Editorial Feature

# What Is Diffraction?

This article provides an overview of diffraction, its historical development, its role in understanding wave-particle duality, and its wide-ranging applications in modern science and technology.

Image Credits: Ivanagott/shutterstock.com

## Understanding Diffraction

Diffraction refers to the bending of waves, such as light, when they encounter obstacles or pass through small openings. The size of the opening relative to the wavelength of light influences the degree of diffraction. When the opening is comparable to or smaller than the wavelength, significant diffraction occurs, causing a noticeable bending of light. If the opening is much larger than the wavelength, diffraction effects are minimal and often invisible.

This phenomenon creates distinctive patterns through the interference of light waves. When wave crests align, they amplify the wave intensity (constructive interference), resulting in brighter regions. Conversely, when crests and troughs coincide, they cancel each other out (destructive interference), leading to darker areas. This interference pattern is key to many diffraction phenomena and applications.

For instance, in the atmosphere, diffraction can create effects such as the silver lining of clouds or coronas around the sun and moon. It is also responsible for colorful bands on CDs and DVDs and influences the resolution of optical instruments.1,2

## Historical Context and Key Discoveries

#### Huygens' Wave Model

In 1678, Dutch physicist Christiaan Huygens proposed that light propagates as a wave through a medium, with every point on a wavefront acting as a source of secondary wavelets, expressed as:

s = vt

Where s is distance, v is speed, and t is time.

Each point on a wavefront emits semicircular waves at speed v, forming a new wavefront tangent to these wavelets. This principle, applicable to all waves, explains why waves diffract around the edges of opaque materials due to the obstruction of some wavelets.

While Huygens' model accounts for linear and spherical wave propagation, reflection, and refraction, it does not fully explain diffraction effects.2

#### Young's Double-Slit Experiment

In 1801, Thomas Young verified the wave nature of light through his double-slit experiment. He observed that light passing through two closely spaced slits created an interference pattern of dark and bright fringes on a screen. This demonstrated diffraction, with light waves overlapping and interfering constructively or destructively.

For constructive interference (bright fringes), the path difference between the two waves must be an integral multiple of the wavelength:

dsinθ = mλ

Where d is the slit distance, θ is the angle from the original beam direction, λ is the wavelength, and m is the interference order (0, ±1, ±2, ...).

For destructive interference (dark fringes), the path difference must be a half-integral multiple of the wavelength:

dsinθ = (m + ½ λ)

Young's observation of multiple bands rather than just two verified Huygens' wave model of light, as particles would not produce such a pattern.3

#### New Horizons

The 20th century saw further developments in the understanding of diffraction with key breakthroughs in X-ray crystallography.

In 1912, Max von Laue and his colleagues demonstrated the wave nature of X-rays by observing diffraction patterns when X-rays were directed through a copper sulfate crystal. This finding confirmed that X-rays interact with the regular arrangement of atoms in a crystal, producing diffraction spots.

Following this, William Henry Bragg and his son Lawrence used these insights to formulate Bragg's Law, which enables the determination of atomic structures from diffraction patterns.

These pivotal discoveries led to the development of X-ray crystallography and spectroscopy, earning von Laue the Nobel Prize in 1914 and the Braggs in 1915 for their contributions to crystal structure analysis.4

#### Diffraction and Wave-Particle Duality

The concept of diffraction played a crucial role in developing our understanding of wave-particle duality in quantum mechanics.

In 1924, Louis de Broglie hypothesized that particles could exhibit wave-like properties, with a wavelength inversely proportional to their momentum, expressed by the equation:

λ = h/p

Where h is Planck's constant, λ is the wavelength, and p is momentum.

He suggested that all matter, regardless of size, could display wave-like behavior under certain conditions.

De Broglie's hypothesis was validated later by the Davisson-Germer experiment in 1927 when Lester Germer and Clinton Davisson observed that electrons bombarding a nickel surface produced a diffraction pattern similar to that of X-rays in crystals.

This result not only confirmed the wave nature of electrons (particles) but also led to the development of electron microscopy and marked a significant shift in physics, showing that the distinction between waves and particles is not absolute and paving the way for quantum mechanics.5,6

## Applications of Diffraction

#### Bio-molecular Structure Analysis

Biomolecular structures are analyzed by directing X-rays at samples to create distinctive diffraction patterns. These patterns generate three-dimensional electron density maps, revealing the complex structures of proteins and nucleic acids and providing insights into functional sites and molecular interactions. This technique was crucial in Watson and Crick's discovery of the DNA double helix structure.7

#### Structural Information of Crystalline Materials

Single crystal diffraction is pivotal for obtaining detailed three-dimensional structural information of crystalline materials. It allows for examining molecular packing, guest molecules in frameworks, intermolecular interactions, and molecular conformations. This method is valuable for studying the nature of static and dynamic disorders in materials.4

#### Piezoelectric Ceramics Inspection

X-ray and neutron diffraction are commonly used to analyze ferroelectric and ferroelastic domain structures in piezoelectric ceramics, understanding the impact of electric fields on material behavior.

The choice between synchrotron X-ray or neutron sources impacts data quality, influencing resolution, sampling volume, and peak profiles. This precision is important for optimizing piezoelectric ceramics, where controlling particle size and distribution is necessary for achieving high efficiency and quality.

By interpreting diffraction patterns and applying models like Bragg's Law, researchers can determine structural parameters such as lattice spacing and phase composition, which are pivotal for enhancing the performance of sensors and actuators.8,9

#### Enhancing Component Safety and Efficiency

Strain-scanning is vital for assessing internal stresses in components to prevent failures and enhance design for safety and durability.

