What are coherent and incoherent sources of light?

Coherent sources are the sources emitting waves with zero or constant phase difference and the same frequency. Incoherent sources have variable frequency and phase differences.

What are interference and diffraction class 12?

Interference is the superposition of waves that emerged from the narrow slits. Diffraction is the superposition of waves of the same wave that emerged from each point on a single slit.

What are the conditions for interference?

The conditions for interference are, Light sources must be coherent sources. To have greater contrast, the amplitude of the electric field vector of the light wave should be the same.

Define Snell’s law.

Snell’s law states that the ratio of the sine of angles of incidence to the sine of the angle of refraction is a constant. It is given by, sin i/ sin r =n 1 /n 2 =constant=μ Here, μ is the refractive index, i is the angle of incidence, r is the angle of refraction.

What is the relationship between the distance of separation between the slit and the angular separation between fringes?

θ=λ/d , where θ is the angular separation between the fringes, λ is the wavelength of the wave, d is the separation between the slit.

What is Brewster’s angle?

At a particular angle of incidence, an unpolarized light ray exhibits maximum polarization. This angle is called Brewster’s angle. It can be represented mathematically as, μ=tani p Here, μ is the refractive index, i p is the polarizing angle Therefore, according to the equation, the tangent of the polarizing angle is equal to the refractive index of the medium.

Why do we see colored fringes in soap bubbles?

The colored fringes in soap bubbles are due to thin-film interference. Light waves reflect off both the front and back surfaces of the thin soap film. These reflected waves interfere, constructively or destructively, depending on their phase difference, resulting in the observed colors.

What is the relationship between interference fringes and wavelength?

The spacing between interference fringes is directly proportional to the wavelength of light. Longer wavelengths produce wider fringe spacing, while shorter wavelengths result in narrower spacing. This relationship allows scientists to use interference patterns to measure wavelengths accurately.

What is the principle behind an anti-reflective coating on camera lenses?

Anti-reflective coatings work on the principle of destructive interference. A thin film is applied to the lens surface with a thickness of one-quarter the wavelength of light. This causes reflected light waves from the top and bottom of the film to be out of phase, resulting in destructive interference and reduced reflection.

Why do thin films, like oil on water, show different colors at different thicknesses?

The colors observed in thin films are due to interference of light reflected from the top and bottom surfaces of the film. As the film thickness changes, the path difference between these reflections changes, causing constructive interference for different wavelengths (colors) at different thicknesses.

What is the physical significance of the Airy disk in diffraction patterns?

The Airy disk is the central bright spot in the diffraction pattern of a circular aperture. Its size determines the resolution limit of optical instruments like telescopes and microscopes. Two point sources can be resolved only if their Airy disks are sufficiently separated.

How does the double-slit experiment demonstrate the wave nature of particles?

When particles like electrons are used in a double-slit experiment, they produce an interference pattern similar to waves. This demonstrates that particles can exhibit wave-like behavior, a key principle of quantum mechanics. The interference pattern emerges even when particles are sent one at a time.

How does the refractive index of a medium affect interference in thin films?

The refractive index determines the phase change of light upon reflection at interfaces. For a thin film between two media, the refractive index differences affect whether there's a phase shift upon reflection, influencing the conditions for constructive and destructive interference.

How does the concept of wavefront relate to Huygens' principle in diffraction?

Huygens' principle states that every point on a wavefront acts as a source of secondary wavelets. In diffraction, this principle explains how waves can bend around obstacles or spread out after passing through an opening. The new wavefront is formed by the envelope of these secondary wavelets.

How does the width of interference fringes change with distance from the source?

As the distance from the source increases, the width of interference fringes also increases. This is because the angular separation between fringes remains constant, but the linear separation grows with distance. This effect is important in designing and interpreting interference experiments.

How does the concept of optical path length relate to interference?

Optical path length is the product of the physical path length and the refractive index of the medium. It determines the phase of a wave at any point. In interference, the difference in optical path lengths between interfering waves determines whether constructive or destructive interference occurs at that point.

Why do we sometimes see colored fringes in interference patterns with white light?

When white light is used in interference experiments, different wavelengths (colors) interfere constructively at different positions. This is because the conditions for constructive interference depend on wavelength. As a result, we see a spectrum of colors instead of just light and dark fringes.

How does the refractive index gradient in the atmosphere affect the apparent position of stars?

The atmosphere's refractive index decreases with altitude, causing light from stars to bend as it travels through layers of air. This atmospheric refraction makes stars appear slightly higher in the sky than they actually are. The effect is more pronounced near the horizon, leading to phenomena like the flattened appearance of the setting sun.

How does the phase change on reflection affect thin-film interference?

When light reflects from a medium with a higher refractive index, it undergoes a 180-degree phase change. This phase change is crucial in thin-film interference, as it determines whether the reflections from the top and bottom surfaces of the film interfere constructively or destructively.

How does the concept of temporal coherence relate to the spectral width of a light source?

Temporal coherence is inversely related to the spectral width of a light source. A narrow spectral width (nearly monochromatic light) corresponds to high temporal coherence, allowing for interference over larger path differences. Broadband sources have low temporal coherence and can only interfere over short distances.

