Which type of wave is invisible and can penetrate deeply into tissues?

The correct answer highlights that infrared waves can penetrate deeply into tissues and are invisible to the human eye. Infrared radiation is part of the electromagnetic spectrum and is commonly associated with heat. It is utilized in various applications, such as in medical therapies where it can be absorbed by living tissue, leading to a mild warming effect that can promote healing and relieve pain.

Infrared waves can penetrate several millimeters into biological tissues, making them suitable for certain diagnostic and therapeutic techniques in medicine. Their ability to deliver heat without being seen adds to their usefulness in these contexts, differentiating them from other wave types in the provided options that do not share the same depth of tissue penetration or associations.

In contrast, while radio waves are also part of the electromagnetic spectrum, they generally have longer wavelengths and do not typically penetrate deeply into tissues in the same manner. Microwaves, on the other hand, can penetrate food and other materials through dielectric heating but are less effective in terms of deep tissue penetration for medical applications compared to infrared waves. Ultraviolet waves, although effective for sterilization and disinfection, do not penetrate deeply into tissues and are mostly absorbed at the surface, which limits their use in therapeutic contexts.