A general method for the calculation of refracted ray paths through the atmosphere is developed and adapted for photochemical modeling applications. The adapted method is then used to study the effect of refraction on the twilight radiation field and stratospheric photochemistry. We show that refraction has the effect of increasing the amount of solar flux available for photodissociation in the atmosphere. This is caused by both the reduction of the optical path of the direct solar beam and a significant lengthening of the sunlit day. It is also shown that for polar conditions, concentrations of O(3P), NO3, and NO are significantly affected by refraction. ¿ American Geophysical Union 1995