Jasmine Grover Content Strategy Manager
Content Strategy Manager
Coherent sources are formed when two waves have a constant relative phase or a zero or constant phase difference and the same frequency. The features of the interrelationship between physical quantities of a single wave or between numerous waves are described by coherence. Coherence also allows for stationary interference in waves. Also, interference visibility is commonly used to determine the amount of coherence. For Example: Two parallel slits illuminated by a laser beam can be categorized as two coherent point sources.
| Table of Content |
Key Terms: Diffraction, Wave optics, Coherent and Incoherent sources, Fringe Width, Huygens' guideline
What are Coherent Sources?
[Click Here for Previous Year Questions]
“Coherent Sources are formed when any two sources have a zero or constant phase difference and the same frequency.”
Most light sources around us, such as the bulb, sun, candle, and so on, are a mash-up of a slew of incoherent sources of light. The laser is an example of a coherent source as numerous key sources inside the laser are phase-locked.
Coherent Soures
To investigate the effects of optical phenomena such as interference in a lab, you will require coherent light sources. Two coherent point sources are two equivalent slits illuminated by a laser beam. Furthermore, two light sources are considered to be coherent when they produce light waves of the same wavelength.
| They are said to be coherent if they exhibit the same
|
Two Coherent Point Sources
Also check:
Types of Coherence
[Click Here for Sample Questions]
Coherence is classified into two types:
- Temporal Coherence and
- Spatial Coherence.
These two types are discussed below.
Temporal Coherence
- Temporal coherence is a measure of the average relationship between the value of a wave and itself at any significant pair of moments.
- The measurement of temporal coherence determines how monochromatic a source is.
- It describes how well a wave can interfere with itself at a later period.
- The coherence time is defined as the delay over which the phase or amplitude should be by a substantial amount (and hence the correlation reduces by a considerable amount).
Spatial Coherence
- Take for example, optics or water waves where the dimension of the wave stretches from one to two spaces.
- Spatial coherence is thus defined as the capacity of two points in space of a wave (x1 and x2) to interfere.
Spatial Coherence
- Spatial coherence is defined as the constant cross-relationship between two places in a wave.
- Perfect spatial coherence occurs when a wave has a single amplitude value over an indefinite length.
- The diameter of the coherence area, Ac, can be defined by the significant interference that exists between the range of separation and the two spots.
Coherent Sources
Coherent light sources output a light wave with the same frequency, wavelength, and phase, or have a constant phase difference. When waves superimpose and the positions of maxima and minima are fixed, a coherent source produces continuous interference patterns.
Coherent Sources
Two separate sources are never cohesive, or they cannot be regarded as coherent sources because all of the aforementioned elements cannot exist at the same time.
| Examples Some common instances of coherent sources include:
|
Coherent and Incoherent Sources
Characteristics of Coherent Sources
[Click Here for Previous Year Questions]
The following features are shared by coherent sources:
- The generated waves have a constant phase difference or are in phase with each other.
- The waves are all of the same frequency.
- The amplitudes of the waves should be the same.
Applications of Coherence
[Click Here for Sample Questions]
Radiography is a centuries-old practice. It was only later that it was discovered that the coherence of a next-generation facility beam makes it possible to easily break the normal barrier to absorption and visualize phase characteristics. The X-ray beam contains:
- A high degree of spatial coherence. This signifies that the beam's size and divergence are extremely minimal.
- After monochromatization, there is good temporal coherence.
These features of the beam, which are attributable to its extreme brightness, allow for the development of new approaches in the X-ray field such as Photon correlation spectroscopy, phase-contrast imaging, tomography and in-line holography
Methods of Producing Coherent Source
[Click Here for Sample Questions]
Finding a consistent source of light might be difficult at times. As a result, we can generate such sources in a variety of ways as follows:
1. Splitting the Wavefront
The wavefront is separated into numerous sections. We can utilize various lenses, mirrors, and even prisms. Young's double-slit experiment, Lloyd's mirror arrangement, Fresnel's biprism method, and others are examples of procedures that can be used.
2. Amplitude is divided by two
A coherent source can be made by taking an incoming beam and dividing its amplitude into distinct portions via partial reflection or refraction. Later, these pieces that go along new routes collide, resulting in interference.
Coherence in Microscope
We can employ the Newton ring phenomena or Michelson's interferometer arrangement or configuration.
Incoherent Sources
[Click Here for Sample Questions]
- Incoherent sources are the polar opposite of coherent sources.
- These types of sources generate light at a constant frequency, with random changes in phase between photons.
- All traditional light sources are examples of incoherent sources.
- The transition of an atom's energy states is a completely random process.
- As a result, we have no control over when an atom will lose energy in the form of radiation.
Incoherent Sources
What is Interference?
[Click Here for Previous Year Questions]
- When two light waves from distinct coherent sources are merged, the energy distribution caused by the first wave is disrupted by the second.
- Because of the superposition of the two light waves, this change in light energy distribution is known as interference of light.
- According to the principle of wave superposition, when several wave disturbances pass through a particular location in a medium, the resultant disturbance at that point is the vector sum of the disturbances caused by the individual waves.
Types of Interference
[Click Here for Sample Questions]
The two types of Interference are:
Constructive Interference: When two waves of equal frequency and wavelength clash, the crest of the first wave collides with the crest of the second wave, and the trough of the first wave collides with the trough of the second wave. The troughs and crests of both waves will add together to generate a wave of identical amplitude in this situation. Constructive Interference is the total of the two waves.
