# 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. ### Balmer series experiment

Hello. In data that the professor sent, I see only ##n## and ##\theta##. So I do not know what is d. Can I find ##\lambda## without it? On other thing he mentioned : Suppose that the diffraction grating consists of 300 lines per millimeter. I know that d is the distance between two consecutive...
2. ### B Difference between holographic paper and diffraction gratings?

These photos show pearlescent tape that changes color when observed from different angles. Either it's the full rainbow like in the third, fifth and seventh images I displayed here or just a few colors like in the other images, they still change color and are called "iridescent decorative...
3. ### I Is the uncertainty principle applicable to single slit diffraction?

Hi. I've seen single slit diffraction being brought up as an example of the uncertainty principle: Narrowing the slit restricts the particles more in one dimension, which means the momentum in this dimension is more uncertain, which results in a more spread-out diffraction pattern. I've even...
4. ### Researcher in electron diffraction

Thanks for including me in this forum. I have a few questions on photon-matter interaction, that hopefully someone would be kind to assist in, Johan
5. ### B Explanation for bright fringes in Single Slit Diffraction

In Young's Double Slit Experiment, we were shown the complete derivation for location of fringes, width of fringes etc. on interference by two point sources of light and all was well. In Single Slit Diffraction we were just asked to remember the formulae as they were with little explanation. I...
6. ### B Why does light diffraction create circular patterns?

We see under trees circular light that goes from sun through the leaves gaps. Why does it create a circular shape and not other shapes? why does this diffraction phenomenon create it in circular patterns?
7. ### I Single-Slit Diffraction -- Deriving the relation for the minima

Hi everyone. I'm studying single-slit diffraction, and a question came up: to derivate the relation for the minima (dark fringes), the slit is devided in two parts, and it's assumed the distance between the light rays is a/2. Why is this distance chosen? I was wondering about other options...
8. ### A Diffraction theory, power density and angle of incidence

Hello everybody I am currently looking at diffraction through a rectangular aperture. I am looking at an aperture which is large compared to wavelength, and am looking at diffraction at all angles behind the aperture in a distance which is approx equal to the size of the aperture. I am...
9. ### I How Does Diffraction Influence Shadows and Optical Observations?

I have a couple of doubts about diffraction I'd like to clear up anyway The Sun's shadow cast by a tall pole embedded in the ground is less sharp at the top... How is this related to diffraction? Is it due to the presence of air? On Halliday, Resnick, Krane textbook it also says that...

13. ### I Diffraction of Matter: Examining Bohr's Theory

Does matter (like electrons) diffract at the single slit and create an interference pattern on the screen? If it's not why? Isn't that violation of Bohr's Theory?
14. ### I Understanding Diffraction Condition in Kittle's Intro to Solid State Physics

I am going over the diffraction condition section in Kittle's Introduction to Solid State Physics physics and I am having a hard time understanding why the phase difference angle for the incident wave is positive while the phase angle difference for the diffracted wave is negative. Thank you...
15. ### A Ballentine on Diffraction Scattering

In Chapter 5.4b of Ballentine, a discussion ensues about the analysis of a particle scattering off of a (Bravais lattice) periodic array. I attach pictures here of the full discussion in case anyone wants/needs to refer to it, but I am particularly baffled by the discussion on page 135. In...
16. ### I Questions About Bragg's Law of X-Ray Diffraction

Bragg's law is schematically shown on the picture: Two parallel and plane waves are shown which propagate towards the crystal. For plane waves, wave fronts are flat planes perpendicular to the wave propagation with infinite size. In reality, there are no plane waves. Nevertheless, they are...
17. ### A Diffraction from a set of concentric rings with random phase

I have been considering the properties of a Diffractive Optical Element (DOE) consisting of a very large number of concentric rings of equal (small) width, where the thicknessses of the rings are such as to produce random phase shifts in the range 0 to 2pi. I think I understand the behaviour of...
18. ### Diffraction Effects and Artifacts in Telescopes like the JWST

[Mentor Note -- Discussion spun off from the main JWST thread] Yes, the apparent hexagonal shape of that star is due to the diffraction within the telescope optics (optical filter characteristics may also have played a role). For clarity, we're talking about this star: They are (images...
19. ### Number of bright fringes given by diffraction grating on a screen

(1) In the book I am using the separation of bright fringes is indicated as being ##\Delta y=\frac{\lambda}{d}##, where ##d## is the separation of the slits so on a screen of width ##W## I would see ##\frac{W}{\frac{\lambda}{\frac{1}{N}}}## bright fringes. I don't see why the text of the...
20. ### A Diffraction of spherical wave by plane grating

