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Measurement of Length

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Measurement of Length - Lesson Summary

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We can easily measure length of various entities using appropriate instruments such as a metre scale for length of a cloth piece, a land tape for length of a building, a survey chain for length of roads, etc.

However, lengths to be measured can be as large as distance between two heavenly bodies or as small as the radius of an atom.

To measure large distances, we employ an important method called the parallax method.

If we look at an object with one eye closed, its position seems to be moved a little from the original position.

If you look at a pencil held in front of your eyes against a fixed reference point on a plain wall, with your left eye, closing your right eye and then with your right eye, closing your eye you observe that the position of the pencil seems to change with respect to the fixed reference point on the wall. This is called parallax.

You may call the two eyes are two different viewpoints, since when used separately, they create different lines of sights.

Distance between these two viewpoints or points of observation is called basis. Here, the basis is equal to the distance between two eyes.

We use exactly the same method to measure the distance of a distant star. Instead of two eyes, we view the star from two different observatories on the earth at the same time.

To measure very small distances such as the size of a molecule, special microscopes need to be used. We cannot employ devices like a Vernier callipers or a screw gauge to measure such lengths.

All instruments such as simple microscope, optical microscope or an electron microscope have a certain limitations in measuring lengths. However, we can use a simpler method to estimate the size of a molecule.







Let us take some oleic acid. Oleic acid is a soapy liquid with large molecular size of the order of 10–9 metre.

We need to form a monomolecular layer of oleic acid on water surface so as to measure the thickness or diameter of a molecule of the acid.

We learnt how to measure a tiny molecular distance or a massive stellar distance.

There are various scales of measurement in this range. A brief chart of the measurable distances is shown here:

Some of these units are named for usability purpose.


1 fermi = 1 f = 10–15 m

1 angstrom = 1 Å = 10–10 m

1 astronomical unit = 1 AU (average distance of the Sun from the Earth) = 1.496 × 1011 m

1 light year = 1 ly= 9.46 × 1015 m (distance that light travels with velocity of 3 × 108 m s–1 in 1 year)

1 parsec = 3.08 × 1016 m (Parsec is the distance at which average radius of earth’s orbit subtends an angle of 1 arc second)

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