Monday, 27 Sep 2021
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Monochromatic Light: Example, Uses & Frequently Asked Questions

The light that the Sun emits is white light. It is a polychromatic ray of light. Polychromatic light is a mixture of different monochromatic light rays. Therefore, monochromatic light is a ray of light with a single color. This idea was prevalent throughout the ancient world. It comes from the Greek word “monos” which means one. Astrophysics, astronomy, and even forensic sciences make wide use of monochromatic light.

The wavelength of a particular ray of light determines its color. For monochromatic light, the atoms emit electromagnetic radiation. Only the lights which fall in the visible spectrum of white light have such wavelengths that the eye can see. So, human beings cannot see the light which has a wavelength lower than violet light (ultraviolet rays) or higher than that of red light (infrared rays).

Violet light is present at the higher visible end of the electromagnetic spectrum. It has a very low wavelength but is still visible. When such a light ray passes through the atmosphere, it interacts with different molecules, dust, droplets, etc. The presence of these in the medium absorbs and scatters the light making it visible. Atomic transitions absorb or emit wavelengths which are energy packages.

Certain properties determine which atom will emit or absorb which wavelength of light. They depend both on the physical and chemical properties of the isotopes. Isotopes are atoms of the same element having the same atomic number but different mass numbers. So, atoms either absorb or emit quanta which are little packs of light.

Now how does absorption occur? When light passes through an atom, electrons jump to the outer orbits. This causes absorption. In this process, the quanta are totally absorbed. It is not transferred from one orbit to another in an infinite way.

Definition Of Monochromatic Light

Monochromatic light is defined as light or optical radiation which has one single frequency in the range of the optical spectrum. This is a spatial point that has a periodicity, bandwidth, and electric field strength which is purely sinusoidal. A source that emits a ray of monochromatic light is supposedly monochromatic.

It is exactly the opposite of polychromatic light. Examples of polychromatic light will include thermal radiation. Incandescent bulbs emit such radiation. They also exhibit a wide spectrum of different optical frequencies.

Monochromatic light is more suited for scientific processes. It is applied in optical and photonic calculations. The optical wavelength, as well as frequency of each light, are always the same. For instance, one has to calculate the value of laser beams this way.

Monochromatic Light

Does a Laser Emit Monochromatic Light?

Getting a source to emit monochromatic light was extremely difficult till laser technology came to the foreground. Before this, scientists used metal vapor lamps like mercury vapor or sodium vapor, or gas discharge lamps. These emitted light in the form of very narrow spectral lines and separated them with a monochromator. This process allowed the ray to have little power or intensity.

So, separating each light with the same wavelength from every other wavelength was necessary. This is what an optical monochromator does. It is, in fact, an optical filter that filters out a certain wavelength from the rest. Therefore, no color is seen. However, the rest of the wavelengths of the light are lost.

What is the Meaning Of Quasi-Monochromatic?

No real light source can ever be exactly monochromatic. So, a real light source will never have a bandwidth that is a perfect zero. So, all sources of light including lasers are generally quasi-monochromatic. This means that they have an extremely narrow bandwidth which is almost indistinguishable from monochromatic light. They share nearly the same characteristics too.

For instance, if the bandwidth is much lower than the concerned spectral characteristic in laser technology, the light is quasi-monochromatic. In quasi-monochromatic light, a lot of factors decide its optical bandwidth.

What is Monochromaticity?

Traditionally, monochromatic light meant light of a single color. In the visible optical spectrum, there are various colors of light. Each has a different wavelength. However, monochromaticity cannot function solely based on the color of light. This is particularly because a lot of colors might exist in non-monochromatic light. It can, in fact, include infrared or ultraviolet radiations as well.

Lasers generally emit quasi-monochromatic light. However, lasers can emit monochromatic or quasi-monochromatic light having extremely high optical strengths in comparison to light which bandpass filters emit. These have quite narrow bandwidths as well. Some lasers might display monochromaticity. However, for that, the optical bandwidth of the lasers should be so narrow that they are negligible. A laser that has a bandwidth less than 1 Hertz has a good enough and stabilized single-frequency. The light it emits might attain maximum monochromaticity.

Refraction of Monochromatic Light

Light gets bent or refracted on passing from one medium to another if the two media have different refractive indices. Refraction leads to several interesting everyday phenomena- like a mirage in a desert or bending of a spoon kept inside a glass of water. Refraction enables lenses to focus visible light rays at a single point. The angle of refraction of the monochromatic light at the interface depends on two things-

  • The angle of incidence at the interface.
  • The refractive index between the two different media.

Example of Monochromatic Light Refraction

For example, let us consider a case where a sine wave of red light passes from air to water, a different medium with a higher refractive index. The angle of refraction, in the beginning, is 40.51 degrees. However, when it enters the other medium, the angle can change with the help of the Incident Angle slider. The incident angle slider is set to default at a value of 60 degrees. The angle of refraction will change continuously as the slider is moved to the left, and the right.

A wavelength slider can change the wavelength of the incident light too. Each material has a different refractive index. So, the amount of refraction or bending of light depends on the material you choose too. The refractive index of vacuum or air is nearly a perfect one and let’s say in this case, the angle of incidence lies between 0 to 80 degrees on the interface.

If the angle of incidence is 90 degrees, that is light is normal to the interface, then no refraction occurs even with the change of media. The light passes straight out, perpendicularly. Any other incident angle will lead the light to bend or cause refraction at the interface. The angle of refraction will increase as the angle of incidence increases with respect to the interface.

