![]() I’ll focus the answer on electromagnetic waves, including light. Some commonly observed phenomena are combinations of those two. We need to clarify what are those concepts, and what are not. ![]() I’ve seen many confusions on these concept which I think need to rectify. Feel free to comment here to improve accuracy and clarity. ![]() 2 Credits: 1 Feynman Lectures on Physics 2Optics-Ajoy Ghatak. The best we can do is, roughly speaking, is to say that when there are only a few sources, say two interference sources, then the result is usually called interference, but if there is a large number of them, it seems that the word diffraction is more often used.1 To be more explicit read this passage from Ajoy Ghatak: We should point out that there is not much of a difference between the phenomenon of interference and diffraction, indeed, interference corresponds to the situation when we consider the superposition of waves coming out from a number of point sources and diffraction corresponds to the situation when we consider waves coming out from an area sources like a circular or rectangular aperture or even a large number of rectangular apertures (like the diffraction grating). It is just a quest of usage, and there is no specific, important physical difference between them. Quote Feynman has come from heaven to answer your question! Listen to him: No one has ever been able to define the difference between interference and diffraction satisfactorily. Whenever waves spread out, or bend around a corner, it’s called diffraction. It’s not so different than the thin film interference, you see in oil spills and soap bubbles So to summarise, all the energy distribution business, which causes coloration (in white light) or alternate dark and bright bands (in mono chromatic light) are called interference. Where you see Blue, it’s because the blue light is undergoing strong construction and rest, not so much. But the coloration are caused by interference itself. So in that sense, its called a diffraction grating. If someone asks what causes the colouration? If you say diffraction that’s wrong. It’s called so, because it has lots of slits, and so light undergoes lots of diffraction, but again, eventually the pattern is caused by interference of light. Then we have something called the diffraction grating. So it’s wrong to call something as interference or diffraction pattern. So all patterns (bright and dark regions) are caused by interference of light (by the very definition of it), and in all experiments when slits are used there is diffraction happening at each slit. These waves, eventually meet each other and redistribute their energies, ergo, interfere with each other, which we can see as alternate dark and bright regions. So there is diffraction happening in all of these pictures. When they do, the small opening makes the light diffract (bend around and spread out). In each picture, the light is made to pass through slit/s. It’s because of captions given to the images like these Some are called as interference patterns, and some others are called as diffraction patterns. ![]() The two phenomenon are so different and so nicely defined, why is there a confusion? Forget differences, what is even the similarity between the two? Well the confusion is caused by a lot of teachers and books. If you look at the two definitions, it would make one wonder, what’s the meaning of this question. ![]() But since light is a wave, it diffracts too. Diffraction of sound is so common, that you must have never thought about it. Diffraction refers to the fact that waves can bend around things. When they do they cause an energy distribution which makes the famous bright and dark bands. Quote The name interference itself gives it away. ![]()
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