energy of each wave is quantized. Each frequency requires a minimum energy to excite the vibration. Thus, the total energy of a frequency (ν
E) be a whole number of times (n) the minimum energy hν. That is, E = nhν
ν.
The photoelectric effect Another problem of light-matter interaction would further evidence of behavior corpuscular radiation. Philipp von
Lennard (1862-1947) studied thin layers of metals when illuminated by radiation.
In a classical setting, the light should excite the electrons, and come to pluck with increasing radiation intensity. With increasing intensity, was also pulled forward to the greater electron kinetic energy. But found instead that a minimum frequency needed to boot the metal electrons. Below this minimum frequency, it was impossible to start electrons, regardless of the intensity of light. Moreover, the kinetic energy of the electrons plucked not dependent intensity, but frequency as well.
Albert Einstein (1879-1950), one of his famous articles in the wonderful year 1905, deduced the solution to this problem. Planck independently, reached the same conclusion about the quantum of radiation, including the same value for the constant h. When one of these few came to the surface of the metal, an electron absorbs energy, and if this is enough to leave the metal, it does. Otherwise, it is issued. Thus, only radiation with minimal energy (which depends on the material) is capable of starting electrons. Moreover, when energy fixed, the electrons lose some to leave the material, and the remaining (also a fixed amount) is used to acquire a high speed, or kinetic energy.
with radiation below the minimum frequency, increased intensity of the radiation, ie, a greater number of light quanta, not cause it to boot as electrons because no energy is minimal. On the other hand, radiation with enough energy, produces electrons with a kinetic energy set, and increased intensity, will cause it to pull up more electrons, but all with the same kinetic energy. Is to increase the frequency of light with increasing energy electron kinetic uprooted.
This approach is very easily understood if one considers the radiation and fixed-energy particles collide with electrons to snatch the material.
Compton effect The last of the experiments that highlight the particle properties of light is Compton scattering, discovered by Arthur Compton (1892-1962), when working with X-ray scattering In the scattering, detected as the wavelength of the scattered radiation increased, ie the radiation energy lost. Once again, the phenomenon is understandable if one takes the radiation and particles, and discusses the game against another particle, taking into account the conservation of energy and momentum.
The quantum of radiation has an energy given by its frequency. Also has an angular momentum, as any particle, which depends on its wavelength. In the shock of the energy and angular momentum are transferred to the particle. The result is a loss of radiation energy, which is revealed in its incidence, or increase its wavelength.
The explanation for these phenomena are based on the quantization of light energy, which makes it behave like a particles, called photons. In the next post we will see how matter can behave in turn as a wave
