Question

The main scale of a spectrometer is divided into 720 divisions in all. If the vernier scale consists of 30 divisions, the least count of the instrument is (30 division of vernier scale coincide with 29 division of main scale) –

• A. 0.1⁰
• B. 1''
• C. 1'
• D. 0.1''

Answer 1

Answer: (C)

1'

Answer 2

LC = 1 MSD – 1VSD

LC = $\left( 1 - \frac { 29 } { 30 } \right)$ MSD = $\frac { 1 } { 30 }$ MSD

720 MSD = 360⁰

1 MSD = $\frac { 360 ^ { \circ } } { 720 }$ = 1/2⁰

So LC = $\frac { 1 } { 30 }$ × $\frac { 1 ^ { \circ } } { 2 }$ = $\frac { 1 ^ { \circ } } { 60 }$ = $\frac { 60 ^ { \prime } } { 60 }$ = 1'