Textbook examples usually involve a plane monochromatic wave that is diffracted by a plane grating. If one places an ideal focusing lens behind the grating one will get a diffraction pattern in the back focal plane of the lens. The geometric size of this diffraction pattern is proportional to...
21. ### B Number of fringes in a 2-slit diffraction pattern

When looking at a diffraction pattern made by two slits, and trying to calculate wavelength, what do we count as fringes? For example, in this picture are there five total fringes or, like, 25? I've been trying to set up a lab for my high school physics 2 class and the math says that every...
22. ### I with this Circular Aperature Diffraction Problem please

I know that in order for the two lights to be distinguishable from one another they have to be separated by an angle of at least theta = 1.22(wavelength)/(width of aperture). I tried drawing the given picture below and then using trig to find L in terms of d/2 and theta/2. However, this ended up...
23. ### A Calculating Fresnel Diffraction Pattern from Circular Apertures

I am attempting to calculate the Fresnel difraction pattern from different diameter circular apertures for specific source to aperture and aperture to sensor distances. I'm generally following the procedure given in Klaus D. Mielenz "Algorithms for Fresnel Diffraction at Rectabngual and Circular...
24. ### Fresnel diffraction from square: on axis intensity

I'd appreciate if someone could check whether my work is correct. The ##x##-##y## symmetry of the aperture separates the Fresnel integral:\begin{align*} a_p \propto \int_{-a/2}^{a/2} \mathrm{exp}\left(\frac{ikx^2}{2R} \right) dx \int_{-a/2}^{a/2} \mathrm{exp}\left(\frac{iky^2}{2R} \right) dy...
25. ### Question on Resolving Power of Diffraction Grating

I have doubts about my work for the second part of the question, where I am asked if resolution will be possible or not. For the first part, I calculated No. of lines N=6.43*10^2 lines/cm For the second part, I have attached below a snapshot of my neatly written work. I request a PF member to...
26. ### I Umbrella fabric and diffraction pattern

It is sometimes said that a distant street lamp seen through a dark coloured umbrella fabric appears like dotted diffraction pattern, as in figure A. However, I have tried several umbrellas but all of them produce a blurred cross, as in figure B. The umbrella fabric superficially looks like a...
27. ### I Using Diffraction (i.e., Fresnel Zone Plate) to defocus/diverge light

I am wondering if it is possible to use principals of diffraction to cause a collimated beam of light (laser) to become divergent. I see that zone plates are most always used for focusing the light from a source, unless they are used in reverse. This is why zone plates are seemingly always...
28. ### I Calculating Intensity in Single Slit Diffraction Experiments

In a single slit diffraction experiment, when we want to calculate the intensity of light on a screen located very far away from the slit, usually Huygens' principle is adopted as a model to perform the calculations. It is assumed that the width of the slit consists of an infinite number of...
29. ### Calculating the Number of Lines for a Diffraction Grating

A spectral line of wavelength λ = 4,750˚A is actually a doublet, of separation between the lanes 0, 043˚A . a) which is the smallest number of lines a diffraction grating needs to have to separate this doublet in the 2nd order spectrum? To be honest, i don't know what to do. I first thought...
30. ### Do Radio Transmitters cause diffraction?

I am very confused as to how diffraction even happens. I am thinking that radio transmitters are those tall towers. (My country doesn't have those, our lines are underground). Where is the 2nd antenna, and how does the diagram look like? Does it look like: (The dashes are to fill up space)...
31. ### A Magnifying diffraction patterns and real images

I've been playing around with geometric optics a bit and started wondering whether it's possible to magnify both the real image and the diffraction pattern with the same lens setup. With a single lens both become larger when the focal length is increased (as along as it stays below the object...
32. ### Derivation of single slit diffraction formula

I want to ask several questions regarding to the text: 1) Why do we find the minima of the diffraction? Why not the maxima?2) "Figure 25.32b shows two rays that represent the propagation of two wavelets: one from the top edge of the slit and one from exactly halfway down" Why do we take point...
33. ### I Single-slit diffraction experiment and the conservation of energy

In a single-slit diffraction experiment, a monochromatic light of wavelength ##\lambda## is passed through one slit of finite width ##D## and a diffraction pattern is observed on screen. For a screen located very far away from the slit, the intensity of light ##I## observed on the screen in...
34. ### I How Can One Derive the Diffraction Pattern Formula from a 1D Aperture?

