Why does William Herschel's experiment show red light as warmer than blue if blue light is higher energy?
Here is an explanation of Herschel's experiment http://www.ipac.caltech.edu/outreach/Edu/Herschel/backyard.html
In short, Herschel placed multiple thermometers on the light separated by a prism. This led to the discovery of infrared light as he placed his control thermometer outside of the visible spectrum on the red side.
You may see similar apparatus to his here:
As you can see, the red thermometer is reading a higher temperature.
My guess is that blue is scattered more, therefor the thermometer for red absorbs more light.
Best Answer
The main reason is due to the fact that the prism refracts light in such a way that the "blue" part is more spread than "red" part. So that overall the energy hitting the thermometer is greater in the infrared and red part than on the blue part of the spectrum.
Edit: I have just seen your edit. You're right. There you can see the details.
Here's a quote from the website:
The website also provides some graphs and in one you can see that the red part (~650 nm) of the spectrum is about 3 times more concentrated than the blue one (~450 nm), while the infrared part is even more concentrated.