What is Diffraction: Definition and 1000 Discussions

Diffraction refers to various phenomena that occur when a wave encounters an obstacle or opening. It is defined as the bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a secondary source of the propagating wave. Italian scientist Francesco Maria Grimaldi coined the word diffraction and was the first to record accurate observations of the phenomenon in 1660.

In classical physics, the diffraction phenomenon is described by the Huygens–Fresnel principle that treats each point in a propagating wavefront as a collection of individual spherical wavelets. The characteristic bending pattern is most pronounced when a wave from a coherent source (such as a laser) encounters a slit/aperture that is comparable in size to its wavelength, as shown in the inserted image. This is due to the addition, or interference, of different points on the wavefront (or, equivalently, each wavelet) that travel by paths of different lengths to the registering surface. However, if there are multiple, closely spaced openings, a complex pattern of varying intensity can result.
These effects also occur when a light wave travels through a medium with a varying refractive index, or when a sound wave travels through a medium with varying acoustic impedance – all waves diffract, including gravitational waves, water waves, and other electromagnetic waves such as X-rays and radio waves. Furthermore, quantum mechanics also demonstrates that matter possesses wave-like properties, and hence, undergoes diffraction (which is measurable at subatomic to molecular levels).

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  1. velkyr

    Rayleigh criterion (two wavelengths) + diffraction grating

    Homework Statement "A source emits light with two monochromatic components of wave-lengths λ1 = 510.50 nm and λ2 = 510.90 nm. Using the Rayleigh criterion, find the minimum number of slits of a grating that must be illuminated by a beam from the source in order to resolve these components."...
  2. D

    I Laser diffraction through single slit

    Will a laser diffract through a single slit if the laser beam width is smaller than the slit? Such that the laser beam can pass though without touching the sides.
  3. L

    Diffraction, Amplitude and Phase

    Hi, Today I was told that phase and amplitude on the screen in a far field of a mask consisting of hole are more or less the same. So, I wanted to check myself, but didn't find any resource for this. All the resources are, as usual, talk about intensity distribution in the Fourier domain, but...
  4. RJLiberator

    Calculating Light Spot Size on the Moon for a Diffraction-Limited Laser Beam

    Homework Statement A laser is a light source that emits a diffraction-limited beam (like waves diffracting through a wide slit) of diameter 2 mm. Ignoring any scattering due to the Earth's atmosphere, calculate how big a light spot would be produced on the surface of the moon, 240,000 miles...
  5. A

    A source's effect on diffraction pattern

    I am quite aware of the effect of slit width and wavelength in a single-slit diffraction pattern. However, my teacher has never touched on the effect of a source moving towards or away from the slit. Neither can I find any satisfactory or comprehensible response to this phenomenon. While my...
  6. H

    Diffraction Understanding a solution

    Homework Statement (see attached problem) Homework Equations (see attached problem) The Attempt at a Solution So assuming the area of the hole is circular, halving its diameter, will decrease its area by a factor of 16. Therefore LI will be I/16? However in the solution (see attached...
  7. T

    Questions of single slit diffraction

    Homework Statement I have a question on a practice midterm my professor sent out. I would ideally ask him, but I won't see him until itake my midterm. Instead I am attaching the question and solutions at attachments. THe question of interest is in the attachment "physics question" and the...
  8. S

    What Is the Difference Between Interference and Diffraction?

    I have a confusion regarding interference and diffraction phenomenon, I will be glad if you can post a video or something about interference and diffraction basics. I don't have meaningful concept over their differences. [Mentor's Note: Thread moved to General Physics and thread title edited]
  9. M

    Focal length of diffraction grating for recip. dispersion

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  10. Aurghyadip Kundu

    Bending of Light in diffraction? Is it actually possible?

    I know light has wave particle duality, I understand these two points very well, 1. Light moves as a probability function, taking all the possible paths at once. The alternate paths cancel themselves out, resulting in a particle-like combined motion. 2. When relatively close to an edge, some...
  11. Y

    Far Field Approximation After a Prism

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  12. A

    Photon information after passing the pupil

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  13. I

    Are diffraction patterns additive?

    Lets say I have three surfaces, one with pattern A, one with pattern B and the third with A and B overlapped. Will the diffraction pattern be a simple addition of the diffraction patterns from A and B?
  14. T

    2D Fraunhoffer diffraction

    Hello, My name is Thibaut, I am a student in physics and I currently struggling with something that some of you might find obvious. I did an experiment with a laser 650nm passing through a 2D squared grid. The light interferes and diffracts and a photomultiplier collect only the central spot...
  15. G

    Why is a laser's focus limited by diffraction?

