82
votes
How does light from distant galaxies reach us?
Intergalactic space is estimated to have a mean density of about $1$ molecule per cubic meter.
Air has a density of about $ 3 \times 10^{25}$ molecules per cubic meter.
1 Light Year is about $9 \times ...
- 8,982
59
votes
Why doesn't a light ray bend again when emerging from a lens?
They technically should "bend" because of refraction, and a more accurate drawing would be this:
But drawings like the one that you show usually just tell you the net effect of the lens, i....
- 22.9k
53
votes
Does real life have "update lag" for mirrors?
As the speed of light is finite, sure enough there is some lag, but let's evaluate how big that lag is. Considering that the mirror is 100 meters away, than the lag will be $$2\times 100\: \mathrm{m}/(...
- 683
47
votes
If light propagates like waves, why can't I see around corners?
The bending of waves around corners is known as “diffraction,” and its natural length scale is the wavelength of the diffracted wave. So if you want to block the sound from a speaker playing a middle ...
rob♦
- 71.8k
42
votes
Accepted
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 ...
- 21.6k
40
votes
If you were invisible, would you also be cold?
Not necessarily. Glass is transparent and it can be heated with convection, conduction and some radiation. It will maintain any temperature around it.
- 3,772
39
votes
Why doesn't a light ray bend again when emerging from a lens?
You are right. The drawing shown in your question is quite poor.
Here is a much better drawing, which correctly shows
the refraction of rays on both convex surfaces.
The rays bend towards the normal ...
- 26.9k
33
votes
Does real life have "update lag" for mirrors?
If there were such a lag, you could use a moving mirror to measure the speed of light.
This was first done in the 1920s by Michelson. (See e.g. this link which hosts papers from 1924 and 1927.) ...
rob♦
- 71.8k
33
votes
Why is it so dark during a solar eclipse?
The most significant difference is that in a total eclipse the moon obstructs the sun's light outside of earth's atmosphere whereas at sunset, the light is obstructed by the horizon within the ...
- 476
30
votes
Why do microwaves penetrate flesh when visible light does not at the same intensity?
Why exactly if microwaves have less energy per photon, with visible
light at the same high intensity, would they be more capable of
penetration?
The short answer is it is the frequency not the ...
- 56.2k
30
votes
If you were invisible, would you also be cold?
Although the premise of this question is, well, questionable, I’ll answer because invisibility cloaks do exist. Of course, as of now, they work imperfectly, in impractical ways, and for only a narrow ...
- 10.6k
29
votes
Can sound travel through light?
This is an interesting question, and the answer turns out to be yes. Ordinarily we say that photons do not interact with one another (that is, they pass right through one another without colliding), ...
- 51.9k
27
votes
How does light from distant galaxies reach us?
The passage of a photon is only "dampened" if it interacts with something
You assume that objects tire when travelling. This is wrong even for heavy objects as we have known since Newton (or ...
- 2,373
26
votes
Accepted
Radiation from Sticky Tape
Triboluminescence. The exact mechanism in sticky stuff is still uncertain, but it's not just light. It makes X-rays. Fractoluminescence is the version that applies only to fracturing crystals (e.g. ...
- 23.2k
25
votes
Accepted
Aren't all objects luminous in a sense?
Black bodies are in equilibrium with their surroundings - they absorb radiation from their surroundings and then re-emit it. Luminous bodies have internal energy sources, i.e., there is energy ...
- 39.3k
24
votes
Can you actually stand inside a rainbow? I see people claiming you can
It's impossible to "stand inside a rainbow" because there is no such place.
A rainbow (or, more accurately, a series of rainbows) is the result of multiple reflections from light rays coming ...
- 2,539
23
votes
Alternate sky colors?
Rayleigh scattering results in scattering inversely proportional to the fourth power of the wavelength. So, no matter what the composition of the scatterer is, the blue will be scattered more than the ...
- 6,568
22
votes
Light of laser doesn't spread?
If you take a light source such as a hot filament (not a laser) and put the filament one focal length away from a parabolic reflector, then you will get an accurately collimated beam if the filament ...
- 47.1k
20
votes
Why does light have multiple frequencies?
Light comes with so many wavelengths because it is made of so many photons. A typical lightbulb puts out something on the order of $1\ \mathrm{W}$ of power in the visible spectrum, while individual ...
- 4,769
19
votes
Accepted
If frequency of photons is a continuous spectrum, wouldn't the chance of a photon having the exact right frequency to excite an electron be zero?
There is a natural line width which is determined by the lifetime $\tau$ of the excited state. This means that you only need to be within some frequency interval of $\approx 1/\tau$ about $\omega_{\rm ...
- 41.8k
17
votes
Accepted
Failure of Newton's corpuscular theory and success of photon theory of light
The corpuscular theory was opposed to the wave theory, and was rejected when diffraction experiments confirmed the wave theory. The blackbody spectrum also confirmed the wave theory at low frequencies,...
- 4,176
16
votes
How does light from distant galaxies reach us?
It is dampened and lost on its way here by clouds of dust, and by interacting with certain gaseous chemical compounds in deep space (see lyman-alpha forest). The galaxies we can see from far, far away ...
- 73.8k
16
votes
Accepted
Why is it so dark during a solar eclipse?
Right after you see the sun disappearing below the horizon, the air above you is still illuminated and scatters light back to the ground. That is why it is not completely dark during dusk. During a ...
- 471
15
votes
Can you actually stand inside a rainbow? I see people claiming you can
A rainbow is formed by the reflection and refraction of light from the Sun by water droplets.
This is illustrated in the diagram below.
The process by which this happens is fairly involved and there ...
- 78.1k
14
votes
How does light from distant galaxies reach us?
I suppose that the background of your question is that in our daily experience waves do die down when they travel far. We can overhear a conversation only from so far, waves in a pond have limited ...
- 3,690
14
votes
Can you actually stand inside a rainbow? I see people claiming you can
In a sense, you can. Though that might not mean quite what you think it does. It is a bit like standing in the beam of a flashlight. The video you linked shows this happening - Drove Through the End ...
- 27.2k
13
votes
Why doesn't a light ray bend again when emerging from a lens?
That diagram shows what's called the "thin lens" approximation.
In real life, lenses have thickness, and light rays get refracted both when entering and exiting the material. But in practice,...
- 543
13
votes
If light propagates like waves, why can't I see around corners?
Light traveling in straight lines is an approximation that works very well most of the time. The most common case where it fails is when light travels through a pinhole or slit. Then it bends a little....
- 27.2k
12
votes
If you were invisible, would you also be cold?
Two key points to remember:
Radiation is not the only form of heat transfer. There's also conduction and convection.
Being invisible doesn't only mean that you shouldn't absorb any light. It also ...
12
votes
If light propagates like waves, why can't I see around corners?
TL;DR: Visible light - no, but radio waves - yes.
Electromagnetic waves do bend around the corners, if their wavelength is comparable to the size of the object (such as building, for example). Visible ...
- 39.3k
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