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Interference vs Diffraction (Physics)

Updated on October 13, 2015

What is Interference?

When the light waves coming from a coherent source is allowed to superpose with each other, the distribution of light energy will be non-uniform due to which alternate dark and bright bands (or fringes) can be seen on the screen. This phenomenon of non-uniform distribution of light energies due to superposition of light waves coming from a coherent source is called interference.

Interference from Young's Double Slit Experiment

Straight wavefronts make the two slits act like individual sources of light.
Straight wavefronts make the two slits act like individual sources of light. | Source

What is Diffraction?

However, diffraction is something very different from interference. Diffraction is the bending of light around the corners of any obstacle. In simple words, it is the deviation of light from a true rectilinear path.

Differences between interference and Diffraction

It is the phenomenon of non-uniform distribution of light energy due to the super position of light waves coming from a coherent source.
The phenomenon of bending of light around the corner of any obstacle (disobeying the rectilinear propagation of light) is called diffraction.
The intensity of all the bright bands is equal.
The intensity of all the secondary maxima is not equal.
The width of all the interference fringes is equal.
The width of central maxima is large, and on increasing distance, the width of maxima decreases.
The intensity of dark fringe is completely zero.
The intensity of secondary minima is minimum, but not completely zero as in the case of interference.
It is the effect of superposition of light waves.
It is the super position of wavelets.

An example of interference (which is quite common in our daily life) is the visible colors of the thin soap film. The soap film scatters the light ray into the 7 colors of white light and the air trapped inside the soap bubble acts as the apparatus of Young’s Double Slit experiment and hence, the light interfere to produce colorful fringes.

The main thing to note is that the wavelength of the light (or wave) must be comparable to the width of the slit for both interference and diffraction to occur.

There are no visible and common examples for diffraction of light in our daily practical life. However, it is really practical in case of diffraction of sounds. The sound we produce is a wave and its wavelength is comparable to widths of several objects around us – like door slits, windows and walls. But the wavelength of light is really small, so diffraction of light is not practically visible.

And that is why, we can hear the sound of people from another room (because sound wave is diffracted through doors and walls) but we can’t see them (as light does not diffract through objects of width so large).

Examples of Diffraction


I hope these points were helpful in explaining interference and diffraction, and also the differences between them. If you have more questions or anything you would like to share or ask, kindly post them in the comments below. Thanks!

Interference vs Diffraction Quiz

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