    Why can a laser only be focussed to within its wavelength? It says due to it being diffraction limited, what about focussing lenses and mirrors they don't involve diffraction. Please explain
  16. A

    Optics : interference and diffraction

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  17. P

    Diffraction of red LED light picture with spots

    I would like to know what the dots are. This reminds me of X-ray diffraction, but I am imaging a sphere of liquid on the end of a microcapillary nozzle (2um inner diameter glass nozzle with sputtered gold outer coating on nozzle). I am taking a picture of a small glass nozzle with a metal...
  18. L

    Two slit diffraction and energy conservation

    Hi all, I have a small misunderstanding about the energy conservation in diffraction from 2 slits. First, I understand the energy conservation of interference from 2 slits. If intensity from each slit is I, then I have intensity of 2I after slits plane. Interference is given by: So at bright...
  19. W

    Modern Physics Question: Davisson and Germer Expt. beam diffraction

    Homework Statement In the experiment of Davisson and Germer (a) show that the second- and third-orderdiffracted beams cannot occur and (b) find the angle at which the first-order diffracted beamwould occur if the accelerating potential were changed from 54 to 60 V. (c) What...
  20. chikou24i

    Materials and x-ray diffraction

    What is the material that doesn't diffract x rays ?
  21. G

    Diffraction of Light with Moving Slit

    If plane waves of light in a vacuum are incident on a slit that is not stationary with respect to the frame of reference, but it is moving either backwards or forwards with a certain speed, what will the diffraction "field" look like?
  22. G

    I Diffraction Redshift and Emission Theory

    In the emission theory of light, light waves can move at any speed. We can still apply the Doppler effect, but to the best of my knowledge, only the frequency changes, not the wavelength. The pattern for a diffraction grating only depends on the wavelength right? And we have observed...
  23. Raymond Potvin

    B Diffraction of light, the mechanism

    Hi everybody, At wiki, I was astonished to read this about diffraction of light : "In Quantum Mechanics, it is a remarkable fact that the mechanism of diffraction is based on radiating particles of a medium, while quantum mechanical waves have no transmitting medium. As being part of the...
  24. P

    Rate of Change of Diffraction Pattern with Increasing Distance Between Slits

    Homework Statement Monochromatic electromagnetic radiation of wavelength λ nm falls on double slits, creating a diffraction pattern on a screen L m away, Suppose now that the distance between the slits begins to increase at a constant rate: dd/dt = x μm/s. Assume that everything else...
  25. A

    Diffraction pattern for large number of particles

    Why does a large number of identical particles randomly distributed produce a diffraction pattern same as that of a single particle?
  26. J

    Estimate diffraction angle - homemade spectrometer

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  27. C

    Diffraction Grating Relationship Question

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  28. Alettix

    Estimation of data stored on a CD

    Hello! I would be very happy if you could lend me a helping hand with the following problem. :) 1. Homework Statement We know that a CD is read with a laser with a wavelenght of λ = 1 μm. a) Estimate the data stored on the CD. b) We know that the CD contains 80 minutes of sound. How fast is it...
  29. I

    Diffraction & Interference - interference fringes per cm

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  30. DoobleD

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  31. B

    Diffraction Grating Homework: Find the Distance of Bright Fringes

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  32. DoobleD

    2 slits : diffraction always "enveloppe" of interference ?

    In the double-slit diffraction experiment, two interferences are observed simultaneously : diffraction interference from each single-slit, and double-slit interference (where in the double-slit interference, diffraction is ignored because we considered theoretical slits way smaller than...
  33. M

    Analyzing diffraction pattern with ImageJ

    Hey! I have a very unusual question. It is about using ImageJ to analyze diffraction patterns from TEM. 1. Homework Statement To calculate lattice parameter I need Miller Indices from planes I can recognize in the picture, wavelength of electrons and camera length. In our instructions for...
  34. TheOfficialAB

    Crystal phase transition, Diffraction peak splitting problem

    Homework Statement [/B] Hi there! This is a question from a practice problem sheet I got from the lecturer of my Condensed Matter 1 course. Below are the normal vectors to the {111} and {112} lattice planes: Homework Equations [/B] Bragg Condition: \begin{equation} n \lambda = d \sin...
  35. J

    Diffraction and Wavelength Dependency

    I have found lots of forum threads on this but I still can't see the connection between wavelength and diffraction. Here is a picture I found: If you increase the wavelength then all is this will do is just increase the time between the crests that arrive and I don't see how that can have...
  36. I

