Question

What are mole and candela

Answer 1

The SI system has two types of units of measure:

1. SI base unit : these are units that are defined by the physical properties of real objects or systems. For example, the second is defined by frequency of radiation emitted by cesium atoms. The seven base units are meters, kilograms, seconds, amperes, kelvin, moles, and candelas.
2. SI derived unit : these are units that are derived by multiplying and dividing base units. For example, the SI unit for speed is m/s.

So let's take a look at the SI definition of the mole: The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. The mole is defined in terms of the physical properties of a real object, the carbon atom. Therefore it is a base unit. You cannot get moles by multiplying and dividing the other base units.

The candela is a measure of luminous intensity, meaning the perceived brightness of light. This is different from the radiant intensity (i.e. the amount of power per solid angle) because the sensitivity of the human eye varies with wavelength. Here is the SI definition of candela: The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×10^12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. So the candela is defined in terms of a physical property: the human eye's perception of light at a particular wavelength and radiant intensity. Therefore it too is a base unit. Like the mole, it cannot be derived by multiplying and dividing other base units.