Unit of Length - SI Unit and Other Common Units

Edited By Vishal kumar | Updated on Jul 02, 2025 05:01 PM IST

Here, in this article we are going to discuss SI unit of length, unit of length, standard unit of length, what the standard unit of length is, what is the SI unit for length, what the SI unit of length is, what is the standard unit of length, what SI unit of length is, what is the unit of length, basic unit of length, the basic unit of length is, what is the basic unit for length, all units of length, which unit is not for length, name the SI unit of length, What SI unit of length is called, define length in physics, what the standard unit of length is called, meter is the standard unit of, fundamental unit of length in the metric system etc.

Unit of Length - SI Unit and Other Common Units

SI unit of length

The SI unit of length is meter (m).

Define length in physics

We define length in physics as the length of the path travelled by light in a vacuum in 1⁄299792458 seconds.

What is the standard unit of length?

The standard unit of length is meter (m).

CGS unit of length

The CGS unit of length is centimeter (cm).

MKS unit of length

The MKS unit of length is meter (m).

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Standard unit of length

There are several ways to measure distance and the device needed to make the measurement depends on the range of length to be measured and requirement of the level of accuracy. Now, let us take an example of a meter scale. The divisions on the scale are made at a separation of 1mm. Hence, the minimum distance it can measure accurately is or . Similarly, the minimum length that a vernier caliper can measure accurately is or . Now let us compare the meter scale used by a tailor to measure the cloth. A tailor uses a measuring tape to measure the dimensions of the body. So, we conclude that the choice of the measuring instrument depends on what we actually want to measure. For example, a vernier caliper is suitable to measure the diameter of a rod or small ball etc.

Range of Length

The size of the proton is of the order of m and the range of the observable universe extends up to a distance of m. We can see that in physics there is a wide range of lengths. Apart from the standard SI units we often require different units to be used in different contexts for convenience. Let us know more about these units.

Astronomical Unit (AU): The average distance between the Earth and the Sun is called 1 AU.

Light Year (ly): One light year is the distance travelled by light in one year with a speed of m/s.

Parallactic second (parsec):- One parsec is the distance at which the average radius of Earth’s orbit subtends an angle of 1 arc second.

1 parsec = m.

1 fermi (f) = 1 f = m. It is used to describe the radius of the nucleus.

1 angstrom = 1m.

1 micron = 1m = m.

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All units of length

Some other units of length are:-

1. kilometer (km)

2. Hectometer (hm)

3. Decameter (dcm)

4. Decimeter (dm)

5. Nanometer (nm)

6. millimeter (mm)

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Other Non-SI Length Units

1. Foot

2. Yard

3. Mile

4. Inch

Special units of length:

1. 1 fermi = 1 f = 1 × m

2. 1 Angstrom = 1 Å = 1 × m

3. 1 astronomical unit = 1 AU = 1.496 × m. It is the average distance of the sun from the earth.

4. 1 light year = 1 ly = 9.46 × m. It is the distance travelled by light in one year, with a velocity of 3 × m/s.

5. 1 parsec = 3.08 × m. It is the distance at which the average radius of earth’s orbit subtends an angle of one arc second.

Also check-

NCERT Physics Notes:

Frequently Asked Questions (FAQs)

1. Which of the following is not the unit of length? (A) meter (B) centimeter (C) kilometer (D) Newton

Option (D) is not the unit of length .Newton is the unit of force.

2. What is length in physics?

We define length in physics as the length of the path travelled by light in a vacuum in 1⁄299792458 seconds.

3. Meter is the standard unit of?

Meter is the standard unit of length.

4. What is the standard unit of length?

The standard unit of length is meter (m).

5. What is the fundamental unit of length in the metric system?

The fundamental unit of length in the metric system or MKS system is meter (m).

6. Length is one of the fundamental quantities. Justify this statement.

The length of an object is measured in metres. One metre is equal to the length of path travelled by light in vacuum in time interval 1/299,792,458 of a second. Since the value is always the same, it is a fundamental quantity.

7. How can we measure the distance of a star which is nearly 50 light years away from us? Can we measure the distance from a star 150 light years away using the same method?

We can measure the distance of a star that is 50 light years away using the parallax method. The star will be observed from two different locations on earth and by measuring the distance between both the locations and angle subtended by the star on both positions, the star distance can be calculated. This method cannot be used to measure a star which is more than 100 light years as the accuracy of the result is not achieved. Thus, we cannot measure the star 150 light years away from us using the same method.

8. What is the SI unit of length and why was it chosen?

The SI unit of length is the meter (m). It was chosen because it's based on a fundamental constant of nature - the speed of light in vacuum. One meter is defined as the distance light travels in 1/299,792,458 of a second. This definition provides a universal standard that can be reproduced anywhere in the world with high precision.

9. How does the definition of a meter relate to the speed of light?

The meter is defined in terms of the speed of light in vacuum. Specifically, one meter is the distance light travels in 1/299,792,458 of a second. This definition links the unit of length directly to a fundamental constant of nature, ensuring its universality and reproducibility.

