Questions related to the perception and measurement of light (primarily in the visible range), its mathematical description, the reproduction of colors by different means, color combinations, etc. Please use the tag [electromagnetic-radiation] if you want to refer to the general form of light.
20
votes
4answers
1k views
Why does the sky change color?
Why the sky is blue during the day, red during sunrise/set and black during the night?
13
votes
2answers
695 views
Does a photon exert a gravitational pull?
I know a photon has zero rest mass, but it does have plenty of energy. Since energy and mass are equivalent does this mean that a photon (or more practically, a light beam) exerts a gravitational pull ...
13
votes
4answers
445 views
What is the mechanism behind the slowdown of light/photons in a transparent medium?
So light travels slower in glass (for example) than in a vacuum. What causes light to slow down? Or: How does it slow down? If light passes through the medium, is it not essentially traveling in the ...
11
votes
2answers
842 views
Is the “How to break the speed of light” minute physics video wrong?
I am referring to this video, on YouTube, by minutephysics, which has quite a lot of views.
In the video it states that if you flick your wrist while pointing a laser that reaches the moon, that the ...
10
votes
6answers
5k views
What determines color — wavelength or frequency?
What determines the color of light -- is it the wavelength of the light or the frequency?
(i.e. If you put light through a medium other than air, in order to keep its color the same, which one would ...
23
votes
3answers
2k views
Why is the sky not purple?
I realise the question of why this sky is blue is considered reasonably often here, one way or another. You can take that knowledge as given. What I'm wondering is, given that the spectrum of ...
9
votes
4answers
2k views
What exactly is a quantum of light?
I am currently trying to learn some basic quantum mechanics and I am a bit confused. Wikipedia defines a photon as a quantum of light, which it further explains as some kind of a wave-packet.
What ...
1
vote
3answers
874 views
Hours of light per day based on latitude/longitude formula
I'm looking for a formula that will return the number of hours per day given a specific location. I was thinking that can be calculated as a difference of sunrise and sunset, but I see that there are ...
9
votes
3answers
357 views
Why does light of high frequency appear violet?
When people are asked to match monchromatic violet light with an additive mix of basic colours, they (paradoxically) mix in red. In fact, the CIE 1931 color space chromaticity diagram shows this ...
13
votes
3answers
16k views
Why does the moon sometimes appear giant and a orange red color near the horizon?
I've read various ideas about why the moon looks larger on the horizon. The most reasonable one in my opinion is that it is due to how our brain calculates (perceives) distance, with objects high ...
9
votes
6answers
3k views
Why can't light escape from a black hole?
Photons do not have mass (that's why they can move at speed of "light").
So, my question is how the gravity of black hole can stop light from escaping?
2
votes
2answers
333 views
Where is a good place to learn (classical) optics?
I need to learn basic optics for a high school academic competition. Does anyone know any good places, preferably free and online, to learn the basics of optics, like lenses, angles of incidence, ...
41
votes
10answers
4k views
Is it possible that there is a color our human eye can't see?
Is it possible that there's a color that our eye couldn't see? Like all of us are color blind to it.
If there is, is it possible to detect/identify it?
11
votes
4answers
432 views
What longest time ever was achieved at holding light in a closed volume?
For what longest possible time it was possible to hold light in a closed volume with mirrored walls?
I would be most interested for results with empty volume but results with solid-state volume may ...
21
votes
7answers
2k views
What is the difference between a white object and a mirror?
I was taught that something which reflects all the colors of light is white. The function of a mirror is the same, it also reflects all light. What's the difference?
Update:
But what if the white ...
14
votes
4answers
4k views
Why glass is transparent?
Once I asked this question from my teacher and he replied "because it passes light", "and why it passes light" I asked and he said "because it is transparent".
Same question again, Why glass is ...
4
votes
2answers
162 views
Is it possible to charge photons
For example there are anti-particles to every particle we know,
Similary in some sense, is there a possibility that we can charge photons..if not what are the reasons and has there been any attempt ...
1
vote
1answer
390 views
If photons are deflected by a strong gravitational field, then how come photons do not have mass? [duplicate]
Possible Duplicate:
Explain how (or if) a box full of photons would weigh more due to massless photons
It has been proved and showed through experiments that light can be bent by the Sun or ...
11
votes
5answers
977 views
Can someone explain the science behind MIT's 230% efficient LEDs?
I was reading Gizmodo the other day and I didn't quite understand the Physics behind this. Could anybody shed some light on how this effect actually works?
1
vote
4answers
642 views
Explanation about black color, and hence color
I'm bit confused about 'black' as a color. As per my knowledge, it is not given in visible color spectra like other colors for example red, violet etc. Also I'm confused with definition of color--does ...
134
votes
14answers
42k views
A mirror flips left and right, but not up and down
Why is it that when you look in the mirror left and right directions appear flipped, but not the up and down?
10
votes
6answers
1k views
Why is air invisible?
I think that something is invisible if it's isolated particles are smaller than the wavelength of visible light. Is this correct?
Why is air invisible? What about other gases and fumes which are ...
8
votes
3answers
467 views
Why isn't light scattered through transparency?
I'm asking a question that has bothered me for years and years. First of all, let me give some context. I'm a layman in physics (college educated, math major). I've read Feynman's QED cover to cover, ...
5
votes
1answer
871 views
Transparency of materials
Is transparency of material has something to do with inter- or intra-molecular bonding? E.g. both graphite and diamond are carbon, but graphite is opaque and diamond transparent.
4
votes
3answers
415 views
Does light change color on its way through a window?
Looking at the refractive index of glass, it's around $1.6$.
