Light: Fringes

1. The problem statement, all variables and given/known data
A plano-convex glass lens of radius of curvature 2.6 m rests on an optically flat glass plate. The arrangement is illuminated from above with monochromatic light of 565-nm wavelength. The indexes of refraction of the lens and plate are 1.6. Determine the radii of the first and second bright fringes in the reflected light.


2. Relevant equations
t=1/4 lamda
x = [tex]\sqrt{2tR}[/tex]


3. The attempt at a solution
I calculated the first fringe to be 8.5703e-4m. I thought that I would be able to calculate the first dark spot using t= 1/2 lamda and subtract the two to obtain the difference. However, I am not getting the right answer. Does anyone know if this is the correct method and equation?
1. The problem statement, all variables and given/known data



2. Relevant equations



3. The attempt at a solution