Energy dispersive diffraction utilizes a polychromatic "white beam" of X-rays to produce diffraction spectra that reveal information about crystal structures. This technique allows for measuring internal strains and is useful for inspecting thick samples and engineering components.

Its non-destructive nature makes it ideal for studying chemical reactions and processes under realistic conditions, optimizing reaction conditions, and monitoring equipment performance.4

#### Mineralogy and Geological Studies

X-ray diffraction (XRD) is used in mineralogy and geology to identify minerals and understand geological formations. It provides detailed structural information that cannot be obtained through traditional physical tests alone and can analyze poorly formed crystals, amorphous materials, or small samples.

XRD helps geoscientists determine mineral compositions, study phase transitions, and investigate geological history. It detects structural defects and variations, providing information on properties and formation conditions.

Additionally, it helps calculate thermodynamic parameters like compressibility and thermal expansion, making it crucial for interpreting mineral chemistry and predicting behavior in both research and practical applications.10

## Future Outlooks

Advancements in diffraction techniques are set to revolutionize various fields of science and technology in the coming years.

For instance, the development of compact diffractive imagers by UCLA researchers demonstrates how overcoming the traditional diffraction limit can revolutionize bioimaging, lithography, and materials science. This innovation enabled capturing features smaller than the wavelength of light, leading to new possibilities in imaging subwavelength structures and optimizing design in various industries.11

In geological and planetary sciences, advanced diffraction techniques are helping to understand planetary climates and compositions. A recent study comparing Martian soil to Earth's subarctic soils highlights how advanced X-ray diffraction methods reveal critical insights into Mars' historical environment, enhancing our knowledge of its potential for supporting life.12

Metasurface technology, particularly with quasicrystal designs, represents another exciting frontier. By combining holographic imaging with unique diffraction patterns, these advanced metasurfaces offer dual functionality that could lead to breakthroughs in ultra-thin devices for high-resolution displays, light-switching applications, and optical security measures. This development promises to streamline device design and enhance light manipulation, paving the way for new, sophisticated technologies.13

As diffraction techniques and technologies evolve, they will optimize performance in critical applications and drive innovation in fields ranging from materials science to planetary exploration.

More from AZoOptics: Understanding Bragg's Law in X-Ray Diffraction

1. University of Illinois. (2024). Diffraction of Light. [Online] University of Illinois. Available at:http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/opt/mch/diff.rxml
2. LibreTexts. (2024). Diffraction. [Online] LibreTexts. Available at: https://phys.libretexts.org/Bookshelves/University_Physics/Physics_(Boundless)/26%3A_Wave_Optics/26.2%3A_Diffraction
3. OpenStax College. (2024). Young's Double Slit Experiment. [Online]. https://openstax.org/books/college-physics/pages/27-3-youngs-double-slit-experiment
4. OptenStax. (2024). Diffraction for Industry. [Online] OptenStax. Available at: https://www.diamond.ac.uk/industry/Industry-News/Latest-News/Synchrotron-Industry-News-Focus-Diffraction.html
5. Norton, JD. (2022). The Quantum Theory of Waves and Particles. [Online]. University of Pittsburgh. Available at: https://sites.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/quantum_theory_waves/index.html
6. Weinert, F. (2009). Davisson—Germer Experiment. Compendium of Quantum Physics. doi.org/10.1007/978-3-540-70626-7_45
7. Galli, S. (2014). X-ray crystallography: One century of Nobel Prizes. Journal of Chemical Education. doi.org/10.1021/ed500343x
8. Rubio-Marcos, F., Fernandez, JF., Ochoa, DA., García, JE., Rojas-Hernandez, R. E., Castro, M., Ramajo, L. (2017). Understanding the piezoelectric properties in potassium-sodium niobate-based lead-free piezoceramics: Interrelationship between intrinsic and extrinsic factors. Journal of the European Ceramic Society. doi.org/10.1016/j.jeurceramsoc.2017.04.045
9. Horiba Scientific. (2024). Piezoelectric Ceramics. [Online] Horiba Scientific. Available at: https://static.horiba.com/fileadmin/Horiba/Application/Materials/Ceramics/AN212_Particle_Size_Analysis_of_Piezoelectric_Ceramics.pdf
10. Lavina, B., Dera, P., Downs, RT. (2014). Modern X-ray diffraction methods in mineralogy and geosciences. Reviews in Mineralogy and Geochemistry. doi.org/10.2138/rmg.2014.78.1
11. Hu, J., Liao, K., Dinç, NU. et al. (2024). Subwavelength imaging using a solid-immersion diffractive optical processor. eLight. doi.org/10.1186/s43593-024-00067-5
12. Feldman, AD., Hausrath, EM., Rampe, EB., et al. (2024). Fe-rich X-ray amorphous material records past climate and persistence of water on Mars. Commun Earth Environ. doi.org/10.1038/s43247-024-01495-4
13. Xu, C., Zhao, R., Zhang, X., et al. (2024). Quasicrystal metasurface for dual functionality of holography and diffraction generation. eLight. doi.org/10.1186/s43593-024-00065-7

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## Article Revisions

• Jul 29 2024 - This article was updated on 29 July 2024

Written by

### Owais Ali

NEBOSH certified Mechanical Engineer with 3 years of experience as a technical writer and editor. Owais is interested in occupational health and safety, computer hardware, industrial and mobile robotics. During his academic career, Owais worked on several research projects regarding mobile robots, notably the Autonomous Fire Fighting Mobile Robot. The designed mobile robot could navigate, detect and extinguish fire autonomously. Arduino Uno was used as the microcontroller to control the flame sensors' input and output of the flame extinguisher. Apart from his professional life, Owais is an avid book reader and a huge computer technology enthusiast and likes to keep himself updated regarding developments in the computer industry.

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