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?

Answer:

(a) Least count is given by –

$1-\frac{9}{10}=\frac{1}{10}=0.01cm$

(b) Least Count in this case is given by –

$\frac{pitch}{Divisions(total)}=\frac{1}{1000}$

Least count = 0.001 cm

(c) In this case, least count = wavelength of light

= 10^-5 cm

Therefore, least count = 0.00001 cm