Question

If the angle at which bands are viewed is 30o in flatness testing with interferometry, what is the possible error?

(a) 0%

(b) 15%

(c) 30%

(d) 45%

I got this question in an international level competition.

Origin of the question is Interferometry Applied to Flatness Testing topic in section Measurement by Light-Wave Interference of Engineering Metrology

Answer 1

Right choice is (b) 15%

Easy explanation: If the viewing angle is 30 degrees then three fringes may be seen as 2.5 that is it will cause error of 15 %. If the viewing angle is 45 degrees then three fringes may be seen as 1.75 that is it will cause nearly 40% error.