Definition of Dispersion of Light
A white light gets amazingly separated into different colors when it strikes a special medium like glass prisms. This phenomenon of light is known as the dispersion of light. Main reason for this to happen is the differing wavelengths of various colors. The varying wavelength assigns a unique nature of bending to each of them during the refraction process. The varying wavelength gives slightly varying speeds to different colors so that they enter the prism in a distinct way. Finally, they come out causing the beam to spread out into a spectrum of colors.
Finally, colourful patterns of lights commonly known as VIBGYOR are formed which is simply the abbreviated form of colors from shorter wavelength to the highest one: Violet, Indigo, Blue, Green, Yellow, Orange, and Red. The phenomena of dispersion is not just limited to physics, but a beautiful natural evidence. Spectroscopy is one major advancement in physics which totally relies on the theory of dispersion whereas the mesmerizing view of rainbows is the natural art of dispersion. While refraction also means the bending of light, dispersion slightly differs by including the aspect of separation of white light into colors based on wavelength.
Understanding dispersion helps to be clear on the concepts like how light interacts with different materials and also the physics behind creating lenses, prisms, and other optical devices that shape light.
Newtonโs Prism Experiment and the Visible Spectrum
The credit of inventing the term โspectrumโ goes to Sir Isaac Newton for his prism experiment conducted in the 17th century. It provided the foundation for the development of the branch of physics known as โOpticsโ.
In the experiment Newton passed the monochromatic light coming from the sun through a small hole. To obtain better visibility of the diffraction process, he did his work into a dark room and directed the light towards a triangular glass prism. As the light departed from the prism, it scattered into a band of colors which formed a beautiful view of the spectrum on the wall. Newton discovered that white light was not a single object, but rather a mixture of various colors.
Once more, to ascertain his findings, he positioned a second prism in the path of the scattered. Astonishingly, he found the colors again merging to give a white light. He drew his conclusions saying that this occurred due to the result of refraction rather than a prismatic influence.
This novel experiment revealed that all of the visible colors combine to form a white light on the sequence of diffractions that could be achieved singly or as a whole one color, as desired. As like reflection and refraction, dispersion also acts as a strong pillar in the field of optics.
Role of Wavelength and Refractive Index in Dispersion
Dispersion firstly requires refraction to occur and hence depends on the refractive index for this. The second most important factor to specially assign the phenomenon as dispersion is the wavelength. It determines how and to what amount the light gets bent by a certain medium.
As a wave completes its cycle in some periodic patterns, one cycle contains one maximum point and another minimum. Thus the distance between two cycles is measured from their peaks which is often known as the wavelength. Each constituent of a white light undergoes varying degrees of bends at different speeds while passing through a prism being denser than the former air medium because of each color varying wavelength (red has the longest and violet has the least). This phenomenon is called normal dispersion. Hence, this is the reason why violet bends the most and the red bends the least.
Sequence of Colors: VIBGYOR Order Explained
VIBGYORโViolet, Indigo, Blue, Green, Yellow, Orange, Red is the order based on the wavelengths of the colors (from smallest to largest). The wavelength generally ranges from 400 nm (for violet) to 700 nm (for red).
Each of these colors is part of the visible spectrum, which ranges approximately from 400 nanometers (violet) to 700 nanometers (red). The greater the refractive index for a particular wavelength, the more it bends, forming smaller angles with the normal. This is why the deviation is least for the violet color. Same reason clears the question of the red color appearing on top as it has the lowest refractive index among all.
All dispersion phenomena, like rainbows and diffraction experiments, display the same color sequence. Indigo has historically been known as a different hue because of its slightly different refractive index than that of violet. However, it has always been an arguable topic in modern physics as the difference is minimal.
Colors are the arts of nature, therefore wherever color plays its role like artistry, photography, optics and physics, the sequence of this spectrum is very important.
Angular Separation of Colors Through a Prism
Angular separation is the angle by which two different color paths are separated (typically of red and violet). This separation is often visible during two processes:
- When light enters the prism.
- When light leaves the prism.
Since the deviation is greatest between violet and red, their angular separation is also the greatest. This separation therefore depends on the angle of the prism, the refractive index of the medium (prism), and the wavelength of these colors. Thus the angle by which each color is separated by the prism makes the spectrum visible.
The angular separation in a normal prism can also vary from small to larger degrees. The key factor to make scientists able to determine the strength of color, its quality, intensity and hence the composition of a white light is the angular separation of different colors.
Deviation and Dispersion Angles: Key Equations
Angle of Deviation and Dispersion Angles are the major factors characterizing the phenomenon of dispersion.
- Angle of deviation (ฮด): The angle formed by the direction of the incident beam and the emerging ray is known as the angle of deviation (ฮด). It shows the degree of “bent” or “deviated” light as it travels through the prism. For minimum deviation, we have the equation:
ฮด=(ฮผโ1)A [Equation 1]
where:
- ฮด is the angle of minimum deviation
- ฮผ is the refractive index
- A is the angle of the prism.
- Angle of dispersion: It is the difference between the angles of the lowest (violet) and highest dispersed (red) color. It can be calculated by finding firstly the individual deviation angles of each color and then subtracting them from each other as required.
Dispersion angle = ฮดVโฮดR
where:
- ฮดVโ is deviation of violet,
- ฮดRโ is the deviation of red.
For the exact control of light and designing precise optical devices, these factors are the critical requirements.
Factors Affecting Dispersion: Prism Material and Angle
The dispersion of white light is also affected by certain factors and among them the major influencing factors are given below:
- Prism Material: Prism material of course provides various refractive indices that affect the bending of light. For instance, the same glass has various qualities and hence a prism made of flint glass results in more dispersion than crown glass because it has higher dispersion value. Materials with high dispersion values deviate colors more widely.