    Interference - Single slit diffraction

    < Mentor Note -- thread moved to HH from the technical physics forums, so no HH Template is shown > Hi guys, I have a question here which I'm struggling to do and perhaps you could help ; Monochromatic electromagnetic radiation with wavelength λ from a distant source passes through a slit. The...
  37. O

    How a very short λ enable light, not obliterate all shadow? (diffraction)

    Hello all, Guys, I just pointed out that I actually lacking in somewhere in the understanding of the phenomenon of diffraction of light, of which I am aware from no less than 10 years. Recently I revised Huygens wave theory of Light and studied in what ways Newton criticize it. At one of the...
  38. J

    No.of orders of diffraction visible

    1. Homework Statement Homework Equations dsinθ=nλ The Attempt at a Solution I chose D, but I can't seem to see why the number of orders changes. There are more lines per metre, so distance between slits decreases, hence sinθ would increase, which means θ increases too. But this would only...
  39. T

    Optimum gap size for diffraction

    Homework Statement Is diffraction at a maximum when the width of the gap is similar in length to the wavelength of the light? (ie. W ≈ λ, where w=width) Or is it a maximum when the width is smaller than the wavelength? (i.e W < λ)Thanks for the help!Homework Equations n/a (this is a homework...
  40. Vaibhav DixiT

    Fresnel Diffraction through a straight edge

    I read in texts that when a rays of light are diffracted from a straight edge, the phenomenon can be explained using the half period zones. The things that is confusing me is this: "Odd number of half period zones, if exposed, lead to a bright fringe. Even number of half period zones exposed...
  41. Ramesh Manian

    Diffraction in a single slit setting

    In a single slit diffraction setting, if the width of a slit is not much wider than the diameter of a particle such as a photon or an electron, would we still see diffraction bands? If so, is the number of bands / spots you see on the screen across is finite? I am a little confused by the...
  42. T

    Diffraction grating, distances between maxima

    Homework Statement A grating has a line density of 1010 cm−1, and a screen perpendicular to the ray that makes the central peak of the diffraction pattern is 2.5 m from the grating. If light of two wavelengths, 590 nm and 680 nm, passes through the grating, what is the separation on the (flat)...
  43. S

    Width of principle maxima in n slit diffraction

    I was working on derivation to find the witdh of principle maxima in n slit diffraction Angular width=2Δθm=2λ/(Ndcosθm) Where d:distance between slits θm:angle at which mth order principle maxima is present. N:no. Of slits #in the diagram given below why the minima just adjacent to mth...
  44. S

    Calculate N Slit Diffraction: Grating Width 5cm, Slit 0.0001cm, λ 5.5 x 10-5cm

    if I'm given grating width=5cm ,slits of width=0.0001cm separated by a distance of 0.0002cm how many order would be observable at λ=5.5 x 10-5cm? calculate the width of principle maximum? please tell me the approach to do this problem
  45. Fixxxer

    Why Does Maximum Diffraction Occur When Gap Size is Comparable to Wavelength?

    Hi guys, I'm doing an into to diffraction tomorrow, explaining what diffraction is - so light or sound waves spreading out as they pass through a gap, and when it occurs - so it is a maximum when gap size is comparable to wavelength. However I just want to be prepared to answer why this is the...
  46. atyy

    Why does wavelength affect diffraction?

    This question came up in the biology section https://www.physicsforums.com/threads/explain-different-types-of-light-microscopy.833990/. Q1: The longer the wavelength of a wave, the more easily it bend around an obstacle. I do understand the mathematics, but is there any intuition for it? Q2A...
  47. P

    Light: Refraction, Diffraction, Slits, and Energy Law.

    Q1. I do not understand why the light has to bend inorder to undergo refraction. My teachers have explained me that, light bends due to the change in velocity. If I change the velocity of a car by half it's original value, I don't find the cars path changing. It does not seem to make sense to...
  48. genxium

    Slit sizes needed for light diffraction

    I'm reading this tutorial on instructables for implementing the classical double-slit experiment. I've also read this thread for information which contains a very nice answer from @BruceW but still not resolves my confusion about the "sizes". First of all I'd like to make a distinction between...
  49. kelvin490

    Why an array of telescopes is used?

    To increase the resolution of an instrument, smaller wavelength and larger aperture is desirable. It is mentioned in some textbooks that the "effective" diameter of a telescope can be increased by using arrays of smaller telescopes. I just wonder why it is possible because every telescope is...
  50. kelvin490

    X-Ray Diffraction in Crystals: Exploring Scattering & Interference

    In explaining x-ray diffraction of crystals, usually the approach is to consider the atoms absorb radiation and scatter it, at some angles constructive interference occur and bright spots can be observed. It seems that it is different from the diffraction of slits or gratings because in these...
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