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Visible Spectrum

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The electromagnetic radiations can distribute according to their energy values. This distribution is known as electromagnetic spectrum. There are various regions in electromagnetic spectrum which differ in their frequency, wavelength and energy content.

Visible spectroscopy deals with the recording of the absorption of radiations in the ultraviolet and visible regions of the electromagnetic spectrum. The portion of the electromagnetic spectrum lying between 400 and 750nm is the visible region.

Visible Spectrum Definition

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It is the region between ultraviolet and infrared region which can be further subdivided into seven colors. Due to a certain frequency and energy values, it can be seen by human eyes and useful for us. The spectrum of visible region starts from red color and ended at violet color. The red color lies close to infra-red and violet color close to ultraviolet region. As we move from red to violet, the wavelength decreases and energy increases.

Visible Spectrum

Electromagnetic Spectrum Visible Light 

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Various regions of the electromagnetic spectrum with their frequencies and energies are as follows.

S.No
Regions in electromagnetic spectrum
Frequency in hertz
Energy in electron volt
1
Radio wave Less than 3 $\times$ 109
Less than 10-5
2
Microwave
From 3 $\times$ 109 to 3$\times$1012 From 10-5 to 0.01
3
Infrared radiations
From 3 $\times$ 1012 to 4.3 $\times$ 1014 From 0.01 to 2
4
Visible region
From 4.3 $\times$ 1014 to 7.5 $\times$ 1014 From 2 to 3
5
Ultraviolet radiations
From 7.5 $\times$ 1014 to 3 $\times$ 1017 From 3 to 103
6
X-rays
From 3 $\times$ 1017 to 3 $\times$ 1019 From 103 to 105
7
Gamma rays
More than 3 $\times$ 1019 More than 105


Here the ‘eV’ represents the unit of energy that is the electron volt. The electromagnetic spectrum starts from gamma rays, X - rays than move to ultra-violet rays. As we move from gamma rays to radio waves, there wavelength increases which further decrease the energy of radiation. That is the reason; radio ways can be used for satellite purpose and not harmful for human beings due to less energy content of them.

The energy of each segment of electromagnetic spectrum can be calculated with the help of relation between energy, frequency and wavelength as given below.

$\nu = \frac{C}{\lambda}$
and
$E = h\nu$

Where
  • $\nu$ = Frequency of radiation
  • C = Speed of light
  • $\lambda$ = Wavelength of light
  • E = Energy of radiation
The separation of all regions with their frequency, wavelength and energy can be written as below.

Frequency and Wavelength of Spectrum

Hence the frequency and wavelength of radiations are inversely proportional to each other. As the wavelength of radiations increases, it moves toward less frequency value and energy increases with increases with frequency values. On the basis of these energy values, all of these radiations can be classified as ionizing and non-ionizing radiations. Short wavelength radiations like ultra violet, X-rays and gamma rays are ionizing radiations due to high energy value while visible light, infrared and radiation are non-ionizing radiations due to long wavelengths or less energy values. Limits of both types of radiations can be drawn at ultra-violet spectrum.

Other regions of the electromagnetic spectrum are also useful such as radio ways, X-rays and gamma rays. Various applications of electromagnetic radiations depend upon their wavelength and frequencies of radiations. After ultra-violet region, the next region is the visible electromagnetic spectrum which shows around 4.3 x 1014 - 7.5 x 1014 Hz frequency and visible by human eyes.

Solved Example

Question: The Sun is a major source of light. Find out that this light is more visible light or more invisible light.
Solution:
 
Since sun gives all the radiations of electromagnetic radiations, therefore it is more invisible light as a complete electromagnetic spectrum consists of all the radiations like X-rays, gamma rays, microwaves, ultraviolet rays etc. All these radiations cannot visible by human eyes; therefore it is more invisible as visible region is just a small part of this electromagnetic spectrum.
 

Visible Color Spectrum 

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  1. We can remember the sequence of colors of the visible spectrum with the help of abbreviation ‘ROY G.BIV’ or "VIBGYOR" where R-Red, O- Orange, Y-Yellow, G-Green, B-Blue, I- Indigo, V-Violet.
  2. It is a continuous spectrum, therefore we cannot draw a line between any two colors of the visible spectrum or it is not possible to assign any region where a color is going to finish and another is going to start.
  3. Sometimes we are not considering Indigo color as a separate color and the visible spectrum is abbreviated as ‘ROY G.BV’ only.
  4. Overall this is a small region which can be seen by human eyes and lies between 4 x 1014 - 7.5 x 1014 Hz of frequencies or 750 - 400 nm of wavelength or 1.65 - 3.1 eV of quantum energies.

