Which type of light is not visible to the human eye and produces physical effects in the skin?

Infrared light is indeed the correct choice because it lies beyond the visible spectrum of light, which means it cannot be seen by the human eye. This type of light is known for its ability to penetrate the skin and generate heat, leading to various physical effects. Infrared light is often utilized in skincare treatments and modalities, like heating therapies, to enhance circulation, relieve muscle tension, and promote relaxation.

Ultraviolet light, while not visible to the naked eye, is primarily known for its interactions that can lead to skin damage rather than producing beneficial physical effects in the same context. Visible light is clearly within the spectrum that humans can see and would not be considered in this category. Incandescent light, though related to visible light, is also fully within the spectrum visible to the human eye and does not produce the same kind of physical effects in the skin as infrared light does.