Question

The one which is not the unit of length is
A. Angstrom unit
B. Micron
C. Parsecond
D. Steradian

Answer 1

You can start by explaining all the options given in the problem. Remember to mention how angstrom, micron, and parsecond are units of length. Then explain how steradian is not a unit of length.

Complete answer:
Let’s discuss all of the above options one by one.

Angstrom is a unit of length that is used to measure very small distances. It is not the SI (International System) unit of length (the SI unit of length in meters). The symbol of angstrom is $\mathop A\limits^o$ ( $\mathop A\limits^o$ is a Swedish alphabet).
$\mathop {1A}\limits^o = {10^{ - 10}}m$

Micron (also known as micrometer) is a unit of length that is used to measure small distances. It is also not the SI (international system) unit of length. The symbol of micron is $\mu m$ .
$1\mu m = {10^{ - 6}}m$
Also, $1\mu m = {10^4}\mathop A\limits^o$

Parsecond (also known as parsec) is a unit of length that is used to measure very large distances like astronomical objects. One parsec is equal to $31$ trillion kilometers or $3.3$ light-years.
$1pc = 3.08 \times {10^{16}}m$
Also, $1pc = 3.3$ light years

Steradian is not a unit of length, instead, it is a unit of measuring solid angle. A steradian can be defined as the solid angle subtended by a unit area of a sphere at the center of the sphere. A steradian to a sphere is what degree is to a circle.

Hence, angstrom, micron, and parsec are all units of length, only steradian is not a unit of length.

So, the correct answer is “Option D”.

Note:
In the solution above we discussed three units of length i.e. angstrom, micron, and parsecond. But we should remember that no matter what unit we measure the length in, it will not change the length itself but the perspective. By perspective, we mean that if the length is very small then we use angstrom, if it is small then we use micron and it is too big we use parsec.