Exposure to a sufficient quantity of ultraviolet will redden the skin, producing erythema-a sunburn. The amount of exposure necessary to produce this reddening depends on the wave- length. For a 1.0 cm patch of skin, 3.7 mJ of ultraviolet light at a wavelength of 254 nm will produce reddening; at 300 nm wavelength, 13 mJ are required. 

a. What is the photon energy corresponding to each of these wavelengths? 

b. How many total photons does each of these exposures correspond to? 

c. Explain why there is a difference in the number of photons needed to provoke a response in the two cases.