Light – Electromagnetic energy
Light is a type of radiation that moves through space. Animals such as humans are sensitive to light to see and understand the world around them. All the lights seem special, it's really just another kind of electromagnetic energy like microwaves and radio waves. Light normally travels in straight line rays and reflects and refracts in very precise ways as it speeds through the world. Most of the light we see with our eyes is quite weak because it has already refracted things. Not all light is so weak, however, light beams made by lasers are super concentrated and can be powerful enough to slice through metals.
Radiation can also be set to consist of particles or packets of energy called photons. Light is only part of the electromagnetic spectrum that is visible. White light from the Sun is made up of all the visible wavelengths of radiation which can be seen when it is supplied by using a prism. Light like all forms of electromagnetic radiation can be reflected or bounced back and refracted different parts of the electromagnetic spectrum are produced in different ways. Sometimes visible light and infrared radiation is generated by vibrating particles of warm or hot objects.
How Light is Measured?
Light is not a physical quantity, and, therefore, cannot be measured. Hence, when we talk about the unit of light, it does not make much sense in practical terms. However, as light includes several physical quantities like wavelength, speed, intensity, etc., that can be measured, the unit of light can be estimated by using these physical properties. But, before we proceed towards understanding what the unit of light is, let's first have a look at what is light, how can we define it.
Light refers to one of the parts of the electromagnetic spectrum, which ranges from radio waves to gamma rays. As the name suggests, electromagnetic radiations are the fluctuations of electric and magnetic fields, which transport energy from one location to another. Visible lights are not inherently different from other parts of the electromagnetic spectrum but are available with the exception that the human eye can detect them.
Electromagnetic spectrum radiation in another way is described as a stream of photons, the massless particles travelling with wavelike properties, and at the speed of light. A photon is the smallest quantity of energy that can be transported, and it was the realisation that light travels in discrete quanta, the origins of Quantum theory.
Visible light often has wavelengths in the range of 400–700 nanometres (nm), or we can say 4.00 x 10-7 to 7.00 x 10-7 m, amid the infrared rays (with longer wavelengths) and the ultraviolet rays (with shorter wavelengths). This specified wavelength means a frequency range of nearly 430–750 terahertz (THz).
The primary properties of light include propagation direction, intensity, frequency or wavelength spectrum, and polarization, while its speed in a vaccum, 299, 792, 458 metres per second (m/s), is one of the fundamental constants of nature. Light, as with all sorts of electromagnetic radiation (EMR), is found to always move at this speed in a vacuum.
Units and Measures
Light, in general, is measured with two sets of units:
Radiometry emphasises measurements of light power at all wavelengths, and
Photometry measures light with wavelength weighted for a standardised model of human brightness perception. Photometry is useful in quantifying illumination (lighting) intended for human use.
Three main types of unit of lights are as follows:
Candela
Lumen
Lux
Candela:
The candela refers to the base unit of luminous intensity in the International System of Units, i.e., (SI); that is, luminous power per unit solid angle emitted by a point light source in a specified direction. However, luminous intensity is analogous to radiant intensity, but rather than simply adding up the contributions of each wavelength of light in the spectrum of the sources, the contribution of each wavelength is weighted by the standard luminosity function.
The Candela is expressed by taking the fixed numerical value of the luminous intensity of monochromatic radiation of frequency 540 x 1012 Hz, Kcd, to be 683 if expressed in the unit lm W-1, which is equal to cd sr kg-1 m-2 s3.
The frequency chosen in the visible spectrum is near the green, corresponding to the wavelength of around 555 nanometres (nm). If adapted for bright conditions, the human eye is most sensitive near this particular frequency. But, at other frequencies, a significantly more radiant intensity is needed to reach the same luminous intensity, according to the human eye’s frequency response. The luminous intensity for light with wavelength λ is given as
Iv (λ) = 683.002 lm/W. ӯ (λ). Ie (λ),
Where,
Iv (λ) = luminous intensity
ӯ (λ) = photopic luminosity function
Ie (λ) = radiant intensity
If there is more than one wavelength, then one must integrate over the wavelengths spectrum to achieve the total luminous intensity.
For example:
A candle, in general, emits light with a luminous intensity of roughly 1cd.
A 25 W fluorescent light bulb gives out about 1700 lumens, and if that light is radiant equally in all directions, it will have a luminous intensity of:
Iv = 1700 lm / 4 π sr ≈ 135 lm/sr = 135 cd.
If focused on a 20°0 beam, the same light bulb would have an intensity of around 18,000 cd within the beam.
Lumen
The lumen is the SI unit of luminous flux, the rate at which light (visible) is emitted from a source. Luminous flux differs from radiant flux (power) in a way that the radiant flux includes all the emitted electromagnetic waves, while flux is weighted as per a model (a luminosity function) of the sensitivity of the human eye to various wavelengths.
In relation to the candela, the lumen is defined as:
1lm = 1cd · sr.
As a complete sphere has an angle of 4 π steradians, a light source that radiates one candela in all directions holds a total luminous flux of 1 cd x 4 π sr = 4 π cd• sr ≈ 12.57 lumens.
