Which light source is part of the control group for William's experiment?

Wavelength light source is part of the control group for William's experiment

Explanation:

Sir William wanted to conduct an experiment to observe how much heat passed through different colour filters of the sunlight. He directed sunlight to pass through the prism and observed vibgyor and measured the temperature. He found that the temperature increased from violet to red.

Then he was inquisitive to find the temperature beyond the red ray. That's when he found the infrared rays which had higher temperature that was not to be seen through naked eyes. He performed experiments using the light source and found that the wavelength of light are not refracted in a particular pattern.