Video Transcript
Which of the following is the SI
base unit of luminous intensity? (A) Watt per square meter or (B)
candela.
This question is asking us for the
SI base unit of light intensity. Let’s first remember that luminous
intensity is a base quantity. And the word “luminous” refers to
light.
If we think about option (A), watt
per square meter, we have to remember that a watt is a compound unit, specifically
joules per second. And in turn a joule is another
quantity made up of kilogram meters squared per second squared. So option (A) can’t be a base unit
of SI, as it’s a compound unit.
In answer (B), we have the candela,
written cd. The candela is defined as the
luminous intensity of a source that emits a monochromatic radiation of frequency 540
multiplied by 10 to the power of 12 hertz in a specific direction and of which the
intensity is radiated in that direction is one six hundred and eighty-third watts
per steradian. In turn, the steradian is an SI
unit that measures solid angles. Although option (B) is thus made up
of other units, it is itself a base unit. It can be expressed as just one
candela, rather than needing to be divided or have a product taken of it.
Therefore, the correct answer to
“Which of the following is a base SI unit of luminous intensity?” is option (B)
candela.