Then the speed of light $x$ through light should be given by
$$ 1.6 = \frac{3.0\times10^8}{x}, $$
so $x$ is about ...
-1
votes
1answer
184 views
How can you test what color different people perceive? [closed]
If I would show someone a yellow object and ask them, "is this object yellow?"
That person would say "yes".
But I could never know if my perception of the color yellow is the same as that other ...
14
votes
3answers
387 views
What are these rays that appear in photograph of sun?
In many images of light emitting objects we see such rays. Why do they appear ?
What is the math behind their number and direction?
7
votes
4answers
1k views
Why is the speed-of-light “the upper limit” rather than the speed of “particle type X”?
Basically, I can't stop wondering why light (the photon) is so special, compared to all the other particles known (and unknown) to modern day physics.
Could it be that there exists an upper limit on ...
3
votes
2answers
324 views
Why do electrons around nucleus radiate light according to classical physics
As I navigate through physics stackexchange, I noticed Electron model under Maxwell's theory.
Electrons radiate light when revolving around nucleus? Why is it so obvious?
Note that I do not know ...
1
vote
1answer
160 views
Photometer: measured Irradiance L converted to photon rate
I am conducting an experiment in which the power meter reading of $410\,nm$ narrow bandpass stimulus is noted to be 30 $\frac{\mu W}{cm^2}$ at a distance of 1 inch away from the light source.
I wish ...
10
votes
1answer
109 views
If light is linearly polarized, does it have some spatial extent?
If light (a photon) is linearly polarized, say vertically, does it have some vertical spatial extent (perhaps in amplitude)?
9
votes
5answers
389 views
What happens to light and mass in the center of a black hole?
I know that black holes are "black" because nothing can escape it due to the massive gravity, but I am wondering if there are any theories as to what happens to the light or mass that enters a black ...
7
votes
2answers
533 views
What determines the apparent radius of the rainbow?
Let's say I know how to compute the apparent radius of a rainbow from the viewpoint of the observer: take a photo of the scene, measure the distance to a known reference object, and its dimensions. ...
4
votes
2answers
177 views
Can a dot of light travel faster than the speed of light? [duplicate]
Say I have a laser. If I spin the laser so that the beam sweeps in an arc along a very distant object, could that dot travel faster than the speed of light?
In Diagram form:
4
votes
2answers
192 views
3
votes
3answers
370 views
formula for transparency of very thin film of metal
Is there formula that gives transparency of very thin film of given metal (tens of nanometers) to the visible light/light of given wavelength ? Which properties of metals are needed for the formula ?
...
3
votes
1answer
903 views
Purple doesn't occur in rainbow - or does it?
Usually, when asked whether the purple color exists rainbows, an answer similar to this is given:
The purple color is perceived by human eyes via the activation of both red-sensitive and ...
3
votes
4answers
149 views
Rainbow around Sun
From the perspective of a person, a rainbow is formed when the Sun is behind the person, and there is a critical angle made by the rainbow.
However, on several occasions, usually at noon when the Sun ...
3
votes
2answers
549 views
Propagation of light in transparent media: absorption and reemission or scattering?
In the two Phys.SE questions What is the mechanism behind the slowdown of light/photons in a transparent medium? and Why glass is transparent? transparent media were discussed. But I'd like to clarify ...
2
votes
3answers
289 views
What was the motivation behind suggesting the trichromatic theory of vision?
Background
In this thread, I asked whether it is true that the colors red, green and blue, through additive mixture, can make up any color. Turns out they can't.
However, when reading about the ...
2
votes
4answers
484 views
Why are color values stored as Red, Green, Blue?
I learned in elementary school that you could get green by mixing blue with yellow.
...
34
votes
1answer
3k views
Why does a window become a mirror at night?
In day, when you look in the room through the window out, you can clearly see what happens outside. At night when it's dark outside but there's light inside you can look in the window but it becomes a ...
17
votes
3answers
2k views
8
votes
3answers
913 views
Is it possible to reproduce Double-slit experiment by myself at home?
I want to reproduce this experiment by myself. What I need for this. What parameters of slits and laser/another light source it needs? Is it possible to make DIY-detector?
8
votes
3answers
284 views
How can a body be transparent?
How can a body be transparent? I need the theoretical explanation for the same. Has anybody succeeded in doing that practically?
7
votes
2answers
643 views
EM wave function & photon wavefunction
According to this review
Photon wave function. Iwo Bialynicki-Birula. Progress in Optics 36 V (1996), pp. 245-294. arXiv:quant-ph/0508202,
a classical EM plane wavefunction is a wavefunction (in ...
12
votes
1answer
387 views
Is colour, as represented using primary colours, accurate only to humans?
Slightly biological, hopefully physical enough to be answered.
Suppose a magenta hue is represented by a mix of red and blue pigment. This is all very well for a creature with red and blue ...
2
votes
4answers
276 views
How are we able to view an object in a room with bulb..?
This is a very basic question on optics. How are we able to view an object kept in a room with a bulb?
From what I understand, light rays from bulb will hit the object and some colour will be ...
14
votes
6answers
1k views
Why cannot we store light in form of light?
We can store cold (ice),heat (i.e. hot water bag) and electrical charge (batteries). We can even "store" a magnetic field in a magnete. We can convert light into energy and then, if we want, back to ...
9
votes
8answers
2k views
Can there be black light ? I mean is it possible to devise a machine that outputs darkness ?
I understand there are various colours that light can have. But i was wondering why there is no 'black' light. What is the logical explanation for this ? I mean I am expecting an answer that goes ...