I've been trying to derive a formula for diffraction pattern formed by casting a plane-wave through a generic 1D aperture onto a screen distanced ##L## from the aperture. The aperture is described by an opacity function ##f:\mathbb{R} \rightarrow [0,1]## so it can be a single slit, multiple...
35. ### Intensity of central maxima of crystal diffraction

When the crystal above is illuminated with light of intensity ##I_0##, what is the intensity at the central maximum? (The picture shown above is a 4 x 4 unit cell of the crystal) The answer is ##(\frac {5} {16} )^2 I_0##. Why? Apparently, Electric field is proportional to the transparent area...
36. ### Problem on Diffraction Grating and Laser Crystallography

First question: When shining a laser (##\lambda = 630nm##) through a 2D crystal, diffraction patterns are observed. Why? Possible reasons: 1. Atoms and the space between them act as diffraction gratings. 2. Bragg Diffraction, although in this case the pattern would be observed on the same...
37. ### Working out Plane Structure via Powder X-ray Diffraction Multiplicand

Below is the measured values for the Debye rings I obtained. I have to multiply the ratio (which is (sin^2(theta_n))/(sin^2(theta_min))) by a multiplicand until I get an integer. However for the multiplicand and the values I measured I get 1, 3, 13, ??, 4, 8, ??. These should either correspond...
38. ### Inferring Shape of Phasors in Multi-Slit Diffraction

I know that phasors of a single-slit diffraction form a closed polygon or circle, but how could we infer the shape when phasors generated by slits of a multiple-slit barrier?
39. ### Determine horizontal separation of fabric from diffraction pattern

Do I need to use formula to answer this question? Can't I just divided the horizontal distance in the picture by 2. so the horizontal separation of the thread is 54.8 / 2 = 27.4 mm? Thanks
40. ### Treating Oblique Incidence as a Fraunhofer Diffraction Problem

I'm not sure where to start on this as I've only used Fraunhofer when it involves slits, not mirrors. Would I say it was a single slit problem so that D = width of slit (but this doesn't make sense to me because the light is reflecting not transmitting? Or an infinitely narrow slit hence nothing...
41. ### What is the effect on a diffraction pattern when reducing the width of the slit?

Monochromatic light is normally incident on a diffraction grating. The mth order line is at an angle of diffraction angle θ and has width w. A wide single slit is now placed in front of the grating and its width is then slowly reduced. As a result: A. both θ and w increase B. both θ and w...
42. ### I Explaining the Continuous Diffraction Spectrum of a Heated Solid

This is the figure from the book. First of all, from what I know about diffraction, there is an interference pattern but not dispersion of the different colors. If what is happening here can be explained that would be great. Second, the book says the line spectra for different gasses are due to...
43. ### Trivial question about units (Hans Bethe Small Hole Diffraction)

http://www.physics.miami.edu/~curtright/Diffraction/Bethe1944.pdf Hans Bethe's paper Jackson doesn't take any issue with Hans Bethe's paper as far as I can tell when he references it. That leads me to believe the two sets of dipoles are equivalent and there happens to be an "old school"...
44. ### Angular Width Question in Single Slit Diffraction

i feel like subbing the numbers into the equation isn't enough because of the second minima and maxima thing? not sure what to do... would appreciate any help.

46. ### The sources of error in a diffraction grating experiment

In a diffraction grating experiment, what can be the sources of error and also what effect do these sources have on the unknown quantity (wavelength) in the experiment?
47. ### I Van Laue Formulation: X-ray Diffraction from Crystal Structure

When we shine an X-ray beam on a crystal, according to Van Laue formulation, for a sharp intensity peak to be observed on the screen there is a specific direction ##\mathbf{\hat{n}}## , in which for all the X-rays with wavelength ##\lambda## and wavevector...
48. ### Particle analysis of diffraction

I have done a bit of googling but cannot find anything in this regard at all. It seems to all be "thought of" (rather than explained) by the Huygens–Fresnel principle and other spin offs. Can anyone point me to a particle explanation of diffraction for sound waves? Or does anyone here have...
49. ### I Why find the highest order maxima/minima in slit equations?

I have been seeing interference and diffraction problems involving the double slit and single slit equations that ask for things like the "highest order maxima" or minima, which I have learned are basically found by setting the angle to 90 degrees. My question is are there any real world...
50. ### Diffraction of light and conservation of energy.

Suppose that we shined a source of light on a wall with infinitismal small opening. As the opening is infinitismly small, only one ray of light will pass through the opening ( suppose it has an intensity ##I_0##) and this ray of light will diffract into an infinite number of light rays with the...