"...Vit. C does not absorb UV light but exerts an UV-protective effect by neutralizing free radicals, while this effect is not seen with sunscreens. Under laboratory conditions, it has been shown that application of 10% topical Vit. C showed statistical reduction of UVB-induced erythema by 52% and sunburn cell formation by 40-60%...." (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3673383/)
Indian Dermatol Online J. 2013 Apr-Jun; 4(2): 143–146. Vitamin C in dermatology