Which type of UV rays is most penetrating and commonly used in tanning booths?

UVA rays are the most penetrating type of ultraviolet radiation, which makes them particularly effective in tanning. They can reach the deeper layers of the skin compared to other types of UV rays. This deep penetration stimulates the production of melanin, leading to tanning effects, which is one reason why they are commonly used in tanning booths.

In contrast, UVB rays primarily affect the outer layers of the skin and are responsible for sunburns; they play a significant role in vitamin D synthesis but are less effective in achieving a tan. UVC rays are the most harmful but are mainly absorbed by the ozone layer and do not reach the Earth's surface in significant amounts. Visible light is not classified as a UV ray and does not contribute to tanning in the same way that UVA does. Therefore, the use of UVA in tanning booths is primarily due to its ability to penetrate deeper into the skin to encourage a tan.