### Theory:

Amount of substance is a measure of the number of entities or particles present in a substance. The particles present in a substance are atoms, molecules, ions, electrons, protons, etc. Generally, the amount of substance is directly proportional to the number of atoms or molecules present in that substance.

$\mathit{Amount}\phantom{\rule{0.147em}{0ex}}\mathit{of}\phantom{\rule{0.147em}{0ex}}\mathit{substance}\propto \mathit{Number}\phantom{\rule{0.147em}{0ex}}\mathit{of}\phantom{\rule{0.147em}{0ex}}\mathit{atoms}\phantom{\rule{0.147em}{0ex}}\mathit{or}\phantom{\rule{0.147em}{0ex}}\mathit{molecules}$

One can easily count the number of copper coins in the given picture. But, how can one count the number of copper atoms present in a coin?

It is not possible to count the copper atoms since the atoms are invisible to the naked eye.

Mole is the unit used for the measurement of the amount of a substance. It is denoted as mol.
Mole is defined as the amount of substance that contains $$6.023 × 10^{23}$$ entities or particles. The value $$6.023 × 10^{23}$$ is known as Avogadro number.
Luminous intensity
Luminous intensity is the measure of the power of the emitted light, by a light source in a particular direction, per unit solid angle. Candela is the unit of luminous intensity, and it is denoted as 'cd'.
One candela is approximately equal to the light emitted from a wax candle. In other words, an ordinary candle emits light with roughly $$1\ cd$$ of luminous intensity. Measuring device
A photometer, also known as Luminous Intensity Meter, is a device used to measure the luminous intensity in terms of candela. Uses
• A photometer is mainly used in photography to check the intensity of light.
• An umpire, during a cricket match, checks the intensity of light using this instrument.
Luminous flux
Luminous flux, often known as luminous power, is a measurement of the perceived power of light. Its SI unit is lumen.

The luminous flux given by a light source emits one candela of luminous intensity over a solid angle of one steradian is defined as one lumen.
There are two extra units known as derived units in addition to the seven fundamental units.
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