One of the first signs of sunburn is the reddening of the skin (called erythema). As a very rough rule of thumb, erythema occurs if 13 mJ of ultraviolet light of approximately 300 nm wavelength (referred to as UVB radiation) is incident on the skin per square centimeter during a single exposure. How many photons are incident on 1.0 cm2 of skin in this amount of exposure?