Verified Solution • 2025–26 Session moderate

Q1.6: NCERT Physics - Part 1 UNITS AND MEASUREMENT 1.6 Solution: Which of the following is the most precise device for measuring …

⚠️ Common Exam Traps & Conceptual Pitfalls

Students often fail to convert the optical instrument's precision (wavelength of light, approx. $500 \text{ nm}$) into the same unit (mm) as the mechanical instruments, leading to incorrect magnitude comparisons. Always standardize units before comparing least counts to identify the most precise device.

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?

Step-by-Step Verified Solution

**Answer:** (c) The optical instrument. **Step-by-Step Explanation:** Least count of each: - (a) Vernier callipers: 1 mm/20=0.05 mm1 \text{ mm}/20 = 0.05 \text{ mm} - (b) Screw gauge: 1 mm/100=0.01 mm1 \text{ mm}/100 = 0.01 \text{ mm} - (c) Optical instrument: about 500 nm=5×10−4 mm500 \text{ nm} = 5 \times 10^{-4} \text{ mm} The optical instrument has the smallest least count, hence the most precise.

All Questions in this Chapter

Q1.1 Q1.2 Q1.3 Q1.4 Q1.5 Q1.6 Q1.7 Q1.8 Q1.9 Q1.10 Q1.11 Q1.12 Q1.13 Q1.14 Q1.15 Q1.16 Q1.17

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