Question

The unit of Luminous Intensity is:
A. Candela
B. Watt
C. Lumen
D. Ampere

Answer 1

Hint:The luminous intensity belongs to the group of 7 base quantities which are now part of the International System of Quantities that are designated by the International System of Units (SI). All the units in the SI system can be derived from the units of these 7 basic quantities.

Complete step-by-step answer:
The base units that are used to measure the 7 basic quantities are:
A. Length – metre(m)
B. Mass – kilogram(kg)
C. Time – second(s)
D. Electric current – ampere(I)
E. Temperature – kelvin(K)
F. Amount of substance – mole(mol)
G. Luminous Intensity – candela(cd)

Luminous intensity is defined as the wavelength-related power emitted by a power source in a particular direction per unit solid angle. This quantity is based on the sensitivity of the human eye. This is further explained, by the help of luminosity function or the luminous efficiency function that describes the average spectral sensitivity of the human eye perception of the parameter called brightness.
The unit for luminous intensity is candela (cd).
The luminous intensity of a light in candela of wavelength $\lambda$is given by –
$I_v=0.683.\bar{y}(\lambda ).I_e$
where $\bar{y}(\lambda )$ is the standard luminosity function and $I_e$ is the radiant intensity in watt per steradian ${\displaystyle \frac{W}{sr}}$.

Hence, the correct option is Option A.

Note:Luminous intensity must not be confused with luminous flux, which is the total perceived power emitted in all directions. Luminous intensity is the perceived power per unit solid angle. To understand the difference, consider this example:
If a lamp has a 1 lumen bulb and the optics of the lamp are set up to focus the light evenly into a 1 steradian beam, then the beam would have a luminous intensity of 1 candela. If the optics are changed to concentrate the beam into ${\displaystyle \frac{1}{2}}$ steradian then the source would have a luminous intensity of 2 candela. The resulting beam is narrower and brighter, though its luminous flux remains unchanged.