1. The problem statement, all variables and given/known data 2. Relevant equations I = Imax cos^2 ((2pi/lambda)dsin(theta)) .81Imax = Imax cos^2 ((2pi/lambda)dsin(theta)) To shorten up my work I divided out the Imax 3. The attempt at a solution Not sure where I am going wrong. Is the 2.65 deg not the theta I am suppose to use? I divided out the I max, square rooted both sides, took the inverse cos of the left side to cancel out the cos on the right. Then proceeded to do the algebra.