Physics & Politics

Early Quantum Physics

Please read the pages on Background on Atoms          

1. Photoelectric Effect

We have seen that atoms are made of negatively charged electron clouds bound to the positively charged nucleus. In solids the atoms come relatively close to each other. These atoms tend to balance the repulsive forces of the electron clouds of different atoms with the attractive forces of electrons and nuclei of neighboring atoms.  In some conducting solids, such as metals, certain atomic electrons called valance electrons become free to move from one atomic site to another. But even these orphan electrons are bound to the solid as a whole. However, when light falls on a metal it can remove valance electrons from its surface and lead to an electric current. This is called photoelectric effect.

This phenomena was first discovered by Hertz in early 19th century, but its full explanation did not take place until Einstein applied Planck's photon picture of light to this problem. Experimentally observed aspects of this phenomena were:

If the only effect of the light was to provide extra energy for the electron to eject from the metal, then one would expect that photocurrent would depend on the intensity of the light. The more light, the more energy it has to impart to the electron. But this was not at all what the experiment verified. Also, if the light was not intense enough, one would expect that it was a matter of time for the energy it provided that the electron would gain sufficient energy to leave the metal surface. But this also was not the case, as the process was almost instantaneous (occurred in 10 -9 s).

Einstein's explanation was based on Planck's radiation theory with a slight twist: light is quantized. Light of frequency f is made of quantized photons each of energy Ephoton = h fphoton , where h has a value of 6.63x10-34 J.s = 4.136x10-15 eV.s independent of the light, and is therefore known as the Planck's constant. So red light is made of photons that are weaker than the photons that make up blue light. A stream of red light, albeit of high intensity, may not have enough per photon energy to get an electron free from the metal surface. This is indeed what quantization  is all about!

2. Atomic Spectra

Another experimental evidence for quantization of energy is in the discrete nature of atomic spectra. When light emitted from a single species of atom is put through a diffraction grating or prism instead of all a continuum of colors only a few very discrete set of colors are observed. Because the emitted light (photon) carries energy away from the atom, then it seems that atoms can get excited not to any energy, but to discrete energy levels.

Professor Dean Zollman and his Physics Education research group at Kansas State University have developed software that help visualize this concept. Try their web site on atomic emission spectra. See if you can reproduce the observed spectra of hydrogen atom by making transitions among quantized energy state of hydrogen.


3. Wave-particle duality

As we discussed earlier, waves carry energy and momentum through the propagation of a disturbance. Particles carry energy and momentum by traveling from one point to another themselves. But a very important difference between waves and particles is that, until early in the 20th century, only waves were observed to exhibited the phenomena of diffraction and interference.

As we have already discussed diffraction is the phenomena of bending of the wave as it strikes an obstacle or an opening. Interference is when two waves interact to form a new wave form. Unlike two particles that bounce back from each other in a head-on collision, waves pass through each other. In the region that they overlap, they form a new disturbance pattern, i.e. a new wave form.

It was observed that electrons, which were known to be particles also exhibited diffraction. So, it was suggested that perhaps all particles could also act as waves. It was deBroglie who suggested that any object can be represented as a wave with a wavelength given by:

                    l=wavelength = (Planck's constant)/(object's momentum)

where, as we've seen, Planck's constant = h = 6.63x10-34 J.s, and the objects momentum is a product of its mass with its speed. For example, a tennis ball that is traveling at a speed, v, of 30 m/s and has a mass, m, of about 0.2 kg has a momentum, p, that has a value of:

                                ptennis ball = m v = (0.2) (30) = 6 kg.m/s

So, the wavelength of the ball is:

                 l= (6.63x10-34 )/6 = 1.105x10-34 m, this value is smaller than any distance that we can measure! However, an atomic particle, say an electron, can have a wavelength with a "meaningful" value! Try calculating the wavelength of an electron that is moving at 100 m/s. Use the fact that electrons have a mass of 9.11x10-31 kg.

Questions on Early Quantum Physics

Last Modified: Wednesday, September 12, 2007