1. The problem statement, all variables and given/known data A 50W lamp radiates isotropically and has a spectral width of 750 nm and a centre frequency of 600 nm. How much of its total output power is emitted into a solid angle of 10^-6 Sr? What is the diameter of the spot that a beam subtending this solid angle makes on a perpendicular screen placed 1 m from the source? 2. Relevant equations Full sphere 4*Pi Sr 3. The attempt at a solution For the first part of the question, I took the approach of taking the ratio of the solid angle to a full sphere and applying it to the total power radiated from the lamp (10^6/4*pi)*50W, which gave me ~4microwatts. Have I taken the right approach? For the second part of the question, I thought I'd first have to calculate the beam divergence which would lead me to the diameter of the spot, but I'm not given the diameter of the source. I'm assuming the spectral band comes into play here, but I'm hoping someone can give me a nudge in the right direction.