What is the SI unit for measuring the luminous intensity?
Candela
Understanding SI Units and Luminous Intensity
The International System of Units (SI) provides a standardized set of units for measuring physical quantities. It includes base units for fundamental quantities and derived units for others. The question asks about the SI unit specifically used for measuring luminous intensity.
What is Luminous Intensity?
Luminous intensity is a measure of the power emitted by a light source in a particular direction per unit solid angle. It describes how bright a light source appears to be in a specific direction.
Analyzing the Options for Luminous Intensity Unit
Let's look at the provided options and determine which one is the correct SI unit for luminous intensity:
- Ampere ($\text{A}$): This is the SI base unit for electric current. It measures the flow of electric charge.
- Mole ($\text{mol}$): This is the SI base unit for the amount of substance. It is used to count atoms, molecules, and other particles.
- Candela ($\text{cd}$): This is the SI base unit for luminous intensity. It is defined based on the luminous efficacy of a specified radiation.
- Kelvin ($\text{K}$): This is the SI base unit for thermodynamic temperature. It measures the degree of hotness or coldness.
Based on the definitions of these SI units, the unit specifically designated for measuring luminous intensity is the Candela.
| Unit | Symbol | Quantity Measured |
|---|---|---|
| Ampere | $\text{A}$ | Electric Current |
| Mole | $\text{mol}$ | Amount of Substance |
| Candela | $\text{cd}$ | Luminous Intensity |
| Kelvin | $\text{K}$ | Thermodynamic Temperature |
Therefore, the SI unit for measuring luminous intensity is the Candela.
Revision Table: Key SI Base Units
| Quantity | SI Base Unit | Symbol |
|---|---|---|
| Length | metre | $\text{m}$ |
| Mass | kilogram | $\text{kg}$ |
| Time | second | $\text{s}$ |
| Electric current | ampere | $\text{A}$ |
| Thermodynamic temperature | kelvin | $\text{K}$ |
| Amount of substance | mole | $\text{mol}$ |
| Luminous intensity | candela | $\text{cd}$ |
Additional Information on Luminous Intensity and Units
Luminous intensity is distinct from other related photometric quantities like luminous flux (measured in lumens, $\text{lm}$), illuminance (measured in lux, $\text{lx}$), and luminance (measured in candelas per square metre, $\text{cd}/\text{m}^2$). While luminous flux measures the total light power emitted in all directions, luminous intensity focuses on the power per unit solid angle in a specific direction.
The definition of the candela was revised in 2019. It is now defined by fixing the numerical value of the luminous efficacy of monochromatic radiation of frequency $540 \times 10^ {12} \text{ Hz}$ to be $683 \text{ lm}/\text{W}$. This frequency corresponds to green light, near the peak sensitivity of the human eye.
Important Questions from Dimensions of physical quantities
The dimensional formula of force:
- Which of the following combinations of fundamental constants has the dimension of length, $[L^1]$? (Given: Planck constant $h = [ML^2T^{-1}]$, speed of light $c = [LT^{-1}]$, gravitational constant $G = [M^{-1}L^3T^{-2}])$
What is the formula of velocity gradient?
- The dimension of surface tension is ______.
The dimensions of EMF are
