Question

Explain the methods to measure length.​

Answer 1

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The SI unit of length, the metre, was taken to be the distance between two lines marked on the international protoype bar of platinum- iridium alloy kept at the International Bureau of Weights and Measures (BIPM), Sévres, France. This definition of the metre was replaced in 1960 by a more precise one based on the wavelength of certain light and subsequently, by one based on the speed of light in vacuum. Direct measurement of length using various measuring devices is possible for lengths from about 102 m to 10 -5 m. A metre scale can measure up to 10 - m. Vernier callipers, screw gauge and spherometer can measure up to 10 - 5 m. To measure distances beyond the reach of measuring instruments, such as a planet or a star, we use the method of trigonometric parallax

[Note: Two methods are used to measure lengths beyond 102 m:

(i) triangulation and parallax methods involving angular measurements

(ii) reflection methods using radio waves, ultrasound and laser. Both triangulation and reflection methods are used to measure large distances and heights on the Earth. Parallax method is used to measure distances of planets and nearby stars.]

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