What are Coherent Sources?
Physics of coherent sources says that when a light source emits a light wave with the same frequency, wavelength, and phase or has a constant phase difference, we call this light source the coherent source.
You can consider the following figure to understand coherent sources:
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In the above image, you can see that the light waves are coherent here. Now, a question arises: what applications can help us understand this concept in an elaborate manner? We have multiple examples including interference, and parameters like speed, phase difference, amplitude, and so on.
Here, on this page, we will understand all about the coherent sources with certain examples.
Coherent Sources of Light – Parameters
To observe the effects of certain optical phenomena like interference in a lab, you will need coherent sources of light. Two equivalent slits lighted by a laser beam can be said to be two coherent point sources.
Further, two sources of light are said to be coherent when they give light waves of the same.
Phase and amplitude
Wavelength and constant phase difference
Wavelength and intensity
Speed and phase
Interference is one of the key concepts involved in the topic of coherent sources, so what an inference is? Well, the definition of interference of light says that when two light waves from different coherent sources are combined, the energy distribution due to one wave is disturbed by the other. This change in light energy distribution is called interference of light, due to the superposition of the two light waves.
What is Interference – A Key Concept to Understand Coherent Sources
So, what are coherent sources? Well, from the above text, we understand that if the sources have zero or constant phase difference and the same frequency, then the two sources are considered to be coherent. Most of the light sources around us like the bulb, sun, candle, etc. are a combination of a multitude of incoherent sources of light. For coherent sources, the laser is an example, i.e., multiple essential sources inside the laser are phase-locked.
Now, going back to our topic, i.e., interference.
In physics, interference is a phenomenon where two waves form a resultant wave either of high, low, or having the same amplitude.
At any place or moment, you can expect interference but we don't see interference patterns everywhere as most of the sources generate light waves randomly. It means the light waves coming out of the source do not have a constant frequency, phase, or amplitude.
For example, a wide range of frequencies of light, like the colours of the rainbow, is generated by incandescent bulbs. The light out of the bulb randomly gets generated everywhere and in all directions. It means that the starting point of the wave generated may be minimum, maximum, or any point in between. In which the wave will start cannot be predicted; such a source is said to be incoherent.
How does Interference Occur?
Interference occurs when two or more waves of the same wavelength and frequency join each other.
Based upon the principle of superposition of waves, when a number of wave disturbances pass through a given point in a medium, the resultant disturbance created at that point is the vector sum of the disturbance due to the individual waves.
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Types of Interference
Constructive interference and destructive interference are the two types of interference of light waves depending upon the waves colliding.
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Constructive Interference: When two waves of similar frequency and wavelength collide, they will collide in such a way that the crest of the first wave collides with the crest of the second wave, and similarly, the trough of the first wave collides with that of the second wave. In this case, the troughs and crest of both waves will add up to form a wave having equal amplitude. The sum of both the waves is known as constructive interference.
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Destructive Interference: When both waves collide in such a way that 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, then the trough and crest of both waves will cancel each other to form a wave.
The formed wave will be equal to the difference in the amplitude of both waves. If the amplitude of both waves is the same, then after the interference, the amplitude will be zero, this is known as destructive interference.
Based on our above text, we understand that coherent sources are the light sources that have the same frequency, wavelength, and constant phase difference. However, interference is the joining of two light waves. So, when the crests and troughs on both the waves join to add up a new wave, it is called constructive interference. On the other hand, when the crest of one wave collides with the trough of the second wave in such a way that they cancel each other, it is called destructive interference.
FAQs on Coherent Sources
1. What are coherent sources of light in Physics?
In Physics, two sources of light are called coherent if they emit light waves of the same frequency and have a constant phase difference between them. This means the crests and troughs of the waves from both sources maintain a fixed relationship as they travel, which is essential for creating a stable interference pattern.
2. What is the main difference between coherent and incoherent sources?
The main difference lies in the phase relationship of the light waves they emit.
- Coherent sources have a constant phase difference and the same frequency.
- Incoherent sources, like a standard light bulb or a candle flame, emit light with random and rapidly changing phase differences.
3. Can you give a common example of a coherent source of light?
The most common example of a coherent source is a laser. The light from a laser is highly monochromatic (single frequency) and the photons are emitted in a way that they are all in phase with each other. This is why laser light is used in experiments that demonstrate interference and diffraction, such as the Young's Double Slit Experiment.
4. Why can't two separate light bulbs be used as coherent sources?
Two independent light bulbs cannot be coherent because light is produced by the random transitions of electrons inside atoms. In two different bulbs, these atomic transitions occur independently and randomly. There is no way to synchronise them, so the phase difference between the light waves from the two bulbs changes constantly and unpredictably, making them incoherent.
5. What is the importance of using coherent sources in interference experiments?
Using coherent sources is crucial because they are necessary to produce a stable and observable interference pattern. When sources are coherent, the positions of maximum brightness (constructive interference) and maximum darkness (destructive interference) remain fixed over time. Without coherent sources, the interference pattern would shift so rapidly that our eyes would only see a uniform illumination.
6. How can we create two coherent sources from a single light source in a lab?
A common method is called division of wavefront. In this technique, you take a single source and pass its light through two narrow, closely spaced slits (like in Young's experiment). The light waves emerging from these two slits originate from the same parent wave, so they act as two new sources that are perfectly in phase with each other, thus becoming coherent sources.
7. What would happen if we performed an interference experiment with incoherent sources?
If you used incoherent sources, interference would still technically occur at every instant. However, because the phase relationship between the sources is random and changes billions of times per second, the interference pattern would also change just as quickly. The positions of bright and dark fringes would fluctuate so fast that you wouldn't be able to see a pattern at all. Instead, you would just observe an average, uniform brightness on the screen.
