# NCERT Class 11 Physics Solutions: Chapter 2-Unit and Measurement Part 4

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Question: 2.5

A new unit of length is chosen such that the speed of light in vacuum is unity. What is the distance between the sun and the earth in terms of the new unit if light takes 8 min and 20 s to cover this distance?

• Distance between the Sun and the Earth:

• Given that in the new unit, speed of light = 1 unit

• Time taken,

Question: 2.6

Which of the following is the most precise for measuring length:

(a) A Vernier calipers 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 instruments that cam measure length to within a wavelength of light?

A device with minimum count is the most suitable to measure length.

(a) Least count of Vernier calipers

= 1 standard division Vernier division (VD)

Vernier calipers look like

(b) Least count of screw gauge =

Screw gauge looks like

(c) Least count of an optical device = Wavelength of light

Hence, it can be inferred that an optical instrument is the most suitable device to measure length.