1. The problem statement, all variables and given/known data Using Planck's law, calculate the spectral radiance of 400nm sunlight arriving at earth. Assume 1/3600 steradians of view and 5800 K of temperature. 2. Relevant equations Planck's law 3. The attempt at a solution I am not the best at typing formulas. Lets start what we are raising e to the power of. We have the Planck constant times the speed of light over the wavelength times the Boltzmann constant times temperature. Planck constant = 6.63E-34 speed of light = 3E8 wavelength = 400nm = 400e-9 Boltzmann constant = 1.3806488E-23 6.63E-34 * 3E8 / (400e-9 * 1.3806488E-23 * 5800) = 6.2095993290031215 Raising e to that power gives me 497.50187677476521, subtracting 1 gives me 496.50187677476521 So I have 2 * h * c2 / y5 divided by that value Where h is my Planck constant as above c2 is the speed of light squared, as above y5 is my wavelength to the fifth, as above Which gives 2 * 6.63E-34 * 3E8 * 3E8 / (400e-9 * 400e-9 * 400e-9 * 400e-9 * 400e-9) which is 11654296874999994 Divide by what I had above gives 23472815351083 watts / square meter / steradian or 6520226486 watts / square meter or 6.5 gigawatts / square meter. As I am not instantly vaporized as I write my answer, I know I made a mistake. But where?