Answer
Because a photon of ultraviolet radiation has more energy, it can be harmful to the skin.
Work Step by Step
We can write an expression for the energy of a photon:
$E = h~f$, where $f$ is the frequency.
Ultraviolet radiation has a shorter wavelength and a higher frequency compared with visible light. Since ultraviolet radiation has a higher frequency, then a photon of ultraviolet radiation has more energy than a photon of visible light. Because a photon of ultraviolet radiation has more energy, it can be harmful to the skin.
