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. Decimal

    Huygens-Fresnel principle of diffraction

    Homework Statement This question does not concern a homework problem but I don't really understand the Huygens-Fresnel principle of diffraction. My book states that an assumption is made that a wavefront acts as a source of secondary wavelets. They continue with the following derivation. This...
  2. ZHIHZ

    Diffraction Help: Is This Diffraction?

    Is this diffraction??Help..sorry if I've posted in the wrong place.
  3. Decimal

    [Optics] Find maximum order number, Fraunhofer diffraction

    Homework Statement This problem concerns a single slit diffraction experiment where Fraunhofer diffraction is observed on an observing screen via a positive converging lens with focal distance f. The diffraction pattern has a central maximum of width dy, and the vacuum wavelength of the light...
  4. D

    Single Slit Diffraction and Interference Maxima

    In a single slit experiment if the condition of interference maxima be asinΘ=nλ where n=1,2,3,4... and the condition of diffraction minima is also the same Will it not cause any effect on the intensity of the interference maxima as both are at a same distance from the central maximum on the...
  5. A

    Single Slit and Diffraction Grating Formulas

    Hi everyone. I am having some difficulty understanding the formulas of single slit diffraction and diffraction gratings. This is partly because the formulas that I have been given in class do not reflect what I have found online entirely. As we know, the formula for single slit destructive...
  6. A

    Diffraction Grating and White Light Problem

    Homework Statement White light containing wavelengths from 400 nm to 750 nm falls on a grating with 6000 lines/cm. How wide is the first-order spectrum on a screen 2.0 meters away Homework Equations dsinθ=mλ for constructive interference dsinθ=(m+1/2)λ for destructive interference Δx = λL/d...
  7. W

    What Causes Equal Interference Amplitudes in Light Diffraction Patterns?

    Here's a picture that was in my textbook - they are the patterns of interference and diffraction of a light wave. I don't get why the interference amplitudes are all equal. Aren't they supposed to be higher in the middle and lower as they move away from the center??
  8. W

    Interference/Diffraction formulas

    Homework Statement So I'm trying to determine the slit separation of a 2-D grid (as I understanding it, basically a grid with a lot of parallel, equally spaced slits in the x and y directions). I know I have to use some form of the formula d*sin(theta) = m*lambda, and I'm having some trouble...
  9. T

    I Laue Diffraction Intensity Distribution

    I am trying to experimentally determine the atomic scattering factors for NaCl using Laue photography. For an NaCl crystal, the intensity of the wave scattered at the unit cell is proportional to (4⋅(fCl+fNa))2 if h,k,l are even and to (4⋅(fCl-fNa))2 if h,k,l are odd. fNa and fCl are the atomic...
  10. D

    Diffraction and gravitational lensing

    Hello. And i have a dumb question about light. When the diffraction is studied, we look at where the light is coming. But not where the light comes from. When the gravitational deviation is studied, we look at where the light comes from, but not where the light comes. What do you think...
  11. T

    Maximum frequency if there is to be one diffraction order

    Homework Statement A naval towed-array sonar comprises a line of ##80## transducers, equally spaced over a total length of ##120 m##, that is towed behind a ship so that it lies in a straight line just below the surface of the water. An adjustable phase delay can be introduced electronically...
  12. S

    I Plotting a far-field Intensity distribution

    Hey, I'm attempting to plot a far-field intensity distribution using theoretical values, however I'm having difficulty with calculating the intensity using the following equation: $$I = I_o \frac {sin^{2}{b}}{b^{2}} \frac {sin^{2}{Ny}}{sin^{2}{y}}$$ where: $$y = \frac {kdX}{2f}$$ $$b =...
  13. S

    Fraunhofer diffraction experiment- neural density filters

    I'm currently carrying out an experiment with Fraunhofer diffraction. It involves shining a laser beam through neural density filters, a lens and a diffraction grating, to create a diffraction pattern which is then picked up with a CCD camera, to find the intensity of the maximal peaks. However...
  14. Const@ntine

    Barrier of Diffraction: Different Wavelengths, Same Color?

