1. The problem statement, all variables and given/known data Monochromatic light of wavelength 550nm is used to illuminate normally a narrow slit. A converging lens of focal length 50cm is used to focus the diffraction pattern onto a screen. If the seperation between the first minima is 10.0mm What is the width of the slit. The ans is 0.055mm. 2. Relevant equations 3. The attempt at a solution by using formula x= λD/ a , i have 10x10^-3 = (550x10^-9) x ( 50x10^-2) / (a) , i get my a = 0.0275mm ... What's wrong with my working?