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(C) device with the smallest least count is the most precise for measuring length.$1.$ Least count of vernier callipers: Assuming $1$ main scale divis

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an optical instrument that can measure length to within a wavelength of light

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(C) device with the smallest least count is the most precise for measuring length.$1.$ Least count of vernier callipers: Assuming $1$ main scale division $(MSD) = 1 \; mm$,and $20$ vernier divisions $(VD)$ coincide with $19$ $MSD$,the least count $= 1 \; MSD - 1 \; VD = 1 \; mm - \frac{19}{20} \; mm = 0.05 \; mm = 0.005 \; cm$.$2.$ Least count of screw gauge $= \frac{	ext{Pitch}}{	ext{Number of divisions}} = \frac{1 \; mm}{100} = 0.01 \; mm = 0.001 \; cm$.$3.$ Least count of an optical device $\approx$ Wavelength of light $\approx 10^{-5} \; cm = 0.00001 \; cm$.Comparing the values: $0.00001 \; cm Since the optical instrument has the smallest least count,it is the most precise device.

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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?