Ultraviolette Catastrophy -Intensity causes melting of objects?

1. christian0710

399
Hi,

Is it correctly understood that it's the intensity of light that causes melting or heating of objects and not frequency or vice versa? I'm trying to understand the ultraviolette catastrophy (what the problem was) and according the the equation for intensity (given below) frequency increases exponentially, when intensity increases, so is this the poblem scientists were facing: if a black body radiates light at high intensity it would produce a frequency that was high enough to melt objects and as the intensity increased the frequency would go to inifnity?

I = (2*f*Kbb*T)/c^2

2. Matterwave

3,860
No.

Frequency of light is not what melts objects. Objects melt roughly due to being high enough in temperature such that the kinetic energy of the molecules of the solid can overcome the inter-molecular forces holding the object in its solid state.

The ultraviolet catastrophe has nothing to do with melting objects. It's that the prediction of black body radiation intensity by Rayleigh and Jeans predicts an infinite total intensity due to the intensity frequency relationship diverging as the frequency grows. It was an experimentally known fact that this is obviously wrong.

3. Andrew Mason

6,895
The ultraviolet catastrophe results from classical electromagnetic theory and from the equipartition theorem of the classical kinetic theory (statistical mechanics). According to classical EM theory, the number of vibrational modes (analagous to classical degrees of freedom) of an EM wave in a black-body cavity resonator should be proportional to the frequency^2. According to the equipartition theorem, all modes should have the equal energy. So the rate at which energy is radiated by the black body should increase rapidly with frequency (ie in proportion to the number of vibrational modes which is proportional to the square of the frequency). Since this is not observed, it is apparent that classical physics is not adequate to explain black-body radiation.

My sense is that the ultra-violet catastrophe is more a failure of the equipartition theorem than of classical EM theory.

AM