    Hey there! We started Lab III last week, but things are a bit... strange. See, those exercises were written back in the early 1900s,and so they ask us to read from a book that was published around 1890 or so. Naturally, the library has only one copy, and it's open for 2hours per week, so it's...
  15. L

    Maxima in a diffraction grating

    Homework Statement A diffraction grating is made up of slits of width 300 nm with separation 900 nm. The grating is illuminated by monochromatic plane waves of wavelength l 600 nm at normal incidence. How many maxima are there in the full diffraction pattern? Homework Equations mλ = dsinθ The...
  16. L

    I X-ray diffraction minima?

    For x-day diffraction maxima we have braggs law 2d*sinθ = mλ (maxima) Is there an analogous law for the minima like 2d*sinθ = (m+1/2)λ (minima?) Thanks!
  17. C

    I Huygens principle from Kirchoff Integral (sign doubt)

    Consider the Kirkoff integral theorem and the Huygens -Fresnel principle/formula (both from Wikipedia): KIT The Kirchoff integral for monochromatic wave is: $$U({\mathbf {r}})={\frac {1}{4\pi }}\int _{S'}\left[U{\frac {\partial }{\partial {\hat {{\mathbf {n}}}}}}\left({\frac...
  18. K

    Diffraction work differently on water waves vs light waves?

    When the water waves go through one wide slit (slit wider than wave length), there's no fringes, the water waves spread all over the sides, like in this pic: http://electron6.phys.utk.edu/light/images1-3/misc3b.jpg or in this pic...
  19. E

    B Help with Physics lab idea (creative)

    Hiya! In school we have tha task to create our own phy lab. I am going to investigate the width of a hair using wave diffraction (a laser maybe?). Does anyone have any creative ideas on how to develop the lab to make it more uniqe and interesting. E.g measuring from different distances. Or...
  20. L

    Single-slit Diffraction Problem

    Homework Statement A spy camera is said to be able to read the numbers on a car’s license plate. If the numbers on the plate are 5.0 cm apart, and the spy satellite is at an altitude of 160 km, what must be the diameter of the camera’s aperture? (Assume light with a wavelength of 550 nm.)...
  21. Kaneki123

    A Question about Diffraction....

    Okay...I have a question that, I have often read the statement in various topics on diffraction that "Diffraction is maximum when slit is comparable to wavelength of wave". Can someone point out what is exactly meant by ''greater or maximum diffraction'' here?...Like if we are going to talk...
  22. D

    Single Slit Fraunhofer Diffraction

    Just to clarify, this isn't homework. I took an Optics course a few years ago, but in the time between then and now, I've lost all my notes. Suppose we have an infinitely tall slit of width a and a parallel screen a distance r_0 away. Let the x axis be parallel to the width of the slit, with...
  23. bknock

    Thin slits and accuracy of Kirchhoff's diffraction formula

    Imagine a single slit with plane light waves incident on it with a screen (ideally far enough from the slits to simplify the math). According to Kirchhoff's diffraction formula, when a very wide slit is doubled, average intensity (averaged over all diffraction angles) doubles, and so does E_peak...
  24. A

    A Multiplication and Addition to get an integer

    Hi, to understand finally the Laue equation for diffraction I am missing something : h*p+k*q+l*r = integer. Given that p,q,r are integers how come h,k,l MUST BE INTEGERS as well? Say p=q=r=2, than h=k=l=1/2 works just fine. I understand that there is something about a common...
  25. Const@ntine

    Diffraction Barrier-Class of Maximum?