Visible Spectrum of Light 

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The visible light spectrum and their frequencies values of different regions are discussed below.
  1. Each color of visible light spectrum has characteristics frequency, wavelength and energy value therefore it can be identified easily by other colors.
  2. The wavelength of red color starts from 700 nm and moves to 630 nm.
  3. The range of orange color is in between 630-600 nm and yellow is close to 600-570 nm.
  4. The green and cyan color lie between 570 – 548 nm of range.
  5. The blue color starts from 480 nm and ended at 400 nm including violet color.
  6. We can calculate the visible light spectrum frequency values with the help of the relation between frequency and wavelength.
  7. Less than 390 nm lies in the ultraviolet region and not visible for human eyes while more than 750 nm represents the infrared region.

Solved Example

Question: The sunlight cannot approach to all parts of earth at the same time. Why?
Solution:
 
The earth spins around its axis, therefore, all the parts of earth cannot be in front of sun at the same time. Unlike earth, sun is an object but bigger than earth and located at a long distance from earth. Therefore sun can light the half part of earth at the same time.
 

Visible Spectrum Wavelength 

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The visible light spectrum wavelengths of different regions of visible spectrum are discussed below.
  1. The wavelength of violet Light lies around 400 nm and scattered more effectively compare to other colors.
  2. Remember, the color of sky is blue as humane eyes are more sensitive to blue light.
  3. The light coming from the Sun is also rich in blue color. Remember, the color of sky is blue as humane eyes are more sensitive to blue light. The light coming from the Sun is also rich in blue color.
  4. The short wavelength of the light makes it invisible for human eyes such as ultraviolet region.
  5. Next region is Indigo light which has the wavelength around 445 nm.
  6. The wavelength of blue light is more than indigo, around 475 nm, therefore can be scattered more in the atmosphere and provide blue color to sky. After blue, green color comes, with the wavelength of 510 nm.
  7. Remember the color of an object is due to the absorption of the entire color excluding the color of that.
  8. For example, the green color of grass is due to absorption of all other colors apart from the green which reflected back. 570 nm of light is related to yellow color which we can easily observe in sodium lamps used in parking lots.
  9. After yellow color, next orange comes whose wavelength is around 590 nm.
  10. The maximum wavelength is associated with red color which is around 650 nm.
Because of more wavelength, scattering of red and orange color is quite less, therefore the color of sunrise and sunset appears red or orange. There are many colors which cannot be seen by human eyes due to high or fewer wavelengths which cannot match with human eyes.

Like the shorter wavelength radiations (ultraviolet) cannot see by us and higher wavelength radiation, infrared is also not visible by human eyes. The wavelength of visible spectrum can be represents as below.
Wavelengths of Visible Region in Nanometre

Solved Example

Question: Write any two disadvantage of visible light in our everyday life.
Solution:
 
  • Since human beings can see only visible light therefore it is not possible for them to see in dark.
  • Because of long wavelength, these radiations cannot cross the skin and we cannot see through skin like X-rays.

 

Visible Spectrum Chart 

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Visible Light Spectrum Chart
The visible light spectrum chart represents all the parts of the visible region with their wavelength or frequency or energy values.
  1. It shows the relation of the visible region with ultraviolet and infrared region also.
  2. Visible region or visible spectrum or optical spectrum of light starts from 400 nm and ended at 700 nm in the electromagnetic spectrum.
  3. Both edges of the visible light spectrum chart blend into the infrared and ultraviolet levels of radiation.
  4. The combination of all the colors of visible light makes white color which can be divided into seven colors with the help of a prism and split the white light in the visible color spectrum which we can observe in rainbow.
  5. In the sky the airborne water particles act as a prism and divide the white light into seven colors.
  6. The visible light spectrum chart represents all the parts of the visible region with their wavelength or frequency or energy values.
  7. It shows the relation of the visible region with ultraviolet and infrared region also.
  8. Visible region or visible spectrum or optical spectrum of light starts from 400 nm and ended at 700 nm in the electromagnetic spectrum.
  9. Both edges of the visible light spectrum chart blend into the infrared and ultraviolet levels of radiation.
  10. The combination of all the colors of visible light makes white color which can be divided into seven colors with the help of prism and split the white light in the visible color spectrum which we can observe in rainbow.
  11. In the sky the airborne water particles act as a prism and divide the white light into seven colors.

