Which of the following is the most precise devise for measuring length ? (a) a Vernier callipers with 20 divisions on the sliding scals, coindiing with 19 main scale divions (b) a screw gauge of pitch 1mm and 100 divisions on the circular scale (c ) an optical instrument that can measure length to within a wave length of light.

Explanation

The device having minimum least count is the most precise device for measuring length
(a) Least count of vernier callipers
`= 1 MSD- 1 VSD`
`=(1-(19)/(20))MSD`
`=0.005cm`
(b) Least count of screw gauge `=("pitch")/("total number of divisions on the circular scale")= 0.001cm`
(c ) Wavelength of light `lambda=10^(-6)cm= 0.000001cm`
Hence, an optical instrument that can measure length to within a wavelength of light (which is approx. equal to `10^(-6)m`) is the most precise device for measuring length.