Lux
The lux refers to the SI unit of illuminance and luminous emittance, measuring flux per unit area, and it is equal to one lumen per square metre. In photometry, it is a measure of the intensity of light that hits or passes through a surface.
A particular amount of light, if it is spread over a comparatively larger area, will illuminate the surface more dimly. Hence, we can say that the illuminance, if the luminous flux is kept constant, is inversely proportional to the area. Lux is related to the lumen in a way that one lux is equivalent to a lumen per square metre and is given as:
1 lx= 1 lm/m2 = 1 cd•sr/m2
For example:
A flux of 1000 lumens concentrated on an area of one square metre lights up that area with an illuminance of 1000 lux. However, spreading out the same flux over 10 square metres would produce a dimmer illuminance of just 100 lux.
Light- Electromagnetic energy
Light is a type of radiation that moves through space. Animals such as humans are sensitive to light to see and understand the world around them. all the lights seem special, it's really just another kind of electromagnetic energy like microwaves and radio waves. light normally travels in straight line rays and reflects and refracts in very precise ways as it speeds through the world. Most of the light we see with our eyes is quite weak because it has already refracted things. not all light is so weak however light beams made by lasers are super concentrated and can be powerful enough to slice through metals
Radiation can also be set to consist of particles or packets of energy called photons. Light is Only part of the electromagnetic spectrum that is Visible. white light from the sun is made up of all the visible wavelengths of radiation which can be seen when it is supplied by using a Prism. Light like all forms of electromagnetic radiation can be reflected or bounced back and refracted different parts of the electromagnetic spectrum are produced in different ways. Sometimes visible light and infrared radiation are generated by vibrating particles of warm or hot objects. The mission of light in this way is called incandescent light can also be produced by fluorescence a phenomenon in which electrons gains and lose energy within atoms
FAQs on Unit of Light
1. What are the primary units used to measure light in Physics as per the CBSE 2025-26 syllabus?
In Physics, light is measured using a system called photometry, which accounts for the human eye's sensitivity. The three primary units are:
- Candela (cd): The SI base unit for luminous intensity, which measures the amount of light emitted by a source in a specific direction.
- Lumen (lm): The unit for luminous flux, representing the total amount of visible light emitted by a source in all directions.
- Lux (lx): The unit for illuminance, which measures the amount of light that falls upon or illuminates a specific surface area.
2. What is the SI base unit of light, and what does it specifically measure?
The SI base unit for light measurement is the Candela (cd). It does not measure the total light output but rather the luminous intensity. This refers to the light power emitted from a point source per unit solid angle in a particular direction, weighted by the sensitivity of the human eye.
3. What is the practical difference between a lumen and lux? Please provide an example.
The key difference is that lumen (lm) measures the total light output from a source, while lux (lx) measures the light's intensity on a surface. For example, a projector emitting 1000 lumens will always have that same output. However, if it projects onto a small 1-square-metre screen, the surface illuminance is 1000 lux. If it projects onto a larger 10-square-metre screen, the same 1000 lumens are spread out, resulting in a dimmer illuminance of only 100 lux.
4. How are the units candela, lumen, and lux mathematically related?
These units are directly related to one another. The lumen is derived from the candela, and the lux is derived from the lumen.
- Lumen from Candela: One lumen is the luminous flux emitted into a solid angle of one steradian by a point source having a uniform intensity of one candela. The formula is: 1 lm = 1 cd ⋅ sr.
- Lux from Lumen: One lux is the illuminance produced when one lumen is distributed over an area of one square metre. The formula is: 1 lx = 1 lm/m².
5. Why is the official definition of the candela based on a specific frequency of green light?
The candela is defined using monochromatic radiation of frequency 540 x 1012 Hz, which corresponds to green light, because the human eye is most sensitive to this part of the visible spectrum under well-lit (photopic) conditions. By pegging the definition to the peak of human visual sensitivity, the entire system of photometric units (lumen, lux) becomes a practical measure of how bright light appears to us, rather than just its raw physical energy.
6. How does measuring light's perceived brightness (photometry) differ from measuring its total energy (radiometry)?
The key difference is human perception. Radiometry measures the total energy of electromagnetic radiation at all wavelengths, using units like the Watt (W). It is an objective measure of energy. In contrast, photometry measures how bright the light appears to the human eye, using units like the lumen (lm). Photometric measurements are weighted, giving more significance to wavelengths like green and yellow where our eyes are most sensitive, and less to deep reds or blues, even if their energy is identical.
7. Is a 'light-year' a unit used to measure light intensity or brightness?
No, this is a common misconception. A light-year is a unit of distance, not of light intensity, brightness, or time. It represents the enormous distance that light, travelling in a vacuum, covers in one Julian year (approximately 9.46 trillion kilometres). It is used in astronomy to measure the vast distances between stars and galaxies.
8. If a light bulb is rated at 1200 lumens, does its lumen value change if you move it from a small closet to a large warehouse?
No, the lumen value does not change. The luminous flux (lumens) is an intrinsic property of the light source itself and remains constant at 1200 lm regardless of its environment. What changes is the illuminance (lux). In the small closet, the 1200 lumens are concentrated on a small area, creating high lux (a bright space). In the large warehouse, the same 1200 lumens are spread over a much larger area, resulting in very low lux (a dim space).
