# Why is there no blackbody radiation in the high frequency section of Planck's curve?

Upon examining the curve describing blackbody thermal radiation, I noticed that the curves approaches (but never reaches) zero when increasing wavelength, but on the other hand it actually does reach zero at high frequency so I was wondering why?

• Good question. This confused many physicist for years and was at the birth of quantum mechanics. Apr 4 at 1:20
• What makes you think it reaches zero? Apr 4 at 4:14
• Let's state the behaviour carefully. Plotted against frequency, intensity scales as $\nu^3/(e^{\beta h\nu}-1)$, reaching $0$ at $\nu=0$ but not at any finite $\nu>0$. Plotted against wavelength, intensity scales as $\lambda^{-5}/(e^{\beta hc/\lambda}-1)$, which is asymptotic to $\lambda^{-5}e^{-\beta hc/\lambda}$ ($\lambda^{-4}/(\beta hc)$) for small (large) $\lambda>0$, so the $\lambda\to0^+,\,\lambda\to\infty$ one-sided limits are both $0$.
– J.G.
Apr 4 at 20:47
• There is a contradiction in the body of your question. May 11 at 11:05

In the frequency domain, the power spectral density has the form $$f^2$$ as the frequency approaches zero and the form $$f^3e^{-f}$$ as the freuency approaches infinity. In neither case does it reach zero at any finite frequency. You can argue about at which end it approaches zero more rapidly. Plotted on a log frequency axis, the high-frequency end of the spectrum would appear to approach zero very abruptly because of the exponential.