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Question 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.