So, if a ray of light strikes a water surface vertically, it will suffer no refraction. With every other angle, however, it will suffer one. The value might be more or less, proportional to the angle of incidence. This is important because the ratio of the angle of incidence to the angle of refraction for a particular pair of media must remain always the same. The ratio gives the value of the refractive index which is a characteristic of the pair of media and can under any circumstance change.

Refractive Indices

The refractive index of any material or medium calculates how faster or rather how slower light passes through it compared to the speed of light in the vacuum. The universal point of the reference is the refractive index of the vacuum. It is supposedly 1. The Refractive index of any other transparent material is denoted as n and calculated as-

n = c/v, where c is the speed of light in vacuum and v in that of the concerned medium.

Vacuum is devoid of any substances which can absorb or bend light. So light travels in it the fastest, and hence the refractive index is 1.0. Therefore, it is understandable that the refractive index of every other transparent material will be more than 1.0. This again can be calculated by various means.

Now, the refractive index of air is 1.0003. As it is almost the same as the refractive index of the vacuum, scientists use it to calculate the refractive index of other transparent materials.

The refractive index of substances is generally considered to be fixed, same throughout. However, more meticulous calculations gather that the value of this index can change with the wavelength or frequency of the concerned color of visible light or any radiation. So, in reality, a material has several refractive indices. Each of these indices can change a little or quite a lot with a change in the wavelength or frequency. Scientists call this phenomena dispersion and it exists for every transparent medium.

The way the refractive index changes with respect to the wavelength decides the degree of dispersion in a material. When the wavelength increases, the refractive index decreases, and hence light bends less and vice-versa. Violet light has the shortest wavelength. Hence it has the maximum refractive index and hence it disperses or deviates the most. It gets refracted at much higher angles than red light which has the highest wavelength. A prism disperses a polychromatic beam of light, splits its contents, and produces the spectrum consisting of 7 different monochromatic light.

Monochromatic Light

What are the Uses Of Monochromatic Light?

Monochromatic light finds various scientific and industrial uses.

Monochromatic light can measure extremely negligible surface width- like one-millionth of an inch. Self-contained units contain helium which produces glare-free light. This has a wavelength of 1/23.2 million times of 1 inch. Lapmaster Wolters Optical flats make such light easily available on reflective surfaces or semi-reflective surfaces to see.

Optical flats and a source emitting monochromatic light help in seeing fringe patterns of interference on a flat surface. A unit that is big enough to cover the surface completely can light up the optical flat properly. Each gas inside the light tube emits light with a different wavelength. These wavelengths of light serve as references for measuring the optical flats, where a light band is equal to half of the wavelength.

Tabletop monochromatic light lamps of CP Series come in 2 different models. The CP line offers them. Each unit is portable. It has hinges and latches to open and close. To keep the electrical cords there is a small storage unit. It comes with a handle to carry. The helium gas contained inside works on electricity which has a potential of 110 volts and a phase cycle of 50/60 hertz. The diffusing lenses of CP-1 and CP-2 measure 11’’ by 14’’ and 6’’ by 10’’ respectively.

The MLS line offers 2 models of monochromatic light – MLS-16 and MLS-8. In each unit, there is a sodium tube. A fabricated sheet of a metal housing with white paint encloses it. The potential of the electricity it gets should be 110 volts with a phase cycle of 50/60 hertz. The diffusing lenses of MLS-8 and MLS-16 measure 8’’ by 15’’ and 8’’ by 28’’ respectively.

Frequently Asked Questions About Monochromatic Light

Which devices make use of monochromatic light?

Ans. Monochromators make turnable monochromatic light available in different optical devices. One can easily measure the monochromatic light after transmission or reflection from a sample.

How to define monochromatic light in an easy way?

Ans. It is a kind of optical radiation that hosts only one frequency in the spectrum of light. Sources that emit monochromatic light are also often known as monochromatic.

Why is monochromatic light needed?

Ans. Each colored light provides a different interference pattern post the double-slit experiment. So, when white light is used, the central maxima becomes white while the first order maxima holds all the color from violet to red. Using monochromatic light fixes this issue.

How does it differ from visible white light?

Ans. It will have a single wavelength. On the other hand, white light is a combination of all the colors in the visible spectrum. Each color in it has a different wavelength- from 400 nm to 700 nm. A monochromatic source is coherent where the waves are in phase. Therefore, sources of visible light, like a bulb, are incoherent.

How does it differ from polychromatic light?

Ans. It contains light of a single wavelength like a laser beam. On the other hand, polychromatic light consists of light rays with different frequencies as well as wavelengths. Therefore, sunlight is an example of a polychromatic beam of light.

What is multichromatic light?

Ans. Visible white light has more than one constituent light. The difference in the wavelengths of the component color lights results in the formation of different colors. This is called multichromatic light.

Is white light monochromatic or polychromatic?

Ans. White light is not made of a single color but various color lights. So, it is polychromatic and not monochromatic.

Give an example of a polychromatic source?

Ans. Real light sources which emit light with different wavelengths, thereby producing different colors are polychromatic sources. Such a source would be the sun. Hence, the white light it emits is polychromatic.

What color light is sunlight?

Ans. Sunlight is an example of white light. However, it is not monochromatic as white light is a combination of 7 different lights.