Plot3D[Exp[-x^2 - y^2], {x, -2, 2}, {y, -2, 2}, ColorFunction -> (ColorData["VisibleSpectrum"][ Rescale[#3, {0, 1}, {380, 750}]] &), PlotLegends -> Automatic] 
There is no legend in this picture.
Plot3D[Exp[-x^2 - y^2], {x, -2, 2}, {y, -2, 2}, ColorFunction -> (ColorData["VisibleSpectrum"][Rescale[#3, {0, 1}, {380, 750}]] &), PlotLegends -> BarLegend[{"VisibleSpectrum", {0, 1}}]] 
BarLegend["VisibleSpectrum"] 
The color of lengend is wrong.
Mathematica 10.0.1. Is this a bug?

BarLegend[{"VisibleSpectrum", {380, 750}}]but no, that doesn't work either... $\endgroup$BarLegendon its own. You can't even set the scaling then as far I as I know. I count this as a bug.. $\endgroup$