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... – C. E. Nov 24 '14 at 05:19BarLegendon its own. You can't even set the scaling then as far I as I know. I count this as a bug.. – C. E. Nov 24 '14 at 12:09