In 1800, Herschel wondered if the different colors of the rainbow were equal in brightness and heat. He used a prism to separate sunlight into it's different colors and thermometers to measure how much "heat energy" was included with each color. He also measured how much "brightness" was associated with each color. He noticed a "brightness" peak at the middle of the spectrum. Red and violet had the least amount of brightness with a "peak" at green and yellow. But for "heat energy", violet had the least and the amount of heat climbed more and more and showed no sign of a "peak" . His observations supported the idea that neither Brightness nor heat were evenly distributed between the different colors. Some colors had more brightness (green and yellow) and a different color had the most heat (red).

But this lead to a new question. Did the "heat" exist even where there was no visible light and did it peak at wavelengths less than red. He did another set of experiments and found this to be the case. He again used a prism to separate sunlight into the colors of the rainbow. These colors were cast on a sheet of paper and a thermometer could be moved from one color to the next to measure the amount of heat. But this time, he moved the thermometers beyond the red light and made measurements on the paper where there was no visible light. He found the maximum amount of heat at wavelengths longer visible red light.

When Herschel selected where to place his thermometer and then measure the temperature, which variable was the Independent variable.

Question options:

A. The time of day

B. The month of the year

C. the temperature rise

D. the placement of the thermometer