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420 votes
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What is Chirped Pulse Amplification, and why is it important enough to warrant a Nobel Prize?

The problem Lasers do all sorts of cool things in research and in applications, and there are many good reasons for it, including their coherence, frequency stability, and controllability, but for ...
Emilio Pisanty's user avatar
82 votes
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A Rainbow Paradox

Yes. It is precisely the larger angle that makes this happen. Because the red is angled "down" more the red droplets are the ones located higher in your field of view.
Dale's user avatar
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75 votes
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Why are red and blue light refracted differently if they travel at the same speed in the same medium?

In general, red and blue light do not travel at the same speed in a non-vacuum medium, so they have different refractive indices and are refracted by different amounts. This phenomena is known as ...
gandalf61's user avatar
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48 votes
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Why does the light passing through a prism get bent in the same direction twice?

The normals in consideration for the incident and emergent rays are different. For simplicity, take a monochromatic beam of light incident on a prism, as shown in this figure: When light is incident ...
Cross's user avatar
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41 votes
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Why doesn’t a normal window produce an apparent rainbow?

It does create the rainbow, but it is almost impossible to notice. When light direction is changed on the glass-air interface - there is always a dispersion : light with different wavelength will ...
BarsMonster's user avatar
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38 votes

How does light, which is an electromagnetic wave, carry information?

You talk about light as if it were a person carrying a clip board writing down things on its way to you. It is a physical phenomenon that gets affected as it propagates. Depending on the various ...
Sidarth's user avatar
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31 votes
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Why is the speed of oceanic waves not a constant like sound?

I think that this question is why sound waves are non-dispersive whereas gravity waves on the surface of water are and also depend on the depth of the water. In fact if the depth of the water is ...
Farcher's user avatar
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29 votes
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Why does ice make such peculiar sounds?

The pitch of the sounds start high and end up low. Is this some kind of doppler effect? No. This is chirp induced by dispersion, which is the acoustic version of the same phenomenon for light. This ...
Emilio Pisanty's user avatar
25 votes
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Why do convex lenses not disperse light like prisms, given that entry and exit points aren't parallel?

They do. It's called chromatic aberration - each different frequency has a slightly different focus point, blurring the image by different amounts for the different colors. Modern lenses of high ...
Sean E. Lake's user avatar
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22 votes

Why is the speed of oceanic waves not a constant like sound?

The dynamical origins of the two are extremely different. Surface waves in water are gravity waves, which means that the restoring force trying to bring peaks and troughs back to the mean height is ...
Emilio Pisanty's user avatar
22 votes

Why does high frequency have high energy?

This was a big surprise when it was discovered. The answer is that when we construct models they agree with experiment if we assume the Planck relation. The phenomena (black body radiation, ...
John Doty's user avatar
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21 votes
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Is it possible to witness a rainbow while facing the sun?

TL;DR: A rainbow is only visible due to the focusing effects of raindrops, and rays do not get focused unless they reflect internally at least once. The key point that gets lost in most introductory-...
Michael Seifert's user avatar
20 votes

What is Chirped Pulse Amplification, and why is it important enough to warrant a Nobel Prize?

As an addendum to @EmilioPisanty’s excellent review, I’d just like to mention one more application of CPA lasers, which may be overlooked from a theorist’s perspective: Ultrafast Spectroscopy ...
Gilbert's user avatar
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19 votes

Do colors differ in terms of speed?

The speed of light is always c in vacuum, when measured locally, independent of the wavelength. Though, in a medium, the index of refraction is n=c/v. Speed of different em radiation in a medium ...
Árpád Szendrei's user avatar
19 votes

How does light, which is an electromagnetic wave, carry information?

When a photon hit the retina, it only has two pieces of information: Its wave length and its position/direction. That is all. But it is not alone. We are bombarded with billions of photons every ...
Stig Hemmer's user avatar
17 votes
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What makes a wave dispersive?

Dispersion of sound in air, with constant temperature and pressure, is very slight, increasing for very short wavelengths, and for very loud noises. Why? Because the rapid sequence of weak compression/...
Peter Diehr's user avatar
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15 votes

Why does high frequency have high energy?

The individual photons have more energy, but this does not mean the total energy in the wave has a higher energy. There are many examples of a high frequency, but a lower amplitude.
jensen paull's user avatar
  • 6,646
14 votes

Why are red and blue light refracted differently if they travel at the same speed in the same medium?

The refractive index is a function of wavelength. It has different values for different wavelengths. The way to show this in the mathematical notation is to write $$ n(\lambda) $$ just as you would ...
Andrew Steane's user avatar
13 votes
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About de Broglie relations, what exactly is $E$? Its energy of what?

For your initial check, you used the formula $c = \nu \lambda$, giving the incorrect formula $E = pc$. This is wrong, matter waves don't travel at the speed of light; instead, $\nu \lambda$ is equal ...
knzhou's user avatar
  • 103k
13 votes
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If different light frequencies travel at different speeds in air, why doesn't the sun have a rainbow halo?

What's wrong with this diagram (aside from the exaggerated angles)? Answer: Nothing, aside from the exaggerated angles. Let's try estimating the magnitude of the effect for dry air, which seems to be ...
Chiral Anomaly's user avatar
13 votes

Why does the light passing through a prism get bent in the same direction twice?

This is how refraction of light in a medium works. The phase velocity $v$ of light changes transitioning from one medium to a different density medium according to its refraction index $n$ and the ...
Markoul11's user avatar
  • 4,147
13 votes

A Rainbow Paradox

The rainbow is a full circle, of which an observer on the ground only sees part. The digram shows that the red is on the outside of the circle, which, when we see only the upper half of the circle, ...
mike stone's user avatar
  • 53.8k
11 votes

Sound frequencies travel at the same speed?

The phenomenon where waves with different frequencies have slightly different speeds is known as "dispersion," because an impulse which begins with lots of different frequencies traveling together ...
rob's user avatar
  • 90.8k
11 votes

Why surface water waves are dispersive?

Fluids have two basic modes of excitation, propagating (sound) modes with a dispersion relation $\omega^2\simeq c_s^2k^2$, and diffusive modes $w\simeq iDk^2$, where the speed of sound $c_s$ and the ...
Thomas's user avatar
  • 18.7k
10 votes

What makes a wave dispersive?

Take a look at Griffiths Introduction to Electrodynamics, particularly the section called "The Frequency Dependence of Permittivity". Dispersion can arise from the constraints, or bound nature, of ...
curiousStudent's user avatar
10 votes

A Rainbow Paradox

Think about seeing an object reflected off of a mirror. You can draw a straight line from your eye to the mirror, and then a straight line from the mirror to the object. If you remove the mirror, ...
JQK's user avatar
  • 1,785
9 votes

Why do convex lenses not disperse light like prisms, given that entry and exit points aren't parallel?

Sean E. Lake's answer is right: convex lenses disperse light like prisms and that effect is known as chromatic aberration - which is easily noticeably by zooming in in the corners of photographs taken ...
Pere's user avatar
  • 2,159
9 votes
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What does phase velocity physically represent, and why can it be superluminal?

The short answer is: group velocity and phase velocity are just terms that help describe how frequency depends on wavelength in a material, and in specific instances can help give us information about ...
el duderino's user avatar
9 votes

Why surface water waves are dispersive?

I'll show how the dispersion relation for surface water waves can be derived. The words about "resonance" in the body of the question are true in the broad sense that any wave can be ...
Chiral Anomaly's user avatar

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