Question

The candela is one of the seven base units in the SI system. Which physical quantity does the candela measure?

Hint:

Candela is one of the seven fundamental SI base units, and it is specifically tied to how bright a light source appears in a chosen direction.

Answer 1

Step 1: The SI system defines seven base units, and the candela, symbol cd, is the base unit used for measuring luminous intensity.

Step 2: Luminous intensity is a measure of how much visible light power a source sends out in a particular direction, per unit solid angle. It tells us how bright a source appears to the human eye when viewed from a given direction, unlike radiant intensity, which measures total power without weighting it for human vision.

Step 3: One candela is formally defined using a monochromatic source emitting light at a frequency of \( 540 \times 10^{12} \) hertz, with a radiant intensity in a given direction of 1/683 watt per steradian.

Step 4: Common examples of luminous intensity in candela include an ordinary candle, which gives roughly 1 candela, while bright torches or car headlights can be rated at hundreds or thousands of candela.

So the candela is the SI base unit of luminous intensity, not of luminous flux, illumination, or luminous efficiency, which are related but separate quantities.