Interference
Destructive Interference: When the crest of the first wave collides with the trough of the second wave, and the trough of the first wave collides with the crest of the second wave. The dip and crest of both waves will then cancel each other out, forming a wave.
The produced wave will have an amplitude equal to the difference between the two waves. If the amplitudes of both waves are the same, the amplitude after interference will be zero; this is known as Destructive Interference.
Also check:
Things To Remember
- If any two sources have a zero or constant phase difference and the same frequency, they are said to be coherent.
- Two separate sources are never cohesive.
- Coherence is classified into two types: temporal coherence and spatial coherence.
- Coherence is the principle behind laser beams and X-rays.
- When two or more waves of the same wavelength and frequency collide, this is referred to as interference.
- Destructive interference is where the produced wave will have an amplitude equal to the difference between the two waves.
Sample Questions
Ques. What is Sustained Interference? (3 marks)
Ans. Continuous or permanent intervention patterns are interference models that are compatible with locations of highest and least intensity across time.
- The conditions for the creation of sustained interference may be stated as :
- Two sources should be coherent
- Two sources should be very narrow
The sources should be near together to create clear and spacious fringes. Light interference that is not short-lived refers to light interference that is persistent (generated by random variations). For a double-slit experiment, a coherent source of monochromatic light (single wavelength) is required. The assumption is that a persistent interference pattern requires a continuous phase relationship between light beams.
Ques. What are the Coherent Sources of Sustained Interference? (2 marks)
Ans. All electromagnetic waves produce interference patterns. Coherent sources generate steady patterns that are easier to investigate. If you need a visible light source, a laser is an obvious choice. At other wavelengths, the radio transmitter feeds several antennas at a distance, resulting in large-scale interference patterns.
Ques. What are the Conditions for the Interference of Light? (3 marks)
Ans. The following conditions must be met for lightwave interference: the source of the waves must be constant, emitting comparable waves with a constant phase difference. The wave must be monochromatic, with a single wavelength.
Furthermore, for persistent interference to occur, the following conditions must be met:
- Coherent light sources
- To provide significant contrast between minimum and maximum, the intensities and amplitudes must be almost equal.
- The source of light should be small enough to be termed a point source of light.
- The interfering sources must be close to each other to produce wide fringes.
- The distance between the screen, the source, and the screen should be large to produce wide fringes.
- The sources must emit light with the same polarisation.
- The sources must all have the same wavelength.
- Constructive Interference is the total of the two waves.
Ques. What are the Prerequisites for Constructive Interference? (2 marks)
Ans. They are as follows:
- A large number of waves must be coherent.
- The wavelengths of the numerous waves should be similar, and
- Polarization of the waves is required. Some argue that the light must be steady, yet interference happens only with coherent light.
Ques. Write down two conditions to obtain the sustained interference fringe pattern of light. What is the effect on the interference fringes in Young’s double slit experiment, when a monochromatic source is replaced by a source of white light? (CBSE 2008) (2 marks)
Ans. Conditions for sustained interference
(i) The two sources of light must be coherent to emit light of constant phase difference.
(ii) The amplitude of the electric field vector of the interfering wave should be equal to have greater contrast between intensity of constructive and destructive interference.
When monochromatic light is replaced by white light, then coloured fringe pattern is obtained on the screen
Ques. Define the term ‘coherent sources’ which are required to produce interference patterns in Young’s double slit experiment. (CBSE 2014) (1 mark)
Ans. Two monochromatic sources, which produce light waves, having a constant phase difference are known as coherent sources.
Ques. State the reason why two independent sources of light cannot be considered coherent sources. (CBSE 2008) (2 marks)
Ans. Two independent sources of light cannot be coherent. This is because light is emitted by individual atoms when they return to the ground state. Even the smallest source of light contains billions of atoms which obviously cannot emit light waves in the same phase.
Ques. In a single-slit diffraction experiment, the width of the slit is made double the original width. How does this affect the size and intensity of the central diffraction band? (CBSE 2012) (2 marks)
Ans. If the width of the diffraction slit is doubled, the size of the central diffraction band will become half and its intensity will become four times of its original value.
Ques. Why are coherent sources necessary to produce a sustained interference pattern? (2 marks)
Ans. Coherent sources have a constant phase difference and, therefore, produce a sustained interference pattern.
These sources are needed to ensure that the position of maxima and minima do not change with time.
Ques. State two conditions required for obtaining coherent sources. (2 marks)
Ans. Two conditions for obtaining coherent sources: (0 Two sources should give monochromatic light.
(ii) Coherent sources of light should be obtained from a single source by some device.
The fringe width (dark and bright) is given by
\(\beta = \frac {\lambda D} {d}\)
Hence, it is the same for both dark and bright fringes So they are equally spaced on the screen.
Ques. State the difference between interference and diffraction of light. (2 marks)
Ans.
| Interference | Diffraction |
|---|---|
| Interference is due to superposition of two distinct waves coming from two coherent sources. | Diffraction is due to superposition of the secondary wavelets coming from different parts of the same wavefront. |
| Interference fringes may or may not be of the same width. | Diffraction fringes are not to be of the same width. |
| The intensity of minima is generally zero. | The intensity of minima is never zero. |
| All bright fringes are of uniform intensity. | All bright fringes are not of uniform intensity. |
For Latest Updates on Upcoming Board Exams, Click Here: https://t.me/class_10_12_board_updates
Do Check Out:
Comments