2.6 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 ?

A device with minimum count is the most suitable to measure length.

(a) Least count of vernier callipers

= 1 standard division (SD) – 1 vernier division (VD)

(b) Least count of screw gauge =

(c) Least count of an optical device = Wavelength of light ∼ 10^–5 cm

= 0.00001 cm

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