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.

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2
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1answer
247 views

What's the difference between the equivalence principle and curvature of spacetime?

Calculating using the equivalence principle only accounts for half the deflection of light, whereas the other half is from curvature of space-time. But isn't the equivalence principle the same thing ...
-1
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1answer
99 views

Determine when a light is going into the object [closed]

Please look at this image: (http://mypages.iit.edu/~smart/acadyear/refract.gif) How can I determine when light is going into the object? Actually, if there is, what is the simplest way to ...
3
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4answers
34k views

Why do diamonds shine?

I have always wondered why diamonds shine. Can anyone tell me why?
0
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1answer
1k views

Huygens Principle and principal of rectilinear propagation of light

Suppose I have an wave source and light waves are radiating from it. If I have a point source, then after a time t, with a radius of ct I will have a circular wave front.By Huygens principle each ...
0
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2answers
5k views

If nothing in the universe can travel faster than light, how come light can't escape a black hole? [duplicate]

Possible Duplicate: How does gravity escape a black hole? If nothing in the universe can travel faster than light, how come light can't escape a black hole? I mean, Einstein's relativity ...
1
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1answer
115 views

Where do the high order rainbows appear?

This site shows locations of the high order rainbows up to sixth order. My thought is that, There should be even higher order bows like 7th, 8th, etc. (Though they would be practically impossible to ...
1
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1answer
303 views

Speed of light and virtual particles

After becoming extremely bored while studying for an Afrikaans exam, I started thinking about virtual particles. So, can light (photons) interact with virtual particles (even though they only exist ...
3
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0answers
148 views

Bandgap Spacing in Photonic Crystals

I am doing some self-study on photonics and have encountered the following question: We know that amorphous electronic crystals such as amorphous silicon have a bandgap. Can amorphous photonic ...
0
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1answer
84 views

Is synchrotron radiation considered incandescent?

Synchrotron radiation is produced via the acceleration of charged particles, much like incandescence. However, all information I've seen state that incandescent light is produced exclusively from the ...
3
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1answer
173 views

What the name of the evacuated glass gadget with black and white vanes that turn when a light is applied?

I remember a glass device my physics teacher had at high school which Contained some vanes mounted somehow on a vertical axis, which were all black on one side and white on the other Was in a ...
27
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13answers
39k views

Why is light called an 'electromagnetic wave' if it's neither electric nor magnetic?

How can light be called electromagnetic if it doesn't appear to be electric nor magnetic? If I go out to the sunlight, magnets aren't affected (or don't seem to be). And there is no transfer of ...
10
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2answers
4k views

Why do green lasers appear brighter and stronger than red and blue lasers?

This is mostly for my own personal illumination, and isn't directly related to any school or work projects. I just picked up a trio of laser pointers (red, green, and blue), and I notice that when I ...
0
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2answers
115 views

Dark light filters letting through less light

I'm looking for an explanation on why darker light filters let through less light than lighter filters (secondary/high school level). Although this seems to the layman like the obvious outcome, our ...
1
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1answer
141 views

Ideal four-level Gain Medium (or just any old gain medium)

What exactly is happening in a laser's gain cell when the irradiance is greater than the saturation irradiance? Also can someone offer a clear conceptual treatment of the gain-coefficient and its ...
5
votes
1answer
715 views

why sometimes touching old flickering tube lights starts them properly

In my old house there are two old tube lights. Some times they don't start properly, (specially at evening time, may be it is because of low voltage), they starts flickering i.e. on and off ...
-1
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2answers
347 views

A light and magnetic mirror paradox?

If light is an electromagnetic wave and lightspeed is constant (we ignore spacetime or gravity for this question) why can't we slow down light with a few dozen wellplaced magnets and electricly ...
1
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1answer
67 views

Rays in Symmetric Resonator

I'm having some trouble figuring out how to get started on this question: If I have a symmetric resonator with two concave mirrors of radii $R$ separated by a certain distance, after how many round ...
0
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3answers
1k views

Deriving the Poynting Theorem

I am trying to derive the Poynting theorem. So far, I've only been able to narrow down which equations I think I'll need to do so. These are the equations: Maxwell's Equations: $$ \nabla\times{\bf E} ...
-1
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4answers
115 views

What kind of “camera” and “light” source should I use to detect the intermediate path of a light beam? [closed]

I would like to have an image (in any kind of space), where I see the path of a "light" source. In my understanding the most common, directed source would be a laser pointer. a) Is this correct? If ...
0
votes
2answers
303 views

Why is the sky blue? [duplicate]

Possible Duplicate: Why does the sky change color? Basically what the title says. What mechanisms are significant and how do they contribute to make the sky blue. Also when the sky is not ...
0
votes
2answers
948 views

Wavelength and resolution

I'm reading some texts that seam to assume knowledge of light that I'm not too familiar with. How does wavelength of light relate to the minimum distance span that can be observed (i.e. you cannot ...
17
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3answers
1k 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 ...
0
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0answers
42 views

Reflection of light [duplicate]

Possible Duplicate: What is the difference between a white object and a mirror? When I look at a red object under white light, I see it as red because it absorbs the other colours and ...
2
votes
2answers
1k views

Is there still light in practical darkness? Do photons penetrate everywhere?

I always thought that, even in darkness, there would still be some light, making complete darkness, i.e. complete absence of photons as far as I know, just a theoretical thing. When I tried looking up ...
1
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1answer
106 views

What would the spectral distribution of a greenbody look like?

