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Which of the following is the most precise device for measuring length: a vernier callipers 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?

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

Least count of vernier callipers

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

1 - 9/10 = 1/10 = 0.01 cm

Least count of screw gauge =

Pitch/Number of division = 1/1000 = 0.001 cm

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.

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