Define Diffraction
Diffraction Meaning: It is the process by which a stream of light or wave is spread out as a result of passing via a narrow area or across an edge, generally accompanied by interference between the waveform produced.
Consider a train crossing the tunnel, inside the tunnel the rays of the headlight will remain converged; however, as the train comes out of the tunnel, the same light spreads around the area. This is the diffraction of light.
Here, we have discussed the types of diffraction like diffraction grating, Bragg diffraction, double slit diffraction, and electron diffraction.
Diffraction of Light Definition
The diffraction of light is similar to the concept of using a loudspeaker. In a loudspeaker, you speak through a small hole, but the voice coming out spreads around the vicinity and that too modulated, i.e., the diffraction of sound.
Let’s suppose that you are stuck in a tunnel and there is no one around to help you come out of it, so you try to call people moving around the tunnel but they can’t hear you.
Now, you switch on the flashlight of your mobile phone, and the flashlight emitting at the opened end of the tunnel spreads around. As the light is spreading all around the area, this spreading is the diffraction of light.
If in an electric circuit, electrons passing through a narrow wire approaches a big container (as a wire), these electrons spread all around, and this scattering is the electron diffraction.
Consider the below diagram to understand the cases mentioned above:
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Diffraction Physics
Do you know what happens to water when you put a finger under the tap? Does water revert to the tap or it continues falling down? Or If the direction of water remains the same as before putting the finger under the tap?
What happens to water waves when they encounter a stone along their path? Do they revert or continue moving on their way? What occurs to the shape of waves when they pass through a narrow gap? Do waves possess the same shape as before?
You will get the answer to these questions on reading further. So, keep scrolling through this page.
Diffraction of Waves
A light wave when encounters a hindrance in its way, bends around the corner or edges of the opaque object. So, the spreading of light waves around the corners of the obstacle is the diffraction of waves.
Point to Note:
The condition to achieve diffraction is that the dimensions of the hindrance or of the obstacle must be comparable to the wavelength. When the obstacle is much larger than the wavelength, no diffraction occurs; however, when the aperture is smaller than the wavelength, we find that circular wave fronts are produced.
There are certain types of diffraction; these are as follows:
Types of Diffraction
1. Diffraction Grating
2. X-Ray diffraction
3. Double slit diffraction
4. Electron diffraction
5. Bragg diffraction
1. Diffraction Grating
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The above diagram visually explains the diffraction grating.
A diffraction grating is an optical instrument with a continuous pattern. The form of the light diffracted by a grating relies on the structure/orientation of the elements and the number of elements present, but all gratings have intensity maxima at angle ፀm that are given by the following equation:
d (Sin ፀi + Sin ፀm ) = mλ
Where,
ፀi = the angle at which the light incidences,
d = the separation of grating elements, and
m = an integer that can either be positive or negative
2. X-Ray Diffraction
X-Ray diffraction is a phenomenon in which the atoms of a given crystal-bearing uniform spacing cause an interference pattern of the waves residing in an incident beam of X rays. The atomic planes of the crystal act on the X rays behave the same way as it does with a uniformly ruled grating on a beam of light.
The below diagram shows the X-Ray Diffraction:
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From the above diagram, we see that the X-Ray Diffraction is similar to the concentric circles of magnetic field lines formed around the nail.
3. Double Slit Diffraction
Young’s double-slit experiment demonstrates the wave-particle duality behaviour of the light.
In his experiment, Thomas Young considered a coherent light source, viz: a laser beam that emits from a plate pierced by two parallel slits, and the light passing through the slits is observed on a screen beyond this plate.
The wave nature of light causes the light waves passing through the two slits to interfere (as shown in the image below); therefore, producing bright and dark bands on the screen; however, the light was found to be absorbed in the screen in quanta.
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4. Electron Diffraction
The emission of electrons in the form of waves is electron diffraction. We define electron diffraction as the wave nature of electrons.
In technical or practical terms, we consider it a technique that can be used to study matter by firing electrons at a sample and observe the results as the interference pattern.
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5. Bragg Diffraction
In physics, Bragg's law is known as the Wulff–Bragg's condition. It is a special case of Laue diffraction. It gives the coherent and incoherent angles of scattering from a crystal lattice. When X-rays incident on an atom, they make an electronic cloud move akin to any electromagnetic wave.
