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. BruceW

    Classical light diffraction round corner

    I know how photons are diffracted round a corner in quantum mechanics. But is there also an explanation in classical electromagnetic theory (i.e. by Maxwell's laws?) Imagine there was an EM wave traveling purely normal to a slab of material which is highly attenuating, but which has a small...
  2. E

    Understanding Diffraction in a Double Slit Experiment

    i just don't understand diffraction in a double slit , ? how is it related to interference?? like if i want to find how many bright fringes within the central bright fringe of the diffraction envelope? or how many bright fringes are found between the bright fringes of the first and second of...
  3. B

    How Many Bright Fringes Appear on the Screen in a Diffraction Experiment?

    A diffraction grating with 600 lines/mm is illuminated with light of wavelength 500nm. A very wide viewing screen is 2.0m behind the grating. b.) How many bright fringes can be seen on the screen. MY ATTEMPT: i worked out the angular separation between bright fringes to be 0.305 rad (i...
  4. T

    Non-Coherent Light Diffraction through Double Slit

    Hi, I'm trying to perform a double slit diffraction experiment without using a laser, but instead using a super-bright, white LED. This LED has been placed inside a sealed box with a small pinhole opening (~1/32 inch diameter). This however introduces the problem that the light is incoherent...
  5. F

    Fraunhofer (far field) diffraction

    Im having difficulties showing the spatial distribution of optical intensities observed in the far field region, due to fraunhofer diffraction. I have an arpeture function that is in the form of a delta functon, and i understand that with the delta function situated at the origin the observed...
  6. T

    Diffraction on a body centered lattice, h+k+l even?

    This is in my book: exp[2\pi i(hx+ky+lz)][1+exp(\pi i(h+k+l))]=2exp[2\pi i(hx+ky+lz)]\cos^2[\frac{\pi}{2}(h+k+l)] And in order for the cosine not to be zero, h+k+l must be even when we want to see the reflection. But I think that the result should be exp[2\pi...
  7. M

    Difference Between Interference and Diffraction

    Can anyone tell me the difference between interference in double slit's and single slit diffraction?
  8. C

    Predicting a diffraction pattern without the limiting assumption

    Hello all. I have a relatively simple question. I have learned very recently about wave optics and the resulting diffraction patterns. The experiment by Thomas Young was used to explain the topic and the equations were solved with the assumption that L >> d >> lambda. I was curious how one would...
  9. S

    Diffraction Grating: Calculate Wavelengths

    Homework Statement light from an atomic hydrogen lamp falls on a diffraction grating with 200 lines per centimetre. on a screen 2.5 m away, the distance between green spots of the first and second order spectra is 2.43cm while the distance between a green spot and a red spot both of the first...
  10. E

    Help with deriving this diffraction equation

    Homework Statement Hi! I need help with showing how to derive this formula for the diffraction of a laser: m λ = d (sin(θm) - sin(θi)) where: d = groove spacing m = order of diffraction λ = wavelength of laser θi = angle of incidence θm = angle of diffraction Homework Equations I found this...
  11. T

    Dark spot in Fraunhofer diffraction

    Hi, I've been doing Fraunhofer diffraction and I have been getting dark spots in the middle if my patterns, not predicted by the Fourier transform of the aperture. I don't think it's babinets principle and effect is stronger near the edges of the slits. Any ideas? Thanks
  12. J

    Why does light change direction when entering a new medium?

    a wave will be diffracted as it passes by an object. in the case of light passing through a slit experiment, why isn't the probablility function of the associated photon(s) "collapsed" by its interaction with the edge of the slit? a wave will be refracted if it enters a new medium. what...
  13. K

    Single slit diffraction pattern

    Homework Statement A single slit forms a diffraction pattern, with the first minimum at an angle of 40 from the central maximum. Monochromatic light of 410 nm wavelength is used. The same slit, illuminated by a different monochromatic light source, produces a diffraction pattern with the...
  14. Z

    Diffraction in Water: Exploring Snell's Law

    Homework Statement Hi Could someone explain to me... if a diffraction experiment that had a plane grating on it was submerged into water...why the diffracted light would produce extra angles that are wider and narrower than if the experiment had been carried in air?Homework Equations Snells...
  15. F

    Crystal diffraction using Bragg's Law

    Homework Statement You wish to study a material with a cubic crystal structure with three orthogonal sides of length a = 0.3nm. What is the longest possible wavelength of the X-ray source you could use to reveal the (211) Bragg peak of the crystal? Homework Equations nλ =...
  16. P

    Single and Double Slit Diffraction and Interference

    I have no idea where to start with these questions. I am pretty sure I have to use these equations 1. theta = tan^-1(x/D) 2. wsintheta = nlambda 3. dsintheta = nlambda Which order m of in the two-slit interference pattern will give the least uncertainty in the slit spacing, d? a) m = 1 b)...
  17. E

    Single slit interference and Diffraction Grating

    Lets say I have a double slit interference experiment set up. Now if I cover one slit what CHANGES would be observed ? I know I would see fewer bright fringes and that the intensity of the bright fringes would fall. Is there anything else ?In a different scenario I have a diffraction grating...
  18. K

    How Wide Must a Diffraction Grating Be to Resolve Specific Emission Lines?

