What is the SI unit for luminous intensity?

• A. Joule
• B. Candela
• C. Newton
• D. Watt

Answer: (B)

The SI unit for luminous intensity is the candela. This unit measures the power emitted by a light source in a particular direction, weighted by the sensitivity of the human eye to different wavelengths of light. The candela is defined based on the monochromatic radiation of frequency 540 terahertz and carries a specific luminous intensity of 1/683 watts per steradian.

Understanding luminous intensity is crucial in fields such as lighting design, photometry, and visual ergonomics, as it helps in quantifying how bright a light source will appear to the human eye. The candela, therefore, serves as a standard for comparing intensity levels, ensuring consistency in measurements related to illumination.

The other choices represent different physical quantities: the joule is a unit of energy, the newton is a unit of force, and the watt is a unit of power, which measures the rate of energy transfer. Each of these units serves a distinct purpose within their respective fields, but none are applicable to measuring luminous intensity.