1858 - 1947

Wien identified the temperature-depended

displacement of the peak in the emission spectrum

of a glowing solid in 1893. The formula for the total

emission spectrum of a photon has only been

deduced in 1901 by Max Planck. He discovered that

the wave- and radiation characteristics of a photon

depends on the energy transition of the related

electron according the relation: E = hf. In this

formula, which is called Planck's law, E is the

amount of expelled energy in Joule, f is the

frequency of the emitted (electromagnetic) radiation

in Hertz and h equals 6,626 x 10^-34 Joule-second

(Js), being Planck's constant. In daily life it is quite

easy to imagine the unit Joule. 1 Joule corresponds

with 1 Watt during 1 second so in one minute a

40-Watt incandescent lamp converts 40Wx60s =

2400 Joule of energy into heat and light. In relation

to individual electrons however, the unit Joule turns

out to be unmanageable large and therefore often

the unit electron Volt (eV) is used instead. One eV is the amount of energy that is

required to make an electron overcome a potential difference of one volt. When using

electron Volts, the relation E = hf remains unchanged but E is now expressed in eV

instead of Joule and since one electron Volt corresponds with 1,602 x 10^-19 Joule,

h is transformed to 4,136 x 10^-15 electron Volt-seconde (eVs). An energy transition

of 1,24 eV then results in an electromagnetic radiation with a frequency of f = E/h =

1,24 eV/(4,136 x 10^-15) eVs or about 300.000 GHz. The wavelength of this radiation

follows from the equation λ = c/f in which λ is the wavelength in meters, f

the frequency in Hertz (Hz) or vibrations per second and c is the speed of light, about

300.000 km/s. 300.000 GHz then turns out to be infrared radiation (IR-A) with a

wavelength of λ = c/f = 1000nm.