What is the Difference Between Normal and Anomalous Dispersion

Normal and anomalous dispersion are two ways light interacts with a material. These depend on how the speed of different wavelengths (colors) of light are affected.

What is the difference between normal and anomalous dispersion? In normal dispersion, shorter wavelengths travel faster through the material than wavelengths, whereas in anomalous dispersion, longer wavelengths travel faster than shorter wavelengths.

Key Areas Covered

1. What is Normal Dispersion
      – Definition, Features
2. What is Anomalous Dispersion
      – Definition, Features 
3. Similarities Between Normal and Anomalous Dispersion
      – Outline of Common Features
4. Difference Between Normal and Anomalous Dispersion
      – Comparison of Key Differences
5. FAQ: Normal and Anomalous Dispersion
      – Answers to Frequently Asked Questions

Key Terms

Normal Dispersion, Anomalous Dispersion

What is Normal Dispersion

Normal dispersion is a phenomenon where light of different colors, or wavelengths, travel at slightly different speeds within a material. This happens because of the refractive index of the material. Refractive index is the measure of how much the material bends light depending on the wavelength.

Shorter wavelengths, like violet and blue, have higher frequencies. These interact more strongly with the atoms in the material. Therefore, light travels slightly slower through the material. Longer wave lengths such as red and orange have lower frequencies. They experience less interaction with the matter inside the material and therefore they travel through the material faster. Most transparent materials exhibit normal dispersion, especially in the visible light spectrum.

Spectrometers and Spectroradiometers use the separating power of normal dispersion to analyze light sources. When light passes through a prism, normal dispersion spreads the light into its component wavelengths, creating a spectrum. The intensity of each wavelength is then analyzed to understand the composition of the light source, identify materials, or study chemical reactions.

Dispersion is used in understanding the material properties. Studying the way light interacts with a material through normal dispersion can reveal information about its structure and composition. Analysis of the amount of dispersion can give information regarding the density, refractive index and the presence of specific elements or compounds. Normal dispersion principles can be used to design optical filters that selectively transmit specific wavelengths of light.

What is Anomalous Dispersion

In anomalous dispersion, longer wavelengths travel faster through the material compared to shorter wavelengths. Anomalous dispersion is less common than normal dispersion. This phenomenon is used in many applications.

Anomalous dispersion is used to create highly selective optical filters. These filters are used in chemical and biological sensing, medical imaging, and optoelectronics. It is also used in tailored laser design. By manipulating the speed of different wavelengths, scientists can ensure optimal laser operation to achieve output characteristics like pulse width and beam quality. Atmospheric remote sensing is another area where anomalous dispersion of light is used to interpret satellite and remote sensing data and develop better atmospheric models.

Similarities Between Normal and Anomalous Dispersion

1. Both phenomena are based on the relationship between a material’s refractive index and the wavelength of light.
2. They arise from the interaction between light and the atoms or molecules within a material.
3. Both phenomena are used in various optical devices.
4. Both are consequences of the specific properties of the material involved.

Difference Between Normal and Anomalous Dispersion

Definition

• Normal dispersion is a phenomenon where light of different colors, or wavelengths, travel at slightly different speeds within a material. Anomalous dispersion is a phenomenon where light of different colors travels at different speeds in a material, with longer wavelengths usually moving faster than shorter ones.

Type of Wavelength

• In normal dispersion, shorter wavelengths travel faster, while in anomalous dispersion, longer wavelengths travel faster.

Refractive Index

• In normal dispersion, the refractive index decreases as the wavelength increases. However, in anomalous dispersion, the refractive index increases with increasing wavelength.

Bending of Wavelengths

• In normal dispersion, shorter wavelengths bend more when passing through a material, whereas longer wavelengths bend more in anomalous dispersion.

Occurrence

• Normal dispersion is the most common behavior for transparent materials in the visible spectrum, such as glass and water. Anomalous dispersion occurs at specific wavelengths or with certain materials, such as some materials in the ultraviolet range.

FAQ: Normal and Anomalous Dispersion

1. What is the normal dispersion of a material?

Normal dispersion describes how a material interacts with different colors of light. Here, the refractive index, and thus bending, increases with higher frequencies (shorter wavelengths) of light.

2. What is an example of a normal dispersion?

A good example of a normal dispersion is the rainbow. Sunlight (white light) enters water droplets, and normal dispersion separates the colors based on wavelength. Violet bends the most, creating the rainbow’s outer arc, while red bends the least, forming the inner arc.

3. What is the most common type of dispersion?

Normal dispersion is the most common type of dispersion. For example, in a rainbow, red is on the outside, and violet is on the inside. This is because, in most materials, shorter wavelengths (like violet) bend more than longer ones (like red).

4. What are the characteristics of normal dispersion?

Normal dispersion occurs in materials where the refractive index decreases with increasing wavelength. This means shorter, higher-frequency waves travel faster than longer, lower-frequency waves.

5. What are the applications of normal dispersion?

This principle of normal dispersion is used in spectroscopes to analyze material composition by looking at the absorbed colors. Normal dispersion principles also can be used to design optical filters that selectively transmit specific wavelengths of light.

Reference:

1. “Normal Dispersion.” Encyclopedia Britannica. 
2. “Anomalous Dispersion.” Encyclopedia Britannica.

Image Courtesy:

1. “Light dispersion conceptual waves” By Lucas Vieira – Own work (Public Domain) via Commons Wikimedia