1. The problem statement, all variables and given/known data A Helium laser, λ = 588 nm, shines on double-slits separated by 1.80 mm. An interference pattern is observed on a screen at a distance R from the slits. The point C on the screen is at the center of the principal maximum of the interference pattern. The point P is the point on the principal maximum at which the intensity of light is half that of the intensity at C. What is the value of the angle θ? 2. Relevant equations I (final) = I (initial) * cos^2( (pi * d * sin(θ)) / λ ) small angle approximation sin(θ) = tan(θ) = θ 3. The attempt at a solution On questions like these I typically find it easiest to pick some dummy value and solve. In this case i set I (initial) to 10. The center of the center maxima is where intensity is the greatest. At this point θ = 0. so, I (final) = I (initial) * cos^2( (pi * d * sin(θ)) / λ ) = 10 * cos^2( (pi * d * sin(0)) / λ ) = 10 * cos^2 (0) = 10 If I want the θ where I (final) = I(initial0 / 2. I need to find θ where I (final) = 10 / 2 = 5 so (with small angle approximation), I (final) = I (initial) * cos^2( (pi * d * sin(θ)) / λ ) 5 = 10 * cos^2( (pi * (1.8 * 10^-3) * θ) / (588 * 10^-9 ) 0.5 = cos^2( (pi * (1.8 * 10^-3) * θ) / (588 * 10^-9 ) 0.5 = cos^2 (9617.1204 * θ) At this point I plug the above equation in to my calculator and have it solve for θ. According to my calculator θ = 0.03743 deg This seems like a reasonable result; However, the computer kicks it out as incorrect. I am NOT looking for someone to give me this answer. However, ANY help with my method is GREATLY appreciated.