    How wide would the grating have to be for the two emission lines to be observably resolved? What is the m (order of diffraction peak) that is minimum to resolve these lines to be observed??R= Nm= wavelength/change in wavelength w= Nd i know that w=Nd w= (x) (2.5 * 10^-6 m) i...
  19. P

    Single slit diffraction problem

    Homework Statement A single slit 10-4m wide(a) is illuminated by plane waves from helium-neon laser ( l = 6.328*10-7m). If the observing screen is 10m(D) away, determine the width of the central maximum(y). Homework Equations sin(alpha)=(m+0.5)*l/a tan(aplha)~sin(alpha)=y/2*D...
  20. H

    Curved Diffraction - Explaining Why Light Behaves So

    I'm doing some experiences with a laser about diffraction caused by an edge, and I have some question to ask. Unfortunately, I've found very few literature (almost nothing) about diffraction caused by an EDGE. Everyone explains well diffraction caused by slits; someone explains diffraction...
  21. T

    Exploring the Effects of Slit Separation on Interference Patterns

    Homework Statement What is the difference between the pattern resulting from -2 slits, each 0.02mm wide, separated by 0.2mm AND -2 slits, each 0.02 mm wide, separated by 0.4 mm Homework Equations lambda = 2yd / (2m-1)D, m = 1,2,3, etc. D = distance from screen (where pattern is...
  22. H

    Von Laue Condition X ray Diffraction

    Homework Statement The von Laue condition for x ray diffraction can be written dΔκ =2πn where d is a displacement vector between indentical atoms and n an integer What is the smalest angle θ through which x -rays of wavelength 0.12 nm may be scattered by a sample of polonium ,a simple cubic...
  23. R

    Can You Hear Sound in Room Corner by Door?

    Say you have a square room with only one door, and the door is as the center of one side of the square. Suppose you put a stereo just outside the door. Can you hear the sound if you are standing in one of the corners of the room that's adjacent to the door? Fraunhofer diffraction predicts that...
  24. M

    What is the width of the second slit in a single-slit diffraction experiment?

    Homework Statement In a single-slit diffraction pattern on a flat screen, the central bright fringe is 1.37 cm wide when the slit width is 3.48E-5 m. When the slit is replaced by a second slit, the wavelength of the light and the distance to the screen remaining unchanged, the central bright...
  25. jegues

    Central Diffraction Maximum Double Slit

    Homework Statement When a 450-nm light is incident normally on a certain double-slit system, the number of interference maxima within the central diffraction maxima is 5. When 900-nm light is incident on the same slit system, the number of interference maxima within the central diffraction...
  26. S

    Diffraction of x-rays by crystals

    :shy: By looking at the figures showing diffration of x-rays by crystals, it looks more like reflection.. Seems like I am studying reflection... So, where is the diffraction part??
  27. B

    Electron diffraction from crystals

    why is it that in electron diffraction from cystals, theta i and theta r are not necessarily equal?
  28. L

    X-ray Diffraction Homework: Calculate d & a

    Homework Statement X-ray diffraction is made with a copper tube as x-ray source. The tube generates molybdenum x-rays with a wave length of 0.709 A (A=Angstrom). In the interval 40-160 degrees 2theta reflections are found under the following angles: 40.450 degrees 58.408 73.398 86.995...
  29. B

    Diffraction of Light: Explaining Interference & Huygens' Principle

    diffraction takes place when the wavelets are a source of more wavelets that expand in all directions, and the shape of the new wave front is curved. The wavelets of these diffracted, or bent, waves can now travel different paths and subsequently interfere with each other, producing interference...
  30. B

    Diffraction is an evidence that light is wave

    Introduction: diffraction is an evidence that light is wave. Photo electric effect shows that light is particle. Question: What about reflection and refraction? Are they evidences of the wave nature of light or of the corpuscular theory?
  31. Z

    Double Slit Diffraction d/a

    Homework Statement (a) In a double-slit system, what value of d/a causes diffraction to eliminate the third bright side fringe? Homework Equations I(\theta) = I_mcos^2(\beta)(\frac{sin(\alpha)}{\alpha})^2 \beta =\frac{\pi d}{\lambda} sin \theta \alpha = \frac{\pi a}{\lambda} sin \theta...
  32. P

    How does diffraction cause interference

    Light normally doesn't interfere with itself because it is incoherent and unpolarized, right? So how is it that incoherent, unpolarized light passes through a circular aperture and interferes with itself? How does the aperture suddenly cause the light to be able to interfere with itself? In...
  33. N

    What is the Difference Between Diffraction and Interference?

