# Diffraction Definition & Common Examples

• Greg Bernhardt
In summary, diffraction is the spreading, reflection, or bending of a wave when it encounters an aperture, obstruction, or opaque edge. It can also cause interference patterns and has a similar effect as a lens or prism. Bragg diffraction is the reflection of X-rays from a crystal, and it is a natural diffraction grating. Diffraction is observed in various types of waves, with larger wavelengths producing more pronounced effects. It can occur in near-field and far-field scenarios. Diffraction is also influenced by the size and geometry of the openings or obstructions, rather than the speed of the wave. The word "diffraction" comes from the Latin word meaning "to break," referring to the breaking of light into different colors.
Definition/Summary

Diffraction of a wave is the spreading or reflection or apparent bending when it encounters an aperture, obstruction, or opaque edge.

Diffraction by an evenly-spaced series of apertures (a diffraction grating) causes interference patterns and has the same bending or focussing effect as a lens or prism or mirror.

Bragg diffraction is reflection of X-rays from a crystal (a natural diffraction grating).

Equations
Extended explanation

In a physics curriculum, diffraction primarily arises when studying both optics and quantum mechanics.Common examples

Diffraction effects occur for all waves. Common examples studied are electromagnetic waves, sound waves, water waves, and (in quantum mechanics) matter waves or wavefunctions.

Water waves diffracting through a narrow aperature make for a good classroom demonstration, owing to the relatively slow wave velocity and large wavelengths involved.

For optical and other electromagnetic waves, common situations where diffraction effects arise or are studied are:
• The single slit
• The double slit
• The circular aperture
• Diffraction gratings
• Bragg diffraction (x-ray diffraction)
• Laser beams, including those with a Gaussian intensity profile.

Here are simulations of some common diffraction and interference patterns for visible light:

Top: diffraction pattern for a single slit
Middle: interference pattern for a double slit (the slit widths are identical to the slit width in the single slit pattern)
Bottom: diffraction grating pattern (the grating spacing is identical to the slit separation in the double slit pattern)
Near-field and far-field diffraction

For an opening of characteristic size a (the "source"), we distinguish between the near and far fields. For example, a could be the width of a single slit or the radius of a circular aperture.

At large distances from the source (the far field), the diffraction pattern grows linearly with distance but does not otherwise change shape appreciably. This is Fraunhofer diffraction. The pattern is essentially a function of angle, and the intensity diminishes as the inverse-square of distance from the opening. This happens for distances D where

D » a2

At closer distances (the near field), the observed diffraction pattern may change shape and size at different distances from the source, but does not grow linearly with distance. This is Fresnel diffraction, and occurs for distances

D < a2
Wavelengths of different wave types

Diffraction patterns are a function of the wavelength and geometry of the openings or obstructions, and do not depend on the speed of propagation of the wave. We may then order different types of waves by their wavelength, noting that larger wavelengths have more pronounced diffraction effects.

Wavelength (m), wave type

17, Sound in air (20 Hz, low frequency)
0.77 , Sound in air (440 Hz, "Concert A" frequency)
0.017, Sound in air (20 kHz, high frequency)
4-7 × 10-7, Visible light
10-10-10-9, x-rays (10 keV - 1keV)
1 × 10-10, de Broglie wavelength of a 100V electron
3 × 10-11, de Broglie wavelength of air molecules (20 C)
2 × 10-35, de Broglie wavelength of a bowling ball (6 kg, 6 m/s)

For diffraction to produce a 1 degree (far-field) spread in a beam -- a modest but clearly observable effect for visible light -- an aperture size of approximately 100 wavelengths is required. This is in contrast to a common misconception that the aperture size must be closer in size to the wavelength in order to observe diffraction.

Note that, due to the small wavelengths indicated above, observing diffraction for de Broglie wavelengths is difficult for microscopic objects and impossible (for all practical purposes) for macroscopic objects.

Latin derivation:

"diffraction" comes from the Latin frango frangere fregi fractum, meaning to break: this refers to the breaking of light into fractions, or different colours.

* This entry is from our old Library feature, and was originally created by Redbelly98.

Last edited by a moderator:
Thanks for the overview of diffraction!

## What is diffraction?

Diffraction is the bending or spreading of waves around obstacles or through openings. It is a characteristic of all types of waves, including sound, light, and water waves.

## How does diffraction occur?

Diffraction occurs when a wave encounters an obstacle or passes through an opening that is similar in size to its wavelength. The wave will bend and spread out, creating a pattern of interference.

## What are some common examples of diffraction?

One common example of diffraction is the way sound waves spread out around a corner or through a door. Another example is the way light waves diffract through a narrow slit, creating a pattern of light and dark bands known as diffraction patterns.

## What is the difference between diffraction and refraction?

Diffraction and refraction are both phenomena that occur when waves encounter obstacles or pass through openings. The main difference is that diffraction refers to the bending and spreading of waves, while refraction refers to the change in direction and speed of waves as they pass through a different medium.

## What are the practical applications of diffraction?

Diffraction has many practical applications, including in medicine, astronomy, and telecommunications. For example, X-ray diffraction is used to determine the structure of molecules in medicine and materials science, while radio waves are diffracted to improve the range and quality of telecommunications signals.

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