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Difference Between Diffraction and Interference - A Complete Guide

Difference Between Diffraction and Interference - A Complete Guide

Vishal kumarUpdated on 02 Jul 2025, 04:45 PM IST

This article includes the definition of interference and diffraction, the difference between interference and diffraction, diffraction examples; Young’s single slit experiment and Young’s double-slit experiment.
Note: Interference meaning in Tamil is குறுக்கீடு, interference meaning in Bengali is হস্তক্ষেপ, and deflection meaning in Tamil is விலகல்

Difference Between Diffraction and Interference - A Complete Guide
Difference Between Diffraction and Interference

Interference and diffraction of light:

Huygens’s principle failed to explain the wave nature of light. In 1801, the famous physician Thomas Young with his experiment explained the wave and particle nature of light.

What is the interference of light?

By Young’s experiment, the superposition of light from the two-slit causes an interference pattern. The phenomenon of superposition of light waves is called interference of light.

Destructive interference and Constructive interference of light

Interference forms a wave having higher or lower amplitude. Hence, two types of interference of light are obtained by this experiment. They are constructive and destructive interference. When two waves overlap each other and form a wave with a higher amplitude is called constructive interference. When the waves overlap and cancel out each other then it is called destructive interference. If the phase difference between the waves is an even integral multiple i.e., 2Π, 4 Π… then constructive interference is observed whereas if the phase difference is an odd integral multiple of pi, i.e., Π, 3 Π, 5 Π, etc then it is destructive interference.

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What is the diffraction of light?

The bending of light around the narrow obstacles or openings and encroachment to the geometrical shadows is known as the diffraction of light.

Diffraction of light

Figure 2 Diffraction of light

Diffraction is observed in all kinds of waves such as electromagnetic waves, sound waves, water waves, etc. Fringes having light, dark, and coloured bands are produced by the diffraction of light. Diffraction gives the concept of reflection, refraction, and interference.

From Bragg’s law, the relation between the wavelength of light λ and the spacing of the crystal plane d is observed.

Bragg's law

Figure 3 Bragg's law

nλ=2dsinθ Where n is the integer and θ is the reflected angle.

Therefore, if the wavelength of light is proportional to the size of the opening, the bending of light is observed. The bending of light is visible if the opening is comparable to the wavelength of light. The bending is invisible if the opening is larger than the wavelength of light. Therefore, the direction of light depends on the wavelength of light and spacing between the slits.

Related Topics Link,

Difference between interference and diffraction

The two main phenomenons of wave optics interference and diffraction are differed by the following facts:

  • Interference is the wave formed from two discrete sources that resulting discrete wavefronts whereas diffraction is the secondary waves formed from discrete parts of the same wave.

  • In interference, the fringe width is identical while in diffraction fringe width is different.

  • More number of fringes is seen in interference and it is less in number in the case of diffraction.

  • Similar intensity is observed in all the points on maxima in the case of interference but in diffraction the intensity is variable.

  • In interference, a region having minimum intensity is dark as the value is close to zero. While in diffraction, intensity deviates for different positions as intensity doesn’t have zero value.

  • Interference sources have a maximum of two sources as reference sources while diffraction sources have more than the sources that are considered as reference sources.

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In this way we can differentiate between interference and diffraction.

Also Read :

Young’s single slit experiment:

In a single slit experiment, the monochromatic light is passed through a single slit. The pattern obtained on the background is similar to the slit used. As we move away from the central maximum, the width and intensity in single slit diffraction decrease.

Young’s double-slit experiment:

In a double-slit experiment, Young showed both the particle and wave nature of light. In this, two slits are used for the monochromatic light to pass through. Then, the pattern is observed in the background. The wave nature of light is responsible for the light to travel through slits and overlap each other causing light and dark bands on the background. In his experiment, he used two coherent light sources to cause the interference of light constructively or destructively.

Types of diffraction:

There are two types of diffraction. They are,

  • Fresnel diffraction: Both the light source and the background screen should be at a finite distance from the slit as the incident waves are not parallel to each other.

  • Fraunhofer diffraction: The incident light waves are parallel and so the light source and screen are at infinite distance from each other.

NCERT Physics Notes :

Important formulas in wave optics:

Young's double-slit experiment

Figure 4 Young's double-slit experiment

  • Separation of nth order bright fringe from the central fringe is,

yn=Dnλ/d , n=1, 2, 3……

Here, D is the distance between slit and screen, λ is the wavelength of the wave, d is the distance between two slits.

  • Separation of nth order bright fringe from the central fringe is,

yn=(2n-1)Dλ/2d , n=1, 2, 3…..

