The problem: 1000 photons all having an energy of 100 keV hits a copper plate 2 mm thick. How many of these photons can be detected on the other side by a detector that absorbs all radiation? The incident angle is 30 degrees with respect to the plane of the copper plate. If the angle had been 90 degrees I would have had no problem calculating this -- it would just have been N = N0*e^(mu*L), where N0 is the number of incoming photons, mu the linear attenuation coefficient, and L the thickness of the copper plate. But now that it says 30 degrees, I'm out of ideas.