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.
Atom_Diagram.svg
 
Generally, the amount of substance is directly proportional to the number of atoms or molecules present in that substance.
 
AmountofsubstanceNumberofatomsormolecules
 
One can easily count the number of copper coins in the given picture.
 
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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.
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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.
 
candle-1750640_1280.jpg
 
Measuring device
A photometer, also known as Luminous Intensity Meter, is a device used to measure the luminous intensity in terms of candela.
 
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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.
Reference:
https://upload.wikimedia.org/wikipedia/commons/thumb/8/80/Atom_Diagram.svg/768px-Atom_Diagram.svg.png
https://cdn.pixabay.com/photo/2014/08/24/07/24/pennies-426011_1280.jpg