    Homework Statement A Diffraction Barrier has 4200 openings per 1cm. A screen stands 2.00 m opposite of the Barrier. Say that for a certain class m, the maximums that correspond with two different wavelengths (589.00 nm & 589.6 nm) abstain from each other by 1.54 mm. What is the value of m...
  26. J

    Explaining Wavelength and Diffraction in Edexcel Physics A-level Q17 (b) i

    http://pmt.physicsandmathstutor.com/download/Physics/A-level/Past-Papers/Edexcel-IAL/Unit-2/January 2015 (IAL) QP - Unit 2 Edexcel Physics A-level.pdf This is the link to the paper if you can't see the image Q17, part (b) i Please explain This is what the markscheme and examiners report say I...
  27. J

    Bragg Law & Diffraction Condition

    Hello. I am reading "Introduction to Solid State Physics" by Kittel and there is a derivation in the textbook that I am understanding. This should be a fairly simple question but I am unable to see it. 1. Homework Statement In Chapter 2, it derives the Bragg law using the diffraction condition...
  28. K

    I High Resolution X-Ray Diffraction: Explained

    Hello What does it mean when people say high resolution xray diffraction? What is the difference between regular Xray diffraction and high resolution X-ray diffraction? Thank you
  29. Eagertolearnphysics

    Diffraction grating problem

    Homework Statement Yellow sodium light, which consists of two wavelengths, λ_1 =589.0 nm and λ_2 = 589.89 nm, falls on a 7500 lines/cm diffraction grating. Determine (a) the maximum order m that will be present for sodium light, (b) the width of grating necessary to resolve the sodium lines...
  30. G

    Diffraction of Sound Waves Through Slit

    Hi there, I have a scenario in which different frequencies will be played behind a curtain with a 2m opening. I would like to calculate the angle of diffraction for different frequencies played by the piano. One equation that I came across through research is Fraunhofer's Single Slit equation...
  31. Z

    The Kirchhoff diffraction formula with small wavelength

    Homework Statement The fresnel-kirchhoff diffraction formula could explain diffraction,I think it should also produce the result with relatively small wavelength and large width slit in which case there is no diffraction. Homework Equations...
  32. sarvesh upadhyay

    A X-Ray Diffraction: Why Doesn't It Eject Electrons?

    X-ray has enough energy to eject the electron in the atom, molecule etc. But in the x-ray diffraction electron does not eject, only oscillate with the same frequency as X -ray,why? Why does not x-ray eject the electron?
  33. K

    I Coherent X-ray Diffraction Imaging Explained

    Hello I am confused what Coherent X-ray Diffraction Imaging is with real and reciprocal space. Would someone be able to explain this to me like I am 5 years old because I am really not getting it.
  34. J

    Abbe theory: diffraction pattern to primary image

    I have been attempting to unravel Abbe’s theory, of the role of diffraction in microscopic vision, in a ‘nuts and bolts’ sort of way, meaning in terms that I can understand: diffraction, interference and so on. While Fourier transforms are clearly at the heart of the process the assertion that...
  35. clamatoman

    Diffraction Grating - all visible light?

    Homework Statement Find the smallest grating spacing that let's you see the entire visible spectrum. 400nm to 700nm comprises the visible light spectrum. no other information is given. Homework Equations nλ=d sin θ The Attempt at a Solution I am not sure how to start as all i have been given...
  36. KennedYeti

    Diffraction Grating and Visible Light

    Homework Statement "You're using a diffraction grating o view the 400 to 700 nm visible spectrum. Suppose you can see the spectrum through the third order. a) Show that the first and second order spectra never overlap regardless of grating spacing. b) show that the second and third order...
  37. M

    What are the errors in Young's Double Slit experiment?