  • Fringe width of bright and dark fringe is,

β=Dλ/d

  • The angular position of nth order,

  1. Dark fringe= yn/D=(2n-1) λ/2d

  2. Bright fringe= yn/D= nλ/d for n=1, 2, 3…..

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Commonly Asked Questions

Q: What is the fundamental difference between diffraction and interference?
A:
Diffraction is the bending of waves around obstacles or through openings, while interference is the superposition of two or more waves. Diffraction occurs with a single wave source, whereas interference requires at least two wave sources or wave fronts.
Q: Can diffraction occur without interference?
A:
No, diffraction always involves interference. When waves diffract, they spread out and interfere with each other, creating a diffraction pattern. The observed pattern is a result of both diffraction and interference effects combined.
Q: What is the difference between constructive and destructive interference?
A:
Constructive interference occurs when waves combine to form a larger amplitude wave, resulting in brighter regions in an interference pattern. Destructive interference occurs when waves combine to cancel each other out, resulting in darker regions or nodes in the pattern.
Q: Why don't we observe diffraction effects in our daily lives with large objects?
A:
Diffraction effects are most noticeable when the size of the obstacle or opening is comparable to the wavelength of the wave. Since visible light has very small wavelengths (400-700 nm), diffraction effects are not easily observable with large objects in everyday life.
Q: How does the wavelength of light affect diffraction?
A:
The amount of diffraction increases with increasing wavelength. Longer wavelengths diffract more than shorter wavelengths when passing through the same size opening or around the same size obstacle. This is why red light diffracts more than blue light.

Frequently Asked Questions (FAQs)

Q: How does the principle of stationary phase apply to diffraction and interference phenomena?
A:
The principle of stationary phase states that the main contribution to a wave integral comes from regions where the phase is stationary. This principle is used to simplify the analysis of diffraction and interference patterns, especially in the far field, by identifying the dominant contributions to the wave field.
Q: What is the role of coherence length in interference experiments?
A:
Coherence length is the maximum path difference over which interference can occur. It's determined by the spectral width of the light source. For interference to be observed, the path difference between interfering waves must be less than the coherence length. This concept is crucial in designing interferometers and understanding their limitations.
Q: How does the principle of superposition apply to interference and diffraction?
A:
The principle of superposition states that the net displacement at any point is the sum of the individual wave displacements. This principle underlies both interference and diffraction phenomena, allowing us to calculate the resultant wave pattern by adding the contributions from all wave sources or parts of a wavefront.
Q: Why do we see interference patterns in thin soap films but not in thick glass plates?
A:
In thin films, the path difference between reflections from the top and bottom surfaces is small enough for the reflected waves to remain coherent and interfere. In thick plates, the path difference is much larger, exceeding the coherence length of the light, so no stable interference pattern forms.
Q: What is the significance of the Rayleigh criterion in optical systems?
A:
The Rayleigh criterion defines the minimum angular separation at which two point sources can be resolved. It states that two points are just resolvable when the central maximum of one diffraction pattern coincides with the first minimum of the other. This criterion is crucial in determining the resolving power of optical instruments.
Q: How does the principle of least time (Fermat's principle) relate to diffraction?
A:
Fermat's principle states that light follows the path of least time between two points. In diffraction, this principle explains why light bends around obstacles or spreads out after passing through an opening. The observed diffraction pattern represents the sum of all possible paths that satisfy this principle.
Q: What is the physical significance of the Fresnel zones in diffraction?
A:
Fresnel zones are concentric regions on a wavefront that contribute alternately constructively and destructively to the field at a point. They help explain diffraction patterns by dividing the wavefront into regions that interfere constructively or destructively. The concept is particularly useful in understanding near-field diffraction.
Q: What is the difference between primary and secondary maxima in diffraction patterns?
A:
In a diffraction pattern, the primary maximum (central maximum) is the brightest and widest. Secondary maxima are the smaller, less intense peaks on either side of the primary maximum. The intensity and width of these maxima depend on the nature of the diffracting aperture or obstacle.
Q: How does the concept of wave packets relate to interference and diffraction?
A:
Wave packets are localized groups of waves that represent particles in quantum mechanics. The interference and diffraction of wave packets demonstrate the wave-particle duality of matter. The spread of a wave packet over time relates to the uncertainty principle and affects the coherence of the waves.
Q: What is the significance of the Babinet's principle in diffraction theory?
A:
Babinet's principle states that the diffraction pattern of an opaque body is identical to that of a hole of the same size and shape in an opaque screen, except for the forward beam intensity. This principle simplifies the analysis of complex diffraction problems by relating complementary apertures.
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