What is the correct formula for average wavelength to describe surface roughness? (Where, Ra is the arithmetic mean deviation from the mean line of profile)

A. 2π x Ra / Mean slope

B. 2π x Ra x Mean slope

C. 2π / Ra x Mean slope

D. 2π / Ra / Mean slope

The question was asked in an international level competition.

My doubt is from Terminology as per Indian Standards in division Measurement of Surface Finish of Engineering Metrology

The correct answer is A. 2π x Ra / Mean slope

Easy explanation: Rt is the maximum height of peak to valley within the assessment length. Rt measurement is valuable for providing guidance for planning metal-cutting operations and analysing finish. Average wavelength = 2π x Ra / Mean slope.