[Physics Class Notes] on Diffraction Pdf for Exam

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:

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

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 ፀ_m that are given by the following equation:

d (Sin ፀ_i + Sin ፀ_m ) = mλ

Where,

• ፀ_i = 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:

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.

     

                  

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.

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.