Question

The unit of luminous efficiency of electric bulb is:
(A) watt
(B) lumen
(C) lumen/watt
(D) lux

Answer 1

Hint
Luminous efficiency is the measure of how well the light source produces visible light. It is equal to the ratio of luminous flux to power i.e.
luminous efficiency$= \dfrac {luminous}{power}$.
As the unit of luminous flux is lumen and the unit of power is watt then we can easily find out the power of the luminous efficiency.

Complete step by step solution
Luminous efficiency is a measurement commonly used in the lighting industry that indicates the ability of a light source to emit visible light using a given amount of power. It is a ratio of the visible energy emitted to the power that goes into the bulb from the electrical line (visible energy emitted is also known as luminous flux, and the units are measured in lumens).
Now, a lumen can be defined as, A lumen (lm) is the total amount of light emitted by a light source. It’s a unit of luminous power. $1 lumen = 1$ candela (luminous power per unit solid angle) × 1 steradian (unit solid angle).
Therefore, the unit of luminous flux is lumen and the unit of power is watt.
Hence, unit of luminous efficiency is
luminous efficiency $= \dfrac{{lu\min outflux}}{{power}} = \dfrac{{lumen}}{{watt}}$
Hence, option (C) is correct.

Note
Luminous flux or power, the time rate of flow of luminous energy emitted from a light source. Expressed in lm. It's different from the radiant flux. Radiation flux is the measurement of all electromagnetic radiation emitted (including infrared, ultraviolet and visible), which is the total amount of objective light. Luminous flux is the amount of light that the human eye senses. It reflects the sensitivity of the human eye by weighting each wavelength with a luminosity function. So that it is the weighted sum of all wavelengths of power in the visible light band, excluding infrared and ultraviolet.