Why Is Coherence Crucial in Physics?
To understand coherence physics, let us first take a look at the concept of coherent light wave. When two or more light waves are traveling together in such a way that their phase difference with respect to time is constant, then such waves are known as the coherent waves. The property of any light source to measure a definite correlation between any two points with the same frequency and amplitude is known as the coherence of light and the light source is known as the coherent light source.
LASER light is considered to be the best example of the coherent light source. There are no perfect coherent sources. The laser is a highly coherent light source.
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Now let us have a look at coherence physics or coherent meaning in physics. In physics when we are discussing concepts of optics, the behavior of light plays an important role. One of the important parameters associated with the brightness of any light source is its coherence ability. Now, what is coherence? The coherence of light gives the correlation between any two points of light waves which have a constant phase with respect to time is known as the coherence of light, this property is known as coherence and this is the coherence definition Physics. There are two types of coherence depending on the position and time and are given by:
Temporal coherence
Spatial coherence
Types of Coherence
For understanding coherent light meaning with further details, let us start with the discussion of types of coherence in detail.
Temporal Coherence
It is also known as longitudinal coherence. It is a type of coherence depending upon the time.
It is a measure of the correlation of light waves phase at different directions of propagation.
The temporal coherence has explained the monochromaticity of the light source.
The temporal coherence is explained by the lifetime of atoms in their excited state.
This property of light is observed when atoms de-excite from their excited state simultaneously. For this simultaneous de-excitation, the lifetime of atoms must be as large as possible.
The average time period for which the field remains sinusoidal is known as the coherence time or the temporal time. The average length for which the waveforms remain sinusoidal is known as the coherence length.
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Spatial Coherence
It is a type of coherence depending upon the position.
It is a measure of the correlation of the phase of the light wave at different transverse to the direction of propagation.
The spatial coherence has explained the finite size of the light source.
The ideal point source exhibits perfectly spatial coherence, whereas extended point sources are bound to exhibit less spatial coherence.
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Achieving absolute coherence is impossible, rather we can get partial coherence or the degree of coherence in nature.
Incoherence and Incoherent Sources
Incoherence refers to the phenomenon where two or more waves considered are out of phase or the correlation between the two points are not in phase with each other.
The property of light sources such that there will be no correlation between the two emitted waves. All-natural and conventional sources of light are incoherent in nature.
Incoherence explains the random motion of the emitted waves.
Did You Know
While studying quantum optics, the term coherence is often used for the state of light-emitting atoms or ions. In that case, coherence refers to a correlation and phase relationships between the complex amplitudes corresponding to electronic states. This is important, e.g., in the context of lasing without inversion. There is also the term coherent states of the light field, which has yet another meaning.
FAQs on Coherence in Physics: Definition, Types & Importance
1. What is coherence in Physics?
In Physics, coherence is a fundamental property of waves that describes a fixed and predictable relationship between their physical quantities at different points in space or time. For light waves to be coherent, they must have a constant phase difference and the same frequency. This property is essential for waves to produce a stable interference pattern.
2. What are the two main types of coherence?
The two primary types of coherence are based on how the phase relationship is measured:
Temporal Coherence: This measures the correlation of a wave's phase at a single point over a period of time. It is directly related to the monochromaticity of the source—the more single-coloured the light, the higher its temporal coherence.
Spatial Coherence: This measures the correlation of a wave's phase at different points across a wavefront at the same moment in time. It is related to the size of the light source—smaller or more distant sources tend to have higher spatial coherence.
3. How does temporal coherence differ from spatial coherence?
Temporal and spatial coherence describe two different aspects of a wave's predictability. Temporal coherence is about the wave's consistency over time; a wave with high temporal coherence is highly monochromatic (one frequency). In contrast, spatial coherence is about the wave's consistency across space; a wave with high spatial coherence has a uniform wavefront. A small, monochromatic source (like a laser) has both, while a large, multi-coloured source (like a light bulb) has very low levels of both.
4. Why is coherence essential for observing a stable interference pattern?
A stable interference pattern consists of fixed regions of maximum brightness (constructive interference) and minimum brightness (destructive interference). This requires the phase difference between the interfering waves to be constant. If the sources are incoherent, their phase difference changes randomly and rapidly. As a result, the positions of maxima and minima shift too quickly for our eyes to perceive, and the pattern gets 'washed out', leading to uniform illumination instead of distinct fringes.
5. What are some examples of coherent and incoherent light sources?
The most common example of a coherent source is a LASER. It produces highly monochromatic and directional light. Examples of incoherent sources include conventional light sources like an incandescent bulb, a candle flame, or the Sun. In these sources, light is emitted by countless independent atoms, and the resulting waves have random phase relationships.
6. Can two independent light bulbs be used to create an interference pattern? Why or why not?
No, two independent light bulbs cannot be used to create a sustained interference pattern. This is because they are incoherent sources. The light from each bulb is the result of random emissions from billions of atoms. The phase difference between the waves from the two bulbs fluctuates randomly and rapidly, preventing the formation of a stable interference pattern. To observe interference, coherent sources are typically derived from a single parent source, such as in Young's double-slit experiment.
7. What is the practical importance of coherence in technology and real life?
Coherence is not just a theoretical concept; it is crucial for many modern technologies. Key applications include:
Holography: Creating three-dimensional images requires a highly coherent light source (like a laser) to record both the amplitude and phase of the light reflected from an object.
Interferometry: This technique uses the interference of coherent waves for extremely precise measurements, such as testing optical lenses or detecting gravitational waves (LIGO).
Fibre Optic Communication: The coherence of light affects how signals spread and lose quality over long distances in optical fibres.
8. How is the coherence of a light source related to its monochromaticity?
There is a direct relationship between temporal coherence and monochromaticity. A perfectly monochromatic source, which emits light of a single, pure frequency, would have infinite temporal coherence. However, real-world light sources always emit a range of frequencies (a spectral bandwidth). The narrower this bandwidth (i.e., the more monochromatic the light), the higher its temporal coherence. This is why lasers, which are highly monochromatic, are excellent sources of temporally coherent light.
