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Here, the sun is way off to the left and the rays of the sun are parallel and we have a surface that transects this beam of light. The beam's power can be denoted as phi = EA, so the irradiance of the beam can be represented as the power of the beam divided by the area A (on the surface). If we had a real surface that was tilted, we now have a different area but the same power hitting it, so this means we have less power per unit surface area hitting this space.