 $I(\lambda,T)=\frac{2hc^2}{\lambda^5}\frac{1}{e^{hc/\lambda kT}-1}$ or [img]http://upload.wikimedia.org/math/1/0/c/10c96a8f2af7d9631a0f4dde5b817b62.png[/img]
The above equation is energy per unit wavelength per unit solid angle.
[img]http://upload.wikimedia.org/math/3/6/c/36c7c70624f4ed44af2f004c87bf22c4.png[/img] is spectral energy density function with units of energy per unit wavelength per unit volume.
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Question1:
In school, i taught intensity = power / area.
so actually what the y-axis is displaying is intensity/ wavelength ?
[img]http://upload.wikimedia.org/math/3/6/c/36c7c70624f4ed44af2f004c87bf22c4.png[/img] is spectral energy density function with units of energy per unit wavelength per unit [color=blue][b]area[/b][/color] [s]volume.[/s]

Suggestion:
Should the applet called it energy density function with units of energy per unit wavelength per unit [color=blue][b]area[/b][/color]

Question2:
eightpihc2 = 8*pi*h*c*c*1.e40
but i don't understand why [img]http://upload.wikimedia.org/math/3/6/c/36c7c70624f4ed44af2f004c87bf22c4.png[/img] has only 1 c, if you code is 2 c's

Is the image wrong?

Question3:
what is the url of the wikimedia.org? for the picture http://upload.wikimedia.org/math/3/6/c/36c7c70624f4ed44af2f004c87bf22c4.png
maybe i can go read a bit to understand more

Where did you find this equation :)
i look through