In our day-to-day life we observe various phenomena like after the rain a rainbow is formed in the sky, splitting of white light into seven colors, and when water falls on petrol it also shows different colors. These all phenomena are related to the optical phenomenon of light. In this tutorial, we will discuss the two important phenomena i.e. diffraction and interference.
In classical physics, the term diffraction is firstly explained by using the Huygen-Fresnal principle. After this various Scientists make different attempts to explain these terms in their way. It finds applications in the study of biological macromolecules. Whereas interference fins the applications in the radio transmission of the signals.
It is a process in which waves or light rays are expanding by passing through an edge of an obstacle. The pattern forms in this process are known as diffraction patterns. We can easily observe the phenomenon of diffraction in our daily life. for example, the CDs and DVDs show a rainbow pattern when light falls on them. These act as diffraction grating. There are two types of diffraction.
Fresnel’s class: The distance between the source and the aperture or aperture and screen is finite, this kind of diffraction is called fresnel’s class.
Fraunhofer class: In this type of diffraction, the screen is at an infinite distance from the slit.
Fig:1 Diffraction pattern using single slit
Thurth,my own, SingleSlitDiffraction, CC BY-SA 3.0
Diffraction occurs when the size or width of an obstacle is less than the equivalent of the wavelength of the light beam. Higher will be the wavelength of the light wave more will be the diffraction. I.e. $\mathrm{\lambda \ge a}$, where a be the slit width
In this diffraction, we use a single slit of specific width is used to observe the diffraction pattern. It is noted that the width of the slit must be small than the wavelength of light. So let us consider a slit AB of width a and D is the distance of the screen from the slit.
Let P is the point of elevation. The distance between OP is y.
Fig:2 Single slit experiment
From the figure we observe that two rays are propagated towards the point P with a path difference $\mathrm{\frac{a}{2} sin \theta}$
The secondary maxima condition is also known as the condition for bright fringe. we have observed that P is a bright point. Bright fringe is at $\mathrm{\frac{3 \lambda}{2},\frac{5 \lambda}{2},\frac{7 \lambda}{2}}$
$$\mathrm{a sin \Theta =(2n+1)\frac{\lambda}{2}}..........$$
For secondary minima, we observe that P is a dark point then dark fringe is at $\mathrm{\lambda,2\lambda,3\lambda.............}$
$$\mathrm{a sin \Theta =n\lambda}$$
Fringe width of the central maxima is more than the other fringes and the intensity of all bright fringes are different and this can be seen in the image attached given below.
We can see that the central fringe is brighter than the other fringes and it is wider than the other fringes in the experimental image given below.
The phenomenon in which two waves of different amplitude are superimposed with each other and give a resultant wave with a new amplitude. The magnitude of amplitude depends on the type of interference. There are two types of interference.
Constructive Interference: when two waves are propagated in the same direction with the same speed and in the same phase. Then these waves undergo constructive interference. The amplitude and energy of resultant waves are higher than individual waves.
Destructive Interference: when two waves are propagated in the same direction with the same speed but out of phase, these waves form destructive interference. The amplitude of the resultant is less than the individual waves.
There are a few conditions for interference. we will discuss them one by one
The first one is that the source should be coherent. So that the phase difference at the source remains constant.
The light source should be monochromatic.
The amplitude and the frequency of both the waves should be the same.
The light source should be small or narrow.
| Diffraction | Interference |
|---|---|
Wave split at sharp edge or corners of the slit. This phenomenon is called diffraction. It takes place because of the superposition of secondary waves starting from various parts of the same wavefront. | when two waves combine and form another wave this phenomenon is called interference. It takes place because of the superposition of secondary waves, starting from two different wavefronts that must be obtained from a coherent source. |
Does not require a coherent source. | Source should be a coherent source. |
Maxima is varying and the width of the central bright fringe is twice the width of any secondary maxima. | Maxima is constant and all bright and dark fringes are of equal width. |
Intensity of the bright fringe decreases as we move from the central bright fringe on either side. | All bright fringes are of the same intensity. |
Contrast is Weak: Dark fringe region is not perfectly dark | The Contrast is Strong: Dark fringe region is perfectly dark. |
In this tutorial, we have discussed the superposition principle. Also studied interference and diffraction as these two are based on the superposition principle. We have also discussed the types of diffraction and interference. To obtain the diffraction and interference pattern, the light of the source must be coherent. It means light must be propagated in the same direction, with the same speed in the same phase.
Q1. What is monochromatic light?
Ans. Light of a single wavelength is called monochromatic light. It means a light with a single color. For example, monochromatic light of wavelength 400 is violet color.
Q2. Two lights of the same company, same power rating, and same input voltage can act as a coherent source or not?
Ans.No, two independent sources can never act as a coherent source. Because they don't have the same phase difference.
Q3. What is the principle of superposition?
Ans. This principle states that when two waves overlap with each other then the amplitude of a resultant wave is equivalent to the sum of individual waves.
$$\mathrm{Y=Y_1+Y_2}$$
Q4. What is the coherent source?
Ans. The source of light which emits continuous light waves of the same wavelength, and same frequency in the same phase is having a constant phase difference called a coherent source.
Q5. What are the conditions for obtaining a coherent source?
Ans. Following are the condition for a coherent source
The two sources of light should give monochromatic light
The path difference between the light waves from the two sources should be small.
Q6. What is diffraction grating?
Ans: It is an optical tool that split white light into the light component by wavelength. The light of different wavelengths is diffracted by a different angle. A diffraction grating is made up of a large number of slits.
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