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FAQs on Diffraction
1. What is diffraction in Physics?
Diffraction is the phenomenon of the bending of waves as they pass around an obstacle or through an aperture. Instead of continuing in a straight line, the waves spread out. This effect is most prominent when the wavelength of the wave is comparable to the size of the obstacle or opening.
2. What is the essential condition for diffraction of waves to occur?
The primary condition for diffraction to be observable is that the size of the obstacle or aperture (let's say 'a') must be on the order of the wavelength (λ) of the wave. Significant diffraction occurs when λ ≥ a. If the wavelength is much smaller than the obstacle, the wave passes by with negligible bending, appearing to travel in a straight line.
3. How does diffraction differ from interference?
While both phenomena involve the redistribution of wave energy, they arise from different circumstances:
- Origin: Interference is the result of the superposition of waves from two or more coherent sources. Diffraction is the result of the superposition of wavelets originating from different points on the same wavefront.
- Fringe Pattern: In an interference pattern (like in Young's Double Slit Experiment), the bright fringes are typically of uniform intensity and width. In a diffraction pattern from a single slit, the central bright fringe is the brightest and widest, with the intensity and width of secondary fringes decreasing rapidly.
4. What are some real-world examples of diffraction?
Diffraction can be observed in various everyday situations:
- The rainbow-like colours seen on the surface of a CD or DVD are caused by light diffracting from the closely spaced tracks.
- The ability to hear sound from around a corner, even when the source is not visible, is due to the diffraction of long-wavelength sound waves.
- The formation of a bright ring or halo (a corona) around the sun or moon is caused by the diffraction of light by tiny water droplets or ice crystals in the atmosphere.
- Holograms work on the principle of diffraction to create three-dimensional images.
5. What are Fresnel and Fraunhofer diffraction?
Fresnel and Fraunhofer are the two main types of diffraction, classified based on the distances between the source, obstacle, and screen.
- Fresnel Diffraction: Occurs when either the light source or the screen (or both) are at a finite distance from the diffracting obstacle. The incident wavefront is not plane but is typically spherical or cylindrical.
- Fraunhofer Diffraction: Occurs when both the source and the screen are effectively at an infinite distance from the obstacle. This is practically achieved using lenses, ensuring the incident wavefront is plane. Single-slit diffraction studied in Class 12 is an example of Fraunhofer diffraction.
6. What is the formula for the width of the central maximum in a single-slit diffraction pattern?
The linear width of the central maximum (β₀) in a single-slit Fraunhofer diffraction pattern is given by the formula: β₀ = 2Dλ/a. Here, 'D' is the distance between the slit and the screen, 'λ' is the wavelength of the light used, and 'a' is the width of the slit. The angular width is given by 2θ = 2λ/a.
7. Why is the central bright fringe in a single-slit pattern so much wider and brighter than the others?
The central maximum is exceptionally bright and wide because it corresponds to the region where light waves from all points across the slit arrive roughly in phase, leading to strong constructive interference. For the secondary maxima, waves from different zones within the slit interfere destructively, cancelling each other out to a large extent. This results in much lower intensity and narrower bands compared to the central one.
8. How does diffraction limit the resolving power of optical instruments?
Diffraction imposes a fundamental limit on the ability of any optical instrument, like a microscope or telescope, to distinguish between two closely spaced objects. When light from an object passes through the instrument's aperture, it diffracts, creating a blurred circular image (an Airy disc) instead of a sharp point. If two objects are too close, their Airy discs overlap to the extent that they cannot be seen as separate. This is known as the diffraction limit of resolution.
9. Why is it easy to hear someone around a corner but not see them?
This is a classic example demonstrating the role of wavelength in diffraction. Sound waves have long wavelengths (ranging from centimetres to metres) that are comparable to the size of everyday obstacles like doorways and building corners. This allows them to diffract or bend around these obstacles easily. In contrast, light waves have extremely short wavelengths (measured in nanometers). For light to diffract noticeably, the obstacle must be similarly tiny, which is why light appears to travel in straight lines in most macroscopic situations.
