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68 votes

Why exactly does diffraction occur?

For the full math, you can look up 'diffraction' and 'Huygens Principle' but here I will just post a quick observation that is enough to get a good physical intuition. Suppose we are considering ...
Andrew Steane's user avatar
45 votes
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How is momentum conserved in diffraction?

The slits themselves receive a tiny impulse from each photon. If a photon is diffracted to the left, the slits get nudged to the right. Every time a photon changes direction, it requires something ...
Mark H's user avatar
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36 votes
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What did I just photograph? (rainbow way out of place)

As pointed out in the comment by @Rob, this is called a Circumzenithal Arc or colloquially, an "upside-down rainbow". Different to usual rainbows (which appear on the opposite side of the ...
joseph h's user avatar
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29 votes
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Do black holes have Arago spots?

A proper analysis of this should treat light as an EM wave propagating in the curved spacetime background of the black hole. The papers (1) "Wave optics and image formation in gravitational lensing," ...
Chiral Anomaly's user avatar
26 votes

Why exactly does diffraction occur?

The first thing to realize is that waves only appear to travel. But when you look at a fish in the water, it becomes clear that the water only sloshes back and forth. Waves occur because the water ...
MSalters's user avatar
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20 votes

Why do I see three 'beams' when I look to a distant light source at dark?

From physical point of view: One reason is diffraction spike by the mounts (support vanes) of optical reflector of reflecting telescope. Four-fold mounts give four-fold diffraction pattern. However,...
Ng Chung Tak's user avatar
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19 votes

Reverse of diffraction

Reversing diffraction is precisely what is done in Fourier Optics! There, instead of placing a screen after the slit to see the diffraction pattern, a lens is put there instead. If you arrange the ...
KF Gauss's user avatar
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17 votes

Why exactly does diffraction occur?

"Why are waves straight?" is the first question. Let's start with a model of waves where particles don't have much kinetic energy. They just have potential energy. Each location, if it has less ...
Yakk's user avatar
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15 votes
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Differences in Double slit experiment and 2 slit Diffraction

A realistic double-slit has two effects at play: there is single-slit diffraction from each sit (giving rise to the larger envelope) and there is interference between the two beams of light from each ...
Maximal Ideal's user avatar
14 votes
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Why does fringe width in double slit experiment remain constant if slits get narrower?

If I'm misunderstanding anything about your question, feel free to tell me. As I derived in this post, the (one-dimensional) formula for the double-slit intensity (in the Fraunhofer regime) is $$ I(\...
Maximal Ideal's user avatar
13 votes

What is the length of a photon?

A photon is not a point particle in the classical sense of the word (and neither is an electron, or any other fundamental 'particle'). Rather, it is a convenient word for describing some aspects of ...
Andrew Steane's user avatar
12 votes

Why is light with a small wavelength required to have a rectilinear path?

I want to know why the small wavelength of light (smaller than the openings) is required to have a rectilinear path? The simple answer is that with common light sources the effects of diffraction ...
Farcher's user avatar
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11 votes
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What's the origin of the flattened X that appears when of a small (but powerfull enough) light is reflected in our non-powered TV screen?

It looks like a diffraction pattern from the pixels of the screen. In a different SE question, the pattern was four horizontal and vertical "rays" consisting of finely spaced peaks, rather than the ...
Han-Kwang Nienhuys's user avatar
11 votes

Reverse of diffraction

Optics is time-reversal invariant, so the short answer is "yes". As a practical matter it is essentially impossible to set up the exact reversed situation, but we can set up good approximations in ...
dmckee --- ex-moderator kitten's user avatar
11 votes
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What is causing the diffraction pattern on my ceiling?

The rays are white, so it does not look like diffraction. Maybe it is an effect similar to that in camera obscura, so it is a (transformed) image of some outside object. EDIT (Jan 1, 2022): So the OP ...
akhmeteli's user avatar
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11 votes
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Is Kirchhoff's scalar theory of diffraction mathematically inconsistent?

Kirchhoff's scalar diffraction theory is mathematically inconsistent. The reason is as follows. Kirchhoff proved that any function $U$ satisfying the homogeneous wave equation $\nabla^2U+k^2U=0$ also ...
hyportnex's user avatar
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10 votes
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How to derive the intensity formula of a diffraction grating?

(image from Antonine education) The light amplitude $E(\theta)$ into direction $\theta$ can be calculated straight-forward by summing the contributions of all the slits ($n$ from $-N$ to $+N$) and ...
Thomas Fritsch's user avatar
10 votes

What is causing the diffraction pattern on my ceiling?

Since the beams are going up to the ceiling, they must be reflecting from something low outside. Take a look.
R.W. Bird's user avatar
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9 votes
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Which formula for the de Broglie wavelength of an electron is correct?

Often, when dealing with high-energy (relativistic) particles the rest mass of the particle can be neglected when performing calculations. Use your expression for $p$ from relativistic considerations, ...
DarthPlagueis's user avatar
9 votes
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Pink on LCD screens, how is it possible?

When white light is diffracted by a grating, every color gets its own spatial frequency. As soon as you are more than a small distance from the optical axis (where you have a white maximum), you will ...
Floris's user avatar
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9 votes
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Why do diffractions spikes from small aperture not have fringes?

I think your aperture is too big. To get the idea of the scale, for the red light, you say your aperture size is $f/22$, i.e. $\frac{49\,\mathrm{mm}}{22} = 2.227\,\mathrm{mm}$, then the angular ...
Ilya  Lapan's user avatar
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9 votes

Why exactly does diffraction occur?

Your aperture only allows a very short segment of the incoming plane wave to pass through. As the aperture becomes smaller, the segment looks more and more like a point source. A point source emits ...
user45664's user avatar
  • 3,036
8 votes

Which formula for the de Broglie wavelength of an electron is correct?

As the energy of the electrons in that case is much greater than their mass, you can consider the approximation $E \sim pc$. So the formulas are equivalent.
Marta's user avatar
  • 139
8 votes
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Is the single slit experiment a practical example of Heisenberg's uncertainty principle?

Yes, light diffraction may be viewed both as a classical phenomenon and as a quantum mechanical consequence of Heisenberg's uncertainty principle. However, since both explanations work equally well, ...
knzhou's user avatar
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8 votes
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Medical X-Ray - why no diffraction?

Since the lattice spacing is about eight angstroms, the issue isn't any sort of unusual lattice spacing. Instead, the issue is that bones are thick. [Another answer points out that the crystals are ...
rob's user avatar
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8 votes
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Horizontals in a grid do not cast shadow while verticals do. Why is this?

If the light source is like a long fluorescent light, shadows in one direction (along the length of the fluorescent tube) will be blurred and shadows in the other direction will be fairly sharp. Can ...
S. McGrew's user avatar
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8 votes

Why does fringe width in double slit experiment remain constant if slits get narrower?

The path difference in the double slit experiment comes from the distance between the two slits that now act as two coherent sources. Of course strictly speaking, the path difference also depends on ...
Superfast Jellyfish's user avatar
8 votes

How does Davisson and Germer experiment indicate the wave nature of electrons?

The Davisson and Germer experiment used a crystal to scatter electrons. A crystal, as used in this case, is a solid with a periodic structure. So, when the electrons impinge on the crystal and you ...
JQK's user avatar
  • 1,790
7 votes

Does a camera have single slit interference?

Here are a series of photographs which were produce by the same camera and lens arrangement but with a different size of the entrance pupil (aperture of the use-d part of the lens slowly). The ...
Farcher's user avatar
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