An air wedge is formed between two glass plates separated at one edge by a ver fine wire. When the wedge is illumindated from above by 688 nm light, 29 dark fringes are observed. Calculate the radius of the wire. Show answer in micro meters I have tried many different things with this problem and cannot come up with the correct answer. Here is the last thing I did. Since 39 dark fringes are seen; 39 light fringes will also be seen. Where t=0 will be the first dark fringe. Which means fringe 78 is a light fringe, which is right next to the wire separating the two glass plates. The formula is: t = [((m - 1/2)*lamda) / 2] * 1e+6 micro m/m This will give the diameter of t. Divide the diameter by 2 to get the radius. Unfortunately, this does not give the correct answer. What am I doing wrong. The answer I came up with is 13.158 micro meters.