1. The problem statement, all variables and given/known data Light from a helium-neon laser (wave length 633 nm) is used to illuminate two narrow slits. The interference pattern is observed on a screen 3.1 m behind the slits. Twelve bright fringes are seen, spanning a distance of 50 mm What is the spacing, in mm, between the slits? 2. Relevant equations I believe it is: Ym=m(wavelength)/d Where L is the distance from the slits, m is an integer describing how many fringes up or down you are, d is the slit spacing and Ym is the distance to the mth bright fringe. 3. The attempt at a solution Not sure if I'm interpreting the equation correctly. I plugged in 6 for m and 25x10^-3 meters for Ym and came up with the answer 0.469 mm. This is the wrong answer. My logical is that the distance from the center will give you the distance to the 6th fringe, and that would be half the total distance spanned. Is this the correct way to plug in numbers for that equation? Or I might be making some minor mistake because I don't have access to a scientific calculator (lost it today) and I'm also exhausted.