Red and near-infrared light can penetrate several centimeters (close to 2 inches) into your body, and a high-power light with the right wavelengths (especially near-infrared) can even penetrate through the thick and dense human skull to deliver light directly into the brain. Hamblin has given the general rule of red/NIR light penetration of “up to 5cm,” which is almost exactly 2 inches. So that gives you a sense of how deep this light is actually getting into your body.