    Homework Statement I am having troubles trying to figure out systematic and random errors when conducting Young's double slit experiment. Homework Equations NA The Attempt at a Solution I have found one error which could be correct, it is the fact that the laser was not at right angles with...
  38. A

    B Hearing/Seeing Waves: Communicating Without Sight

    Imagine that one person is on one side of a hall, and another person is on the other side. There is a gap in the wall that separates the two people. In that case, how is it possible that one person can hear the other person, but cannot see them unless in a direct line with them?
  39. Charles Link

    The Diffraction Limited Spot Size with Perfect Focusing - Comments

    Charles Link submitted a new PF Insights post The Diffraction Limited Spot Size with Perfect Focusing Continue reading the Original PF Insights Post.
  40. Peter Andrews

    Finding the angular spread of a diffraction minima?

    1. Homework Statement Light of wavelength 6000Å illuminates a single slit of width 10-4m. Calculate the angular spread of first diffraction minima.Homework Equations d*y/D = nλ Y = nλ/a for minima Y = (2n±1)λ/a for maxima Y stands for the position on screen, d is slit width and D is separation...
  41. N

    Double slit experiment with a glass-covered slit of unknown n

    Homework Statement A double-slit experiment uses a helium-neon laser with a wavelength of 633 nm and a slit separation of 12mm. When a thin sheet of glass is placed in front of one of the slits, the interference pattern shifts by 5 fringes. When the experiment is repeated under water, the shift...
  42. Z

    I How Fermat's principle interprets diffraction and interference

    <Moved from the homework section> 1. Homework Statement I have read several chapters of De Brogile's article "the theory of quanta".The motion of a particle could be analogious to a ray in general optics.This is an analogy between Maupertui's principle and fermat's principle. How to use this...
  43. SpiffyPhysics

    Help with diffraction flashlight lab activity....

    I'm updating some poorly written lab activities for an online physics class that I have been given, and I am going through the experiments and doing them myself to better write them. I came across this one, and I have no idea what the results are supposed to be. It comes after the chapter on...
  44. Arup Biswas

    Why subsidary maximum has lower intensity physically in single-slit diffraction?

    In single slit diffraction the subsidary maximum has lower intensity than of central. Yes mathematically from the intensity equation we can see that. But how physically we can explain that? When the central and the 1st maxima has same activity to produce constructive interference( as in same...
  45. S

    Double slit problem with glass block

    Homework Statement Light of wavelength 600 nm passes through two slits separated by 0.20 mm and is observed on a screen 1.0 m behind the slits. The location of the central maximum is marked on the screen and labeled y=0. (I only need help on the last 2 parts, but I will list all of them in...
  46. S

    Diffraction grating problem involving Snell's Law

    Homework Statement A 600 line/mm diffraction grating is in an empty aquarium tank. The index of refraction of the glass walls is [n][/glass] = 1.50. A helium-neon laser (lambda=633 nm) is outside the aquarium. The laser beam passes through the glass wall and illuminates the diffraction grating...
  47. Arget

    Double slit diffraction ripple

    Consider diffraction in a ripple tank from two narrow slits. Answer true (T) or false (F) for each of the following statements. E.g., if the first statement is true and the rest false, enter TFFF. You have 6 tries. If the frequency of the wave source is halved, then the angle between the...
  48. G

    Why is there a wavelength dependence in diffraction?

    Hello community, I'm aware of Huygens construction for helping to explain why you might possibly get diffracted waves if a plane wave passes through a gap (i.e. About the secondary wavelets and superposition bit) but the wavelength dependence bit bothers me because say one wavefront is passing...
  49. CricK0es

    Interference from coherent light sources

    Homework Statement Four equally spaced coherent light sources (n=4) with a wavelength of 500nm are separated by a distance d = 0.1mm. The interference pattern is viewed on a screen at a distance of 1.4m. Find the positions of the principle interference maxima and compare their width with that...
  50. saybrook1

    Question About Diffraction Orders

    Homework Statement Light is normally incident on a double slit system. I'm asked to calculate the number of "orders(maxima)" visible on a screen placed in front of the slits. The light wavelength and slit separation are given. I understand that I can calculate the "order" from m=d/λ but does...
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