10. Why don't we use everyday objects to define units of length anymore?

Everyday objects are not reliable for precise measurements because they can change over time due to factors like temperature, humidity, and wear. For example, the meter was once defined by a metal bar, but this could expand or contract slightly with temperature changes. Using fundamental constants of nature, like the speed of light, provides a more stable and universally reproducible standard.

11. How has the definition of the meter changed over time?

The definition of the meter has evolved to become more precise and universal:

12. What's the difference between a meter and a metre?

There is no difference between a meter and a metre. "Meter" is the American English spelling, while "metre" is the British English spelling. Both refer to the same SI unit of length.

13. How does the metric system relate to the SI system?

The metric system is the predecessor to the SI system. The SI system (Système International d'Unités) is an updated and more comprehensive version of the metric system. It includes the metric units for length (like meters and centimeters) but also defines other base units and derived units for various physical quantities.

14. What's the difference between accuracy and precision in length measurements?

Accuracy refers to how close a measurement is to the true value, while precision refers to the consistency or reproducibility of measurements. A measurement can be precise (consistent) without being accurate (close to the true value), or accurate without being precise. For example, if you measure a 1-meter stick five times and get 99.8 cm each time, your measurements are precise but not accurate. If you get 100.2 cm, 99.7 cm, 100.1 cm, 99.9 cm, and 100.0 cm, your measurements are more accurate on average but less precise.

15. How do scientists measure extremely small lengths, like the size of an atom?

Scientists use various advanced techniques to measure extremely small lengths:

16. How does temperature affect the length of objects, and why is this important for precise measurements?

Temperature affects the length of objects through thermal expansion or contraction. Most materials expand when heated and contract when cooled. This is important for precise measurements because:

17. What's the smallest length that has physical meaning according to current physics theories?

The smallest length that has physical meaning according to current physics theories is the Planck length, approximately 1.6 × 10^-35 meters. This is derived from fundamental constants in nature: the speed of light, Planck's constant, and the gravitational constant. At this scale, our current understanding of space, time, and gravity breaks down, and quantum effects become dominant. It's theorized that lengths smaller than this may not have physical meaning, as the very concept of distance might lose its usual interpretation at such extreme scales.

18. How do we measure the distances to stars and galaxies?

Astronomers use several methods to measure vast cosmic distances:

19. How does the concept of length change in Einstein's theory of relativity?

In Einstein's theory of relativity, length is not absolute but relative:

20. What role does uncertainty play in length measurements?

Uncertainty is crucial in length measurements because:

21. How do we measure the length of curved objects accurately?

Measuring the length of curved objects accurately involves several techniques:

22. How do we account for relativistic effects when measuring distances in space?

Accounting for relativistic effects in space distance measurements involves:

23. How do we measure the thickness of extremely thin materials, like graphene?

Measuring the thickness of extremely thin materials like graphene involves advanced techniques:

24. How does the concept of length apply in fields like computer science or information theory?

In computer science and information theory, length concepts apply in various ways:

25. Why do scientists prefer to use SI units over other systems?

Scientists prefer SI units because:

26. How do you convert between different metric units of length?

To convert between metric units of length, you move the decimal point based on the prefix. Each step between common prefixes (kilo-, hecto-, deka-, base unit, deci-, centi-, milli-) represents a factor of 10. For example, to convert from meters to centimeters, move the decimal point two places to the right (multiply by 100). To convert from kilometers to meters, move the decimal point three places to the right (multiply by 1000).

27. What's the relationship between nanometers, micrometers, and millimeters?

These units are related by factors of 1000:

28. Why do we need such small units of length like nanometers?

We need small units like nanometers to measure and describe very tiny objects or distances, particularly in fields like nanotechnology, molecular biology, and quantum physics. For example, the diameter of a DNA helix is about 2 nanometers, and the size of individual atoms is typically measured in fractions of a nanometer. These small units allow scientists to precisely quantify and communicate about phenomena at the molecular and atomic scales.

29. How does the angstrom (Å) relate to SI units of length?

The angstrom (Å) is a non-SI unit of length equal to 10^-10 meters or 0.1 nanometers. While not an official SI unit, it's still commonly used in some scientific fields, particularly for describing atomic-scale lengths, such as atomic radii or wavelengths of light. To convert:

30. Why don't we use the wavelength of light to define the meter anymore?

The meter was previously defined using the wavelength of light emitted by krypton-86 atoms. However, this definition was replaced because:

31. Why do we still use non-SI units like feet and inches in some countries?

Some countries, like the United States, still use non-SI units due to:

32. What's the difference between a light-year and a parsec?

Both light-years and parsecs are units used to measure astronomical distances:

33. How do we ensure that length measurements are consistent across different laboratories worldwide?

Consistency in length measurements across laboratories is ensured through:

34. What is dimensional analysis and how does it relate to units of length?

Dimensional analysis is a method for analyzing the relationships between different physical quantities by identifying their fundamental dimensions. For length:

35. How do quantum mechanics and string theory challenge our classical notion of length?

Quantum mechanics and string theory challenge classical notions of length:

36. Why is the speed of light in vacuum considered a universal constant, and how does this affect our definition of length?

The speed of light in vacuum is considered a universal constant because:

37. What are some common misconceptions about units of length in everyday life?

Common misconceptions about units of length include: