(Whoops- see answer below./ mbw) Allow me to start with your second question first. Max Planck worked around the year 1900 trying to study light. At that time, it was pretty well understood that light travelled in waves and the wavelength of light varied with the color. What Planck was able to show was that these lightwaves also had a certain amount of energy that varied with the wavelength. He described this using the equation:
E = hc/L = hv, with
h = Planck’s constant = 6.624*10^(-27) erg-seconds
c = the speed of light = 2.998 m/s
L = wavelength of the lightwave
v = frequency of the lightwave
This expression is known as "Planck’s energy" (no- see below/ mbw) and can be used to calculate the energy of a lightwave with known wavelength or vice-versa. It was also a significant enough discovery to earn Planck the Nobel Prize in Physics in 1918.
As for your other question, I’m not familiar with anything called "Planck’s quantum thresholds", but there is an important quantum threshold that is related to the work that Planck did. This was discovered by Einstein while he was studying blackbody radiation, or the emission of light from an object when it is heated.
Einstein discovered that when an atom is struck by a lightwave, it can be forced to release an electron. The kinetic energy of that electron is related to the energy, and thus the wavelength, of the lightwave that struck it. Interestingly, though, he noticed that this didn’t work all of the time. If the frequency of the light was too low, no matter how bright the light was, no electrons would be broken off of the atoms.
Einstein was finally able to figure out that the Planck energy of a lightwave at just this threshold frequency is the amount of energy needed just to break the electron free of the atom. So this energy can’t contribute to the kinetic energy making the electron move. In the end, this can all be expressed as another equation:
E = hv = W + 1/2MeV^2, with
E = hv as before
W = Einstein’s quantum threshold energy, also called the ’work function’
Me = the mass of the electron
V = the velocity at which the electron moves off
And for figuring all this out, Einstein was also awarded the Nobel Prize in Physics, this time in 1921. For more information on this subject and for some of the other discoveries that followed, check out this link
(republished on 07/21/06)