- Prism Angle (Apex Angle): The two refracting surfaces of a prism are joint with a certain angle known as the apex angle which can cause more bending of light while entering and escaping. This is also a main factor to cause greater spreading of light.
Other small influencing factors are:
- Wavelength of the incoming light: Greater wavelength provides greater dispersion.
- Incident angle: How the light enters the prism can have a minimum influence on the bending of light..
All optical designing members, researchers and anyone dealing with dispersion must understand these factors.
Dispersion vs. Refraction: Core Differences
For dispersion to happen, first refraction must take place while refraction is independent of dispersion. Therefore, a slight difference lies between two terms. Refraction makes the monochromatic light bend according to the refractive index of the medium and hence remains trapped within the same medium with the same monochromatic light. However, dispersion requires special 3-D geometrical shapes, which first refract the light and then make them escape out by separating the white light into various colors. These colors deviate according to their wavelength.
Refraction looks only at the difference in medium while dispersion looks for the wavelength of lights. Refraction results in a single white ray but dispersion creates an array of colours. Double dispersion i.e., the process of making dispersed light incident to another prism is capable of giving the original white light.
In a prism, a refraction is again followed by another refraction (entering the medium and coming out of the medium). Thus, dispersion is a special case of refraction but refraction is the main principle controlling all other phenomena of light curves.
Applications in Spectroscopy and Optical Instruments
Major application of dispersion is in spectroscopy where prisms are used to separate light into its component colors. This helps scientists to analyze the composition of any substance as color codes are unique for various substances. For example, light from a star or a moon differs from the sun which can be examined through spectroscopy. Some major applications are given below:
- Astronomy: Spectrographs are embedded with the telescopes to examine the light coming from different celestial bodies and understand their characteristics.
- Chemistry: Since every chemical has its own color imprint, an unknown chemical can be easily recognized.
- Environmental Monitoring: The pollution level in the environment can be measured by checking the concentration of pollutants through their light trapping capacity at various wavelengths.
- Medical Diagnostics: Dispersion is one of the concepts of light interaction used by devices such as pulse oximeters.
Diffraction gratings are the advanced dispersing instruments that are replacing prisms because of their higher accuracy in wavelength separation. However, prisms still remain useful in optical equipment like beam steering, and laser technology because of their longevity and unique features.
Rainbow Formation and Natural Dispersion Phenomena
Rainbow formation is the beautiful dispersion phenomenon observed in nature accompanied by refraction and total internal reflection. Rainbows are refracted by the droplets of rain and again dispersed by them. A chronological explanation of the occurrence of the phenomenon is given below:
- Rays of sun enter into the water droplet and get refracted.
- The rain droplets act as a small prism and splits into seven constituent colors of light.
- The light is strongly deflected and hence creates TIR inside the surface of the droplet.
- The other face of the prism again refracts light and finally light comes out spreading into a spectrum.
This sequence produces an arc-like array in the sky termed as Rainbow. The circular arc of colors seen in the sky is the result of this process. To see a rainbow, the observer needs to be aligned with the sun behind them and the raindrops falling in-between their distance. Each color spread out with different angles of separation (violet at around 40ยฐ, and red at about 42ยฐ).
Other common dispersions may be: Halos of sun and moon (refraction and dispersion caused by ice crystals present in the atmosphere), Iridescent clouds (dispersion of light by water droplets or small ice crystals), Sun dogs (similar to halos) etc. These mesmerizing sights prove that dispersion is not a complex physics theory but a beautiful natural outcome.
Minimizing Chromatic Aberration in Lens Design
The presence of dispersion in some optical devices, especially in lenses will be an issue in viewing, commonly known as chromatic aberration. This occurs when light hues do not converge at the same spot after being incident on a lens. This gives a blurred vision showing colored fringes around the objects.
When a lens built of a single material disperses light, it produces chromatic aberration as the light colors bend uniquely and cannot focus on the same point. Thus, the image quality is diminished and becomes an issue, especially for cameras, microscopes, telescopes etc.
For the correction, following measures are applied:
- Achromatic lenses: Two different glasses having different refractive indices are merged so as to cancel out the chromatic dispersion.
- Apochromatic lenses: It uses more than two lenses to converge light at a point and provide better correction.
- Digital corrections: Image software can reduce the visible effects of chromatic aberration in photos.
These flaws in optical devices can be a serious problem for our vision. Therefore, before constructing any optical devices these problems should be sorted out carefully.
Conclusion
Dispersion is firstly a natural phenomenon which was revealed by the Physics approach taken by Newton. It is grounded by Refraction for its occurrence. The existence of colors comes from the concept of dispersion, which says that the white light we see is also colorful inside. The particle nature of light, one of the reasons for quantum mechanics, also evolved from this concept. Advances in the study of dispersion like the spectroscopy made the study of colors more efficient, which could be utilised in astronomy, chemistry, medical field and various other fields. Prisms and gratings are two main dispersing materials
Although being a colorful natural aspect, dispersion can be disturbing for the vision (Chromatic aberration), if our optical devices are not made taking proper account of refraction and dispersion. Keeping all the ackwards aside, dispersion can be a very useful concept in optics influencing our daily lives to advance physics.
References
Hradaynath, R., & Singh, I. (2015). Lenses, Mirrors and Prisms.ย Encyclopedia of Optical and.
Houstoun, R. A. (1914). The dispersion of a light pulse by a prism.ย Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character,ย 90(618), 298-312.
https://en.wikipedia.org/wiki/Dispersion_(optics)
https://www.geeksforgeeks.org/physics/dispersion-of-light-by-prism/
https://byjus.com/physics/refraction-and-dispersion-of-light-through-a-prism/
