Two readings taken by a slip gauge on a flatness interferometer count 10 and 14 fringes. Second reading is obtained by rotating the set up at 180°. Assuming the wavelength of radiation is 0.8065 μm, the parallelism error b/w two faces of slip gauge is
1
0.5085 μm
2
0.8065 μm
3
0.4032 μm
4
1.7012 μm