What is the physical meaning of the visibility of interference fringes?

Fringe visibility is a measure of the contrast between bright and dark fringes in an interference pattern. It's determined by the degree of coherence between the interfering waves and the relative intensities of the sources. High visibility indicates well-defined fringes and good coherence.

How does the concept of group velocity relate to interference in dispersive media?

In dispersive media, different wavelengths travel at different speeds, affecting the group velocity of wave packets. This dispersion can lead to interesting interference effects, such as pulse broadening or compression. Understanding group velocity is crucial in fields like fiber optics and ultrafast optics.

What is the significance of the van Cittert-Zernike theorem in understanding spatial coherence?

The van Cittert-Zernike theorem relates the spatial coherence of light from an extended source to the Fourier transform of the source's intensity distribution. It's crucial in understanding how spatial coherence develops as light propagates and has important applications in fields like astronomy and imaging.

How does Young's double-slit experiment demonstrate both interference and diffraction?

In Young's experiment, light diffracts as it passes through each narrow slit, spreading out. The diffracted light from both slits then interferes, creating an interference pattern on the screen. This experiment showcases how diffraction and interference work together in wave phenomena.

How does the width of a single slit affect the diffraction pattern?

As the slit width decreases, the central maximum of the diffraction pattern becomes wider, and the intensity of higher-order maxima decreases. Conversely, as the slit width increases, the central maximum becomes narrower and more intense, with more visible higher-order maxima.

What is the difference between Fraunhofer and Fresnel diffraction?

Fraunhofer diffraction occurs when the light source and observation screen are effectively at infinity from the diffracting aperture, resulting in parallel incident and diffracted rays. Fresnel diffraction occurs when either the source or screen (or both) are at a finite distance, resulting in spherical wavefronts.

How does polarization affect interference?

For interference to occur, the interfering waves must have the same polarization or at least components along the same direction. Waves with perpendicular polarizations will not interfere. This property is used in various optical devices and experiments to control interference effects.

Why do we see a rainbow pattern when looking at a CD or DVD?

The rainbow pattern on a CD or DVD is due to diffraction grating effects. The closely spaced tracks on the disc act as a reflective diffraction grating, separating white light into its component colors. Different wavelengths (colors) are diffracted at different angles, creating the observed spectrum.

How does the number of slits in a diffraction grating affect the interference pattern?

As the number of slits in a diffraction grating increases, the principal maxima become sharper and more intense, while the secondary maxima become less prominent. This results in a higher spectral resolution, allowing for better separation of different wavelengths.

What is the difference between a diffraction grating and a prism in separating colors?

A diffraction grating separates colors based on their wavelengths through interference and diffraction, while a prism separates colors through refraction. Diffraction gratings can provide higher spectral resolution and are often more compact, but prisms can handle higher light intensities without damage.

How does the concept of coherence relate to interference?

Coherence is crucial for sustained interference. Two waves are coherent if they maintain a constant phase relationship. Only coherent waves can produce stable interference patterns. This is why laser light, which is highly coherent, is often used in interference experiments.

What is the difference between near-field and far-field diffraction?

Near-field (Fresnel) diffraction occurs close to the diffracting object, where the wavefront curvature is significant. Far-field (Fraunhofer) diffraction occurs at large distances, where wavefronts can be approximated as plane waves. The transition between these regimes depends on the wavelength and aperture size.

What is the principle behind holography, and how does it relate to interference?

Holography uses interference to record and reconstruct 3D images. A hologram is created by interfering a reference beam with light scattered from an object. When illuminated with a similar reference beam, the hologram diffracts light to reconstruct the original wavefront, creating a 3D image.

How do atmospheric conditions affect the twinkling of stars?

Star twinkling is caused by atmospheric turbulence, which creates pockets of air with varying refractive indices. This leads to interference and diffraction effects as starlight passes through the atmosphere, causing rapid changes in the star's apparent brightness and position.

What is the difference between spatial and temporal coherence in wave optics?

Spatial coherence refers to the phase correlation between different points in space on a wavefront. Temporal coherence refers to the phase correlation of a wave with itself at different times. Both types of coherence are important for creating stable interference patterns.

How does the concept of path difference relate to interference patterns?

Path difference is the key to understanding interference patterns. Constructive interference occurs when the path difference between interfering waves is an integer multiple of the wavelength. Destructive interference occurs when the path difference is an odd half-integer multiple of the wavelength.

What is the physical meaning of the diffraction limit in imaging systems?

The diffraction limit represents the smallest feature that can be resolved by an imaging system due to the wave nature of light. It's determined by the wavelength of light and the numerical aperture of the system. Features smaller than this limit appear blurred due to diffraction effects.

Difference Between Diffraction and Interference - A Complete Guide

Physics
Wave Optics
difference between diffraction and interference

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.

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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.

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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 n^th order bright fringe from the central fringe is,

y_n=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 n^th order bright fringe from the central fringe is,

y_n=(2n-1)Dλ/2d , n=1, 2, 3…..

Fringe width of bright and dark fringe is,

β=Dλ/d

The angular position of n^th order,

Dark fringe= y_n/D=(2n-1) λ/2d
Bright fringe= y_n/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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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?

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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