Intensity of light is measured in:
Candela
Understanding the Measurement of Light Intensity
The question asks about the unit used to measure the intensity of light. In physics, particularly in photometry, the intensity of light emitted by a source in a particular direction is a specific physical quantity. Let's examine the provided options to determine which unit is appropriate for measuring light intensity.
Analysing the Given Units
We are given four options:
- Gauss
- Weber
- Candela
- Lumen
Let's look at what each of these units measures:
- Gauss: The Gauss (G) is a unit of magnetic flux density ($\vec{B}$) in the CGS system of units. It is related to magnetism, not light.
- Weber: The Weber (Wb) is the SI unit of magnetic flux ($\Phi$). Magnetic flux is related to the total magnetic field passing through a given area. Like Gauss, it is a unit of magnetism, not light.
- Candela: The Candela (cd) is the SI base unit of luminous intensity. Luminous intensity is defined as the luminous power per unit solid angle emitted by a point light source in a particular direction. This unit directly measures the intensity of light in a specific direction.
- Lumen: The Lumen (lm) is the SI derived unit of luminous flux. Luminous flux is a measure of the total amount of visible light emitted by a source in all directions. While related to light, it measures the total output, not the directional intensity.
Identifying the Correct Unit for Light Intensity
Based on the definitions of these units, the Candela is specifically defined as the unit for luminous intensity, which is the scientific term for the intensity of light in a particular direction. The other units measure magnetic properties (Gauss, Weber) or the total light output (Lumen).
Therefore, the intensity of light is measured in Candela.
Unit Comparisons in Physics
| Unit | Quantity Measured | Field |
|---|---|---|
| Gauss (G) | Magnetic Flux Density ($\vec{B}$) | Electromagnetism |
| Weber (Wb) | Magnetic Flux ($\Phi$) | Electromagnetism |
| Candela (cd) | Luminous Intensity | Photometry (Light) |
| Lumen (lm) | Luminous Flux | Photometry (Light) |
Revision Table: Physical Quantities and Units
| Quantity | SI Unit | Symbol |
|---|---|---|
| Magnetic Flux Density | Tesla | T (1 T = $10^4$ G) |
| Magnetic Flux | Weber | Wb |
| Luminous Intensity | Candela | cd |
| Luminous Flux | Lumen | lm |
| Illuminance | Lux | lx (lm/m$^2$) |
| Luminance | Candela per square meter | cd/m$^2$ |
Additional Information: Photometric Units Explained
Photometry is the science of the measurement of light in terms of its perceived brightness to the human eye. Several units are used in photometry to describe different aspects of light emission and reception:
- Luminous Intensity (Candela, cd): This measures the light power per unit solid angle emitted by a source in a specific direction. It's about how "bright" a light source appears when viewed from a certain angle.
- Luminous Flux (Lumen, lm): This measures the total amount of visible light emitted by a source in all directions (or within a specified solid angle). It's about the total light "output" of a source. A standard 100W incandescent light bulb emits about 1750 lumens.
- Illuminance (Lux, lx): This measures the total luminous flux incident on a surface, per unit area. It's about how much light falls on a surface, or how well a surface is "lit up". 1 lux = 1 lumen per square meter (lm/m$^2$).
- Luminance (Candela per square meter, cd/m$^2$): This measures the luminous intensity per unit area of light traveling in a given direction. It describes how much light passes through, is emitted from, or is reflected from a particular area, and falls within a given solid angle. It's often used to describe the brightness of displays or surfaces.
Understanding the difference between luminous intensity (Candela) and luminous flux (Lumen) is crucial. Intensity is directional, while flux is total output.
Similar Questions
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