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. How does a mirror reverse things?
Mirror doesn't reverse things left to right. Writing looks reversed in a mirror because you have turned it around to the face of the glass. Mirrors actually shift things from back to front along a line through the mirror. In the same way, we see inverted images of ourselves on a spoon if the face of the spoon that is curved inwards is brought closer to us we will see an inverted image. On the other hand, if you bring back the side of the spoon, we will see an erect image. This instance helps us to understand that Mirrors need not be always flat; they can be curved or spherical Mirrors.
2. Explain the dispersion of light with an example of a prism?
When sunlight or white light is refracted through a prism, it splits into many-coloured races. The splitting up of white light into its seven constituent colours on passing through a transparent medium like a glass prism is known as dispersion of light. Different constituent colours of white light are bent by different amounts in a prism; light goes through two surfaces where refraction takes place as the surfaces are not parallel to each other; each colour of light travels in a different direction.
3. Why does white light consist of seven colours?
Light is constituted of seven colours of light; the seven colours are violet (V), Indigo (I), blue (B), green (G), yellow (Y), Orange (O), and red (R); they are collectively known as VIBGYOR. When white light is incident on a prism, it splits into seven constituent colours; the band of colours or any screen is known as spectrum. White light is split into an infinite number of colours. The human eye, being limited, can only distinguish a limited subset of those colours. Most people see about five or six colours in the spectrum. But, Isaac Newton, who first investigated and wrote about the spectrum, decided for purely mystical reasons that there should be seven colours, wrote the number down, and has been copied ever since.
4. Does light be diffracted through gaps?
Yes, light can be diffracted and can also be bent in corners. Light waves spread out when they pass through tiny gaps or holes. The smaller the gap, the more spreading that occurs. You can see diffraction for yourself if you squint your eyes almost closed and stare at a street light. As you close your eyes, you will see light spreading out as it diffracts through the gaps between your eyelashes.
