What is the main cause of skin reddening and sunburn?

The main cause of skin reddening and sunburn is UVB radiation. UVB rays have a higher energy level than UVA and are primarily responsible for the direct damage to the skin that leads to sunburn. These rays penetrate the outer layer of the skin and can actively cause DNA damage, triggering the inflammatory response that results in redness and pain.

Although UVA rays can also contribute to skin damage and have effects that lead to skin aging and some types of skin cancer, UVB is the specific type of ultraviolet radiation that instigates the sunburn phenomenon most notably. UVC rays are mostly absorbed by the Earth's atmosphere and do not significantly impact human skin under normal conditions. Similarly, visible light does not contribute to sunburn, as it does not have the energy to cause similar types of skin damage as UV radiation.