What type of light is primarily responsible for the "browning" effect of tanning?

UVA light is primarily responsible for the "browning" effect of tanning because it penetrates the skin more deeply than other types of ultraviolet light. When UVA rays hit the skin, they stimulate the melanocytes, which are the cells responsible for producing melanin, the pigment that gives skin its color. This stimulation leads to an increase in melanin production, resulting in the browning or darkening of the skin, which is commonly associated with tanning.

In contrast, UVB light, while it plays a role in causing sunburn and stimulating the body's production of vitamin D, is less effective in producing a tan as it primarily affects the outer layer of the skin. Infrared light does not contribute to tanning since it does not have the ability to stimulate melanin production. Visible light, while it can affect skin appearance, does not trigger the tanning process in the same way that ultraviolet light does. Therefore, UVA light is the key factor in achieving the browning effect that many seek from tanning.