421
votes
Accepted
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 ...
- 135k
83
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.
- 105k
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 ...
- 57.6k
48
votes
Accepted
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 ...
- 3,340
41
votes
Accepted
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 ...
- 2,484
38
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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 ...
- 997
31
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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 ...
- 98.8k
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 ...
- 135k
25
votes
Accepted
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 ...
- 22.7k
22
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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 ...
- 135k
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, ...
- 21.8k
21
votes
Accepted
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-...
- 50.8k
20
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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
...
- 12k
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
...
- 29.1k
19
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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 ...
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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.
- 6,755
14
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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 ...
- 60.2k
13
votes
Accepted
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 ...
- 54.7k
13
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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 ...
- 4,328
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, ...
- 55.3k
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♦
- 92k
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 ...
- 18.8k
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, ...
- 1,818
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 ...
- 2,149
9
votes
Accepted
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 ...
- 935
9
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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 ...
- 54.7k
9
votes
Accepted
Why does Cauchy's equation for refractive index contain only even power terms?
Historically, Cauchy derived his equation assuming that light propagated in an elastic aether. The aether theory of light may be wrong, but Cauchy's derivation was pretty much equivalent to simple ...
- 12.2k
9
votes
Why can't thin clouds form rainbows?
Clouds are complicated. There are lots of different types of clouds, which may be composed of spherical or non-spherical water droplets, with uniform or non-uniform sizes; planar or columnar ice ...
rob♦
- 92k
8
votes
How many optical and acoustical branches are in a primitive cell?
TL;DR: We have $3p$ branches in total, corresponding to $3p$ independent modes of vibration. We have $3$ acoustic branches, because we are in $3d$ space, AND, Because we are dealing with an Elastic ...
- 750
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