This is the version of Planck's Equation I am unable to Plot:

L[λ_]:=((8.0*π*2.9979x10^8*2.9979*10^8*6.626*10^-34)/\
λ^5)*(1.0/((E^((2.9979*10^8*6.626*10^-34)/(1.3806*10^-34*\
λ*5000.0)) )-1.0))

• What have you tried so far? – MarcoB Jul 4 at 18:47
• I have used every version of Planck's constant, Boltzmann's constant and the speed of light I can find on the internet. Nothing works. My formula is identical to that used by Zach Heuman in his manipulate demonstration. – OKCarl Jul 4 at 19:31
• Replace x with * and you're all set. – Roman Jul 4 at 19:31
• Replacing x with * yields: 1.49667*10^-15/((-1. + E^(287760./[Lambda])) [Lambda]^5) Unfortunately, still no Plot. What should I use for values on the X and Y axis? – OKCarl Jul 4 at 19:42
• You may have the wrong exponent on your value for $k_B$. Evaluate UnitConvert@Quantity["BoltzmannConstant"] // N to see the value in SI units. Your values of $c$ and $h$ are already in SI units, so use meters for $\lambda$. The final units will be in watts per cubic meter, or watts per square meter of surface area per meter of wavelength, which may be confirmed by evaluating Quantity["PlanckConstant"] Quantity["SpeedOfLight"]^2/ Quantity["Meters"]^5/Quantity["Watts"] // UnitConvert // QuantityUnit – LouisB Jul 4 at 22:05

Using builtin functionalities and SI units:

h = Quantity["PlanckConstant"] // UnitConvert // QuantityMagnitude
(*    132521403/200000000000000000000000000000000000000000    *)

c = Quantity["SpeedOfLight"] // UnitConvert // QuantityMagnitude
(*    299792458    *)

kB = Quantity["BoltzmannConstant"] // UnitConvert // QuantityMagnitude
(*    1380649/100000000000000000000000000000    *)

With[{T = 5000},
Plot[(2 h c^2)/λ^5 1/(E^((h c)/(λ kB T)) - 1), {λ, 0, 5000*10^-9},
AxesLabel -> {"λ [m]", "flux [W/m^3]"}]]


You can see that this matches the built-in plot you get with

PlanckRadiationLaw[Quantity[5000, "Kelvins"], "SpectralPlot"]