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Q. A travelling microscope has $20$ divisions per $cm$ on the main scale while its Vernier scale has total $50 $ divisions and $25$ Vernier scale divisions are equal to $24$ main scale divisions, what is the least count of the travelling microscope ?

JEE MainJEE Main 2022Physical World, Units and Measurements

Solution:

$ 1 MSD =\frac{1}{20} cm $
$1 VSD =\frac{24}{25} MSD =\frac{24}{25} \times \frac{1}{20} cm$
$ \therefore \text { Least count }=\frac{1}{20}\left(1-\frac{24}{25}\right) cm $
$=\frac{1}{20} \times \frac{1}{25}=\frac{1}{500} cm $
$ =0.002\, cm$