Which of the following is the most precise device for measuring length :
(a) a vernier callipers with 20 divisions on the sliding scale
(b) a screw gauge of pitch 1 mm and 100 divisions on the circular scale
(c) an optical instrument that can measure length to within a wavelength of light ?
NCERT Solutions for Class 11 Physics Part 1 Chapter 2 Exercise 6



(c)  -  is the answer.
   Optical instrument has least count so it is the most precise  device for measuring length.