Question

Which of the following is the most precise device for measuring length :

a vernier calipers with 20 divisions on the sliding scale
a screw gauge of pitch 1 mm and 100 divisions on the circular scale
an optical instrument that can measure length to within a wavelength of light ?

Answer 1

A device with minimum count is the most suitable to measure length.
Least count of vernier calipers
= 1 standard division (SD) - 1 vernier division (VD)
= 1 - $\frac{9}{10}$ = $\frac{1}{10}$ = 0.01 cm
Least count of screw gauge = $\frac{\text{Pitch}}{\text{Number of divisions}}$
= $\frac{1}{1000}$ = 0.001 cm
Least count of an optical device = Wavelength of light $\sim$ $10^{-5}$ cm = 0.00001 cm

Hence, it can be inferred that an optical instrument is the most suitable device to measure length.