    Hello, could someone tell me what is the main difference between effects of diffraction and interference? For now, I think that diffraction is name for deviation of waves when they come to obstacle. And interference is phenomenon when two or more waves are in superposition in some point...
  34. C

    Diffraction and phase difference

    Hi, I'm having some trouble understanding the path difference between two parallel rays leaving a single slit, and was hoping you guys could help me. I'm having a hard time understand what path differences exist for each subsection of a slit and for which minimum. For example: -What is the...
  35. K

    Calculating Wavelength for Diffraction Grating Problem

    Homework Statement Light with a wavelength of 475 nm produces a first order maximum 0.214 m from the central bright line for a particular diffraction apparatus. At what distance would you expect a maximum using light of 625 nm? The Attempt at a Solution I am confused. For some reason...
  36. E

    Difference between Bragg and Laue Diffraction?

    Hi! I am confused about the difference between the Bragg and Laue Diffraction. It seems that both arrive at the same result, but the assumptions for both are different?
  37. JJBladester

    Single Slit Diffraction light intensity

    Homework Statement This is the light intensity on a viewing screen behind a rectangular opening in a screen. Is the shape of the opening (1), (2), or (3)? Homework Equations w=(2λL)/a where w is the width of the central maximum, L is the distance to the viewing screen, and a is...
  38. K

    Why Does My Calculation for Electron Diffraction Not Match the Expected Result?

    Homework Statement Electrons are accelerated to an energy of 2.40 keV and then passed through a thin graphite film. The distance between the carbon atoms is 1.42 * 10^-10 m. Homework Equations b) Describe the diffraction pattern on a screen 5.00 cm from the graphite film. We're looking for...
  39. JJBladester

    Diffraction Grating interference pattern

    Homework Statement This is the interference pattern on a viewing screen behind two slits. (image of 7 bright fringes with black fringes between every pair of bright fringes) How would the pattern change if the two slits were replaced by 20 slits having the same spacing d between...
  40. jegues

    Double Slit, Central Diffraction Maximum

    Homework Statement See figure attached for problem statement as well as my attempt Homework Equations The Attempt at a Solution I don't know where to go from here. Can someone help me get started?
  41. D

    Why can this formula be applied for diffraction gratings?

    So for 2 point sources (or 2 slits) the formula for finding the lines of maxima and minima for waves is n(lambda)=dsin(theta) and when n is an integer you would expect to find a maxima. However, I don't understand why this formula also applies to diffraction gratings since there are way more...
  42. D

    Diffraction and superposition.

    This isn't exactly a homework question but just a few conceptual questions I'm confused about. First off on the topic of diffraction I notice many textbooks and other resources say that diffraction is more evident when the width of a slit is close to the wavelength of the wave. However, they...
  43. M

    Diffraction - Calculating Width of a Slit

    Homework Statement I am required to calculate the width of a single slit. I have known distance, L, between the observation screen and the slit, and photo images were taken of the resulting waveform. However, the wavelength is unknown, therefore making many of the equations I'm attempting to...
  44. C

    Can black lines on a mirror cause diffraction of light?

    I watched the Feynman lectures on light . And when he talks about a photon bouncing off a mirror, and that it usually bounces off at the angle it hits, But then he says I'm going to do a dirty trick and paint black lines in the mirror. Like if he paints enough lines the light will diffract...
  45. E

    Diffraction with two wavelengths?

    Homework Statement "White light contains wavelengths from 4.00 x 10^-7 metres to 7.00 x 10^-7 metres. This white light is directed on a diffraction grating ruled with 5.50x10^4 lines/metre. How wide is the first-order spectrum on a screen that is 1.25m away from the grating? Homework...
  46. M

    Electrons diffraction in crystals

    Hello everyone, It's said that the first Brilloin zone is the k-space where electrons are not diffracted. Electrons with k-vectors on the surface of BZ are diffracted. How can we understand in terms of electron particules the behavior of electrons associated waves when these waves...
  47. M

    Refraction, Diffraction, or Interference?

    Homework Statement Identify the following as an example of Refraction, Diffraction, or Interference 1) Rainbow pattern on a compact disk 2) The floating blotches in our eyes 3) Rainbow pattern on a beetle's back 4) The dark bands seen a a window screen Homework Equations...
  48. sophiecentaur

    How long does a diffraction pattern take to form?

    I was idly musing (as you do) and I was thinking of Fermat's principle which says that light takes the 'shortest route'. That, however, must refer to the classical situation. Take the simplest diffraction situation of two slits - or forming the really complex diffraction pattern from a large...
  49. M

    Laser Interference and Diffraction, single and double slit experiment?

    Homework Statement I did the single slit light diffraction lab. The diode laser we used was 630-680 nm. It produced an image very much like the one on this page: http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/fraungeo.html#c1 How does having multiple wavelengths in the laser change...
  50. A

    Wavelength Function and Diffraction Orders Explained

    an someone explain to me the symbol "m" in dsin\theta=m\lambda, where m is the order of diffraction (or whatever it's called). I am using a fluorospectrometer at which the sin\theta is always equal to 1. This instrument can fix the excitation \lambda. The main point is that scattering of the 1st...
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