Which type of UV radiation absorbs and darkens skin to create a tan?

UVA radiation is primarily responsible for causing the skin to darken and develop a tan. It penetrates the skin more deeply than UVB and affects the melanocytes, the cells in the skin that produce melanin. When the skin is exposed to UVA, these melanocytes increase melanin production as a protective response against UV damage, leading to the darkening of the skin, or tanning.

UVB radiation also contributes to tanning and is known for causing sunburn as it targets the upper layers of skin. However, while UVB plays a role in the tanning process, it is not as effective as UVA at inducing immediate pigmentation changes. UVC radiation, on the other hand, is mostly absorbed by the ozone layer and does not typically reach the Earth's surface, making it not relevant to natural tanning.

Thus, the primary contributor to the tanning effect is UVA, which makes it the correct answer in this context.