Difference Between Diffraction And Interference

Diffraction and Interference are two important phenomena studied in Wave Optics. These phenomena are linked to the Wave Nature of the matter. We know that when photons are emitted from the source they start oscillating in space and the locus of all such oscillating particles in the same phase is called Wavefronts. As per Huygens Wave Theory, each point on the Wavefront acts as a source of secondary Wavelets which later add up to give a new Wavefront. The sources of these Wavefronts can be coherent or non-coherent depending upon whether the phase difference is constant or not. These wavefronts interact with each other and with other particles to give rise to the phenomena of  Interference and Diffraction respectively. 

In this article, we will learn about what is diffraction, what is interference and what are the differences between Diffraction and Interference.

Diffraction Definition

The phenomenon in which light bends around the apertures or the corner of the particles spreading into different regions of the geometrical shadow of the aperture is called Diffraction. Diffraction of Light results is due to the superposition of secondary waves from different parts of the same wavefront. For Diffraction to happen the size of the particle or slite must be comparable to the wavelength of the light i.e. d ≈ λ. 

The phenomena of Diffraction can be best observed by Single Slit Experiment. When light passes through a single slit of aperture compared to the wavelength of light then it is expected that screen behind the slit will appear bright and this will be of the size of the aperture of the slit. However, this is not the case, and a larger portion of the screen, greater than the size of the aperture of the slit appears bright. There is a bright spot in the center which is called Central Maxima and on both sides of Central Maxima, there is an alternate bright and dark pattern with decreasing order of intensity.

We generally don’t encounter diffraction in our daily life as the wavelength of light is much smaller than the size of the particles around us. Thus in real life, the interaction of light and larger particles result in shadow which doesn’t happen in the case of diffraction. X-Ray Diffraction helps to determine the particle size of the crystal using Bragg’s Law.

Let’s see some features of diffraction.

Features of Diffraction

There are the following features of Diffraction

• Diffraction happens when light pass through slits or bends around corners of an obstacle
• The size of the aperture of the slit or obstacle must be comparable to the wavelength of light.
• Diffraction from a slit results in a central bright spot
• The diffraction results in unequal fringe width.
• The intensity of the fringes is variable.

Now let’s understand what is interference and what are its features.

Interference Definition

The phenomenon in which two waves traveling in the same direction in the same medium superpose on each other, resulting in different wavefronts is called Interference. Superposition of waves means vector addition of the displacement of waves at any point of time. The interference of two waves causes the resultant intensity of the wave to be different from the intensities of the two waves. Depending upon the resultant intensity the interference can be classified as constructive interference and destructive interference. Interference of the waves can be shown by Young’s Double Slit Experiment.

Constructive Interference

Constructive Interference happens in the case of waves of the same phase i.e. when the crest and trough of one wave superimpose on the crest and trough of another wave respectively. In this case, the intensity of the resultant wave is larger than the intensity of the individual sum of two waves.

Destructive Interference

Destructive Interference happens in the case of waves which are out of the phase with respect to each other i.e. when the crest and trough of one wave align with the trough and crest of another respectively then the resultant intensity is smaller than the resultant intensity of the individual vector sum of displacement of two waves.

Let’s learn some features of Interference

Features of Interference 

There are the following features of Interference

• No slits are required during interference.
• The interference results in equal fringe width.
• The difference between maxima and minima is good.
• The intensity of the fringes is constant separately for constructive and destructive interference.

Learn more about, Interference

Similarities Between Diffraction and Interference

There are following similarities between Diffraction and Interference

• Both Diffraction and Interference are phenomena of waves.
• Both deal with wavefronts.
• The superposition of waves occurs in both diffraction and interference.

Difference Between Diffraction and Interference

There are following differences between Diffraction and Interference are discussed in the table below,

Characterization	Diffraction	Interference
Definition	The phenomena which bend the light around the apertures or obstacles of size comparable to the wavelength of light and spread it into the different regions of the aperture.	The phenomena which superpose two waves traveling in the same direction in the same medium to form different wavefronts.
Result	Superposition of secondary waves from different coherent sources on the same wavefront.	Superposition of two waves in which the sum of the intensities of two waves is different from the resultant intensity of the wave.
Fringe Intensity	Variable fringe intensity i.e. the intensity of fringe decreases on moving away from the central fringe on both sides	Constant fringe intensity i.e. there is no change in the intensity of bright fringes on moving away from the central spot on either side of the fringe.
Fringe Width	Unequal fringe width i.e. the width of the central maximum is twice the secondary maximum	Equal fringe width whether bright or dark
Requirement of Slit	Slits are required for diffraction.	No slits are required for interference.
Difference Between Maxima and Minima	The darker fringes are not perfectly dark hence there is poor contrast between bright and dark fringes.	There is greater contrast between the bright and dark fringes i.e. regions of dark fringes are perfectly dark.
Position of Dark and Bright Fringe	We get the first dark fringe at an angle of λ/a where a is the width of the slit.	We get the first bright fringe at λ/d where d is the distance between two slits.
Direction of Wave	Waves after diffraction change the direction of their propagation	There is no change in the direction of propagation of the wave in the case of the interference

Conclusion

In this article, we learn that diffraction and interference both deal with the wavefronts with a basic difference that diffraction doesn’t create new fronts but interference creates new wavefronts of different intensities. The mandatory condition for diffraction is that there should be a slit or an obstacle of size comparable to the wavelength of light while there is no such condition for interference. Also in the case of diffraction, the fringes are of unequal width and variable intensity while in the case of interference the width of the fringes are equal and are of the same intensity. Thus the contrast between maxima and minima for diffraction is poor while that of interference is good.

FAQs of Diffraction and Interference

Q1. What is Interference?

Answer:

Interference is the phenomenon in which two waves traveling in the same direction superpose together to result in different wavefronts.

Q2. What is Diffraction?

Answer:

Diffraction is the phenomenon in which light bends around the aperture or obstacle of size comparable to the wavelength of light spreading it in different regions of the aperture.

Q3. What is Nature of Fringe Intensity and Fringe Width in Diffraction and Interference?

Answer: 

Fringe width is unequal, and the intensity of the fringe is variable in diffraction while in the case of Interference the fringe width is equal and is of constant intensity.

Q4. What is Wavefront?

Answer: Wavefronts are the continuous point for all the particles of oscillating in the same phase in the medium.

Q5: What are Types of Interference?

Answer:

There are two types of interference namely constructive interference and destructive interference.

Q6: What is Constructive Interference?

Answer:

The interference of waves in which the crest of one wave superposes on the crest of another wave and the trough of one wave superposes on the trough of another wave is called Constructive Interference.

Q7: What is Destructive Interference?

Answer:

The interference of waves in which the crest of one wave superposes on the trough of another wave and the trough of one wave superposes on the crest of another wave is called Destructive Interference.

Q8: Which Experiment is performed to check Interference of Light?

Answer:

Young’s Double Slit Experiment is performed to check the Interference of light.

Q9: Which Experiment is performed to check Diffraction of Light?

Answer: 

Single Slit Experiment is performed to check Diffraction of Light.