Suppose you had a body that behaved like a blackbody for all wavelengths except some range that we would call "green". For those wavelengths it reflects all radiation incident upon it. Am I just ...
4
votes
3answers
271 views

Is the number of rays projected by a source of light finite?

Take a source of light which gives out infinite number of rays, each ray with finite number of photons and each photon with a finite amount of energy, Then, Aren't the number of photons become ...
2
votes
1answer
280 views

Solar energy and optical fiber

I'm not really good at science or physics, just wanted to ask about the idea (doesn't have an opportunity to test this). Will this scheme work and what can be its efficiency?(The goal is to make ...
5
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4answers
3k views

What color would a proton be if it were visible to the human eye?

If a photon hits a proton, would it have a color? What color would it be?
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3answers
1k views

Liquid crystal shutter with >90% transmission?

Today's liquid crystal shutter glasses, when in the "transparent" state, exhibit only 40% light transmission. They work using two polarizer layers, one which is liquid crystal and goes {vertical ...
2
votes
2answers
379 views

What's the Significance of Color Changes Between EM-Spectrums?

As everybody knows that Black objects are black because almost all the light that falls on them is absorbed into the material. Little or no light is reflected back toward our eyes, therefore we see ...
7
votes
2answers
878 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 ...
5
votes
1answer
186 views

What would a length contracted object look like?

I have multiple sub-questions but they are related. What would the object look like if in were passing by? What would a star look like if we were traveling near $c$? Would the perspective be a a ...
6
votes
3answers
884 views

Why the red-shift of distant galaxies is considered to be the effect of expanding spacetime?

Why it's not explained just by Doppler redshift caused by faster movement of those galaxies billions of years ago when that light was emitted? Would the speeds of the galaxies necessary for Doppler ...
0
votes
1answer
316 views

Transmission of Gaussian Beam Through Graded-Index Slab

The $ABCD$ matrix of a glass graded-index slab with refractive index $n(y)=n_0(1-\frac{1}{2}\alpha^{2}y^{2})$ and length $d$ is $A=\cos(\alpha d)$, $B=\frac{1}{\alpha}\sin(\alpha d)$, $C=-\alpha ...
2
votes
0answers
148 views

Help With Difficult Deductive Proof [closed]

Suppose we have a Gaussian beam with a complex envelope expressed by the following equation 1: $$\tag{1} A_G(x,y,z) = \frac{A_1}{q(z)} e^{-ik \frac{x^2 + y^2}{2q(z)}} $$ where $$ q(z) = z+iz_0 $$ ...
2
votes
2answers
193 views

Why energy saving light bulbs contain rare earth?

Reading a french article talking about a recycling unit of rare earth from energy saving light bulbs near Lyon - France (here is the official press release of the Salvay company), I was wondering : ...
0
votes
2answers
154 views

Is an electric lamp a transducer? [closed]

Silly thought. A transducer, by definition, is a device that converts variations in one form of energy to another. An electric lamp converts electricity into visible light - the brightness may vary ...
3
votes
1answer
619 views

Different colors in LED/LCD display when viewed from different angle?

I want to know why different colours appear when viewed with a different angle. Can anyone tell me why?
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2answers
301 views

Pulsed Spherical Wave

Can somebody help show me how a pulsed spherical wave has a wavefunction of the form U(r,t) = (1/r)a(t-r/c), where a(t) is an arbitrary function, r is the radius of the spherical wave, t is time, and ...
4
votes
3answers
9k views

Why can't light pass through walls but sound can?

When I sit in a room I can hear voices coming from the adjacent room but the light in adjacent room does not enter my room i.e. sound waves travels through the wall but light waves can't. Why?
9
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5answers
2k views

Why is Light invisible?

Why can't we see light? The thing which makes everything visible is itself invisible. Why is it so?
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1answer
311 views

Are all effects that change color perception examples of redshift/blueshift?

As defined by Wikipedia: In physics (especially astrophysics), redshift happens when light seen coming from an object that is moving away is proportionally increased in wavelength, or shifted to ...
0
votes
1answer
123 views

Light in the absence of colloids?

If Earth had no colloids (Don't take LIFE into account) or any kind of scattering medium (like atmosphere) that promote to Tyndall effect, how would we able to receive the sun's rays? In the other ...
8
votes
4answers
11k views

What is the color of a mirror?

A mirror couldn't be white, as then you wouldn't be able to see your reflection so clearly. It wouldn't be transparent, as that then won't reflect. So what color is it?
2
votes
1answer
6k views

Why does your reflection suddenly turn upside down while slowly walking away from a curved mirror?

Many people should have realised, when looking into a concave curved mirror (or even a rather reflective spoon in that fact) at a close distance, you will see a slightly distorted reflection. But as ...
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0answers
49 views

What is the main downfall of using QPSK rather than DQPSK in fiber optics?

I've read that QPSK works theoretically in order to encode two bits depending on the phase of the signal. However, practically a reference for the carrier signal would be needed due to some ambiguity ...
15
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3answers
788 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?
-4
votes
1answer
99 views

Photonics: Slab As a Lens [closed]

The question can be found here: http://gyazo.com/fc4d26cd35e6ce368ad2a8ed504f1dcc The refractive index it references can be found here: http://gyazo.com/94fd2f3b5ea7da9226c3acd56b0024c1 I'm not ...
5
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2answers
10k views

Why does earth look blue from outer space?

I know it's more than 70% water. But what has it got to do with earth's colour ?
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1answer
452 views

Is there a simple model explaining Faraday effect?

I find magneto-optical effects fascinating, and especially the Faraday effect. But most sources only give a phenomenological description, while I want a deeper explanation of its mechanism. Is there a ...