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?

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Solution

The most precise device is the one with the minimum least count.
(a) Vernier Calliper:
20 divisions on the sliding scale
L.C = $\frac{1}{20} m m = 0.05 m m$
(b) Screw Gauge:
pitch 1 mm and 100 divisions on the circular scale
L.C = $\frac{1 m m}{100} m m = 0.01 m m$
(c) Optical instrument:
L.C. is within a wavelength of light.
The wavelength of light ≈ 589 nm
Hence Optical instrument is the most precise.

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