Visible Spectrum Frequency

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The frequency of the visible region lies from 4 x 1014 - 7.5 x 1014 Hz. All the colors of the visible region are composed of different frequencies and wavelength, therefore associated with different energy values and color. The wavelength of the visible region lies from 780 nm to 390 nm. The highest wavelength is close to infrared, and lowest one is close to ultraviolet region. As the wavelength increases, the frequencies of radiation decrease with energy values.

The human eyes can detect only visible region, not the close region like ultra violet or infra red one. But it does mean that other radiations are not useful. Honeybee uses ultraviolet region while snake can see infrared region only. Both of these animals cannot detect the visible region like human beings. The ultraviolet and infrared regions are harmful for human beings.

The ultraviolet radiations are used for welding purpose but can be harmful for human eyes. These radiations cause sun burn, irritation and grit in eyes. The different color of various organic and inorganic compounds is due to absorption of certain radiations and reflects others. The color of compound depends upon the frequency of reflected radiations.

Solved Example

Question: Is there any relation between Compton Effect and visible light?
Solution:
 
For the Compton scattering, the energy of the photon should be very much greater than the binding energy of an electron. Generally the threshold frequency of metals is equal to the energy of ultraviolet light, not visible light. Therefore the Compton Effect does not observe with visible light.
 

Visible Spectrum Range 

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  • The range of the visible spectrum is from 700 to 400 nm while UV visible spectrum lies from 200 to 800 nm range where 200-400 nm represents the ultraviolet region.
  • The frequency value of the visible region lies between ultraviolet and infrared regions.
  • Let’s discuss about the frequency and wavelength values of different colors of visible regions.
  • The frequency of red color lies between 400 to 484 THz and wavelength is around 620-750 nm.
  • It is the lowest frequency value in the entire visible region.
Other frequency values and wavelength for different colors are as follows.

Color Frequency
Wave length
Orange
484 THz
590 nm
Yellow
508 THz
570 nm
Green
526 THz
495 nm
Blue
606 THz
450 nm
Violet
668 THz
380 nm

Out of these colors, indigo is not considered in visible spectrum which lies around 425-450 nm and 670-700 THz of frequency value and is made a subset of violet color. Because of low range it’s difficult to distinguish indigo color in the spectral band. Three colors; gray, white and black do not include in the visible spectrum and considered as non-spectral colors. White is the combination of all the colors and black shows the deficiency of any color. The combination of two colors can create a new shade and hues. The color of various organic compounds depends upon their absorption of the certain frequency of light.

For example, the yellow color of quinone is due to its aromatic structure. Similarly, the presence of double bond and conjugation or resonance can also affect the color of organic compounds. The deep red or orange color of carbonyl compounds is due to conjugation of double bonds with lone pair or carbonyl group or aromatic ring. The color of organic compounds can be detected with the help of a spectrometer.

Similarly, the human eyes also functions as a spectrometer and can analyze the light. When we see sunlight, it looks like a uniform and homogenous but it is not correct. It is composed of a broad range of radiation and mainly rich ultraviolet, visible and infrared radiations. When we pass the visible component of it through a prism, it separates into seven colors at different degree of angle according to their wavelength and frequency values. We must remember that wavelength is the distance between adjacent peaks or troughs. It generally designates in meters, centimeters or nanometers.
  • The frequency shows the number of wave cycles from a fixed point in per unit time and represents in cycles per second, or hertz (Hz).
  • The frequency and wavelength of radiation can show the type of color and its position in the visible spectrum.
  • For example; 420-430 nm of light is associated with yellow color while 500-520 nm of light represents red color.
  • Similarly, the absorption of green color is close to 400 nm. The particular color of compounds and dyes make them useful for various purposes such as in decoration, painting and others.
Dyes are generally organic or can be inorganic compounds such as kermesic acid, crocetin, the blue and indigo dye, crimson pigment, and the yellow saffron pigment. The dibromo derivative of indigo is named as punicin which is useful for the coloring of robes of the royal and wealthy. Another example is carotene which is a deep orange color dye and widely found in plants. The color of these organic and inorganic compounds is because of absorption of certain color and reflection of others. The reflected color is responsible for the color of the compound. The presence of conjugation between pi-electrons and lone pair or only pi-electrons absorbs some radiations and imparts color to compounds.

Solved Example

Question: Find out the reason of ultra violet and visible absorption of Cytochrome-C.
Solution:
 
The Cytochrome –C contains amino acids with aromatic groups which can involve in conjugation which is in the range of ultraviolet and visible region.