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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809 views

How far can the human eye see? Any ending point or infinite?

How far can the naked human eye see if unobstructed? (straight ahead on perfectly flat land). Is there a stopping point or is it infinite when you look in the sky? Is there any situation that the ...
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1answer
706 views

Why do most metals appear silver in color with gold being an exception from a scattering and EM viewpoint?

Related: Why are most metals gray/silver? After reading Johannes’ impressive answer to Ali Abbasinasab question of why do most metals appear silver in color with the exception of gold (and copper), ...
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2answers
416 views

How can a green laser be reflected as orange?

My friend recently bought a high-powerd pocket laser pen (from China). It is green, and reflects off most surfaces as green. On some orange surfaces, it reflects orange instead. That is to say, we ...
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2answers
161 views

why does a mirror show what is in front of it?

the only answer I can think of is that light is reflected from the objects in front of the mirror (visible color) and then reflects again off of the mirror to our eye, but im not quite satified with ...
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1answer
84 views

Looking for a resource that explains all colors and shapes of stars and galaxies

I've been looking at some of the astronomy pictures of the day from NASA. They all have content that is indicative of certain situations or events. Supernovas have the scattered lines look, new stars ...
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2answers
934 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 ...
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1answer
87 views

Why are reflected objects blurry?

While this may seems like a strange question, I'll try to explain it the best I can. Like 75% of Americans, I wear glasses and contacts to correct my worsening vision. I'm not far sighted so objects ...
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2answers
108 views

The speed of light as it approaches a massive body

No matter how fast you go, you will aways perceive the speed of light as constant. Taking that as a fact, the special relativity theory was formulated. Now, for what I understand about general ...
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2answers
97 views

Why does the colour wheel wrap around?

Red colour bares 430-480THz and Violet bares 670-750THz according to Wikipedia. What frequencies are the colours between red and violet? Will they be higher than red? or lower than violet? What ...
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1answer
144 views

What determines the form of the intensity curves in Laser-Induced Fluorescence (LIF) measurements?

What determines the form of the intensity spectra of different particle species in Laser-Induced Fluorescence (LIF) measurements? See e.g. I figure that bigger particles have more ways to get ...
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2answers
134 views

Colors of light

White light is always said to contain all the different wavelengths of light. Why, then, can we 'make' new colors simply by adding wavelengths? Is it just a matter of our perception, that, when two ...
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1answer
345 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 ...
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1answer
498 views

How to set up a very simple experiment in optics?

This might come across as a very rudimentary question. My fundamentals of Optics are weak. In the optics chapter of my physics text book I saw diagrams each depicting an object on the left and a lens ...
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1answer
414 views

simple test/measurement to quantify water opacity

I am performing some measurements on liquid surfaces and I want to quantify how these mesures depend on the liquid opacity by performing it with different concentrations of colorant. To allow ...
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1answer
362 views

What are the coloured bands in this “Pale Blue Dot” picture?

You can see Earth in the right most band in this image: What are these bands doing in space?! Can somebody please explain the coloured bands in this image?
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1answer
300 views

Lunar twilight and sixth magnitude stars

Summary: when the Moon is x degrees below the horizon, it interferes with stargazing the same as astronomical twilight would. What is x (as a function of the Moon's phase)? We define civil, ...
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1answer
85 views

Why do we see the sun's light color different from the moon's light color?

Why do we see the color of the sun's light as red or orange, while we see the moon's light as white, although it's reflected from the sun?
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2answers
239 views

Is the frequency/wavelength of light modified when multiple light sources are combined?

Let's say I light a wall with two spotlights: One red and one green one. Where they overlap, I'll see a yellow area at the wall. My question is, whether this is caused by an modification of the ...
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2answers
154 views

What is the energy distribution of light if it has an infinite length?

What is the energy distribution of light if it has an infinite length? I have read in one of the answers here on phys.SE that light has actually an infinite length. But then what is the energy ...
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2answers
45 views

How should I interpret an article about light being slowed and compressed?

I've found an article describing a method that allowed the light to be "slowed and compressed"... That sounds really strange, which made me google a lot and find this PDF document. However for me, ...
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1answer
64 views

How far can diffused undersea light travel and still be perceived by the human eye?

Example: If one SCUBA diver looks past another SCUBA diver horizontally into negative space, how far away is the most distant emission of 'blue' light waves? Consider the sight angle in question to ...
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3answers
1k views

Seeing colors: photons vs waves

As an atmospheric physics major I am familiar with electromagnetic radiation in the atmosphere and what dictates what wavelength objects will emit at. When observing radiation in the atmosphere it is ...
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2answers
133 views

Decomposing light into frequency spectrum

Light hits a charge coupled element. The wavelength of the light somehow is translated into a color picture. Where can I learn about methods (algorithms) to decompose light hitting a CCD into ...
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3answers
230 views

Question on Radiance equation

The radiance equation is $$ L = \frac{d}{dA} \frac{2(\phi)}{dW cos(\theta)} (watt/srm^2) $$ where $\phi$ is the flux. I am thinking, should not be the cosine term on the numerator instead of the ...
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1answer
127 views

Angle of rays leaving a light source

I'm working on ray tracing and I'm trying to understand the impact of the angle at which a light-path intersects the surface of a light source on the amount of light that source delivers to the path ...
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3answers
1k 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 ? ...
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1answer
50 views

How do you transform between theoretical Hertzsprung-Russell Diagrams and Colour-Magnitude Diagrams?

When discussing stars, theorists tend to use effective temperature $T_\text{eff}$ and luminosity $L$ (on logarithmic scales). Observers, on the other hand, usually talk about observed colours and ...
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2answers
985 views

Are Colors Emitted at Specific Temperatures?

There are quite a few nagging questions I have been having over the years, I do not require a full explanation, just some guidance in my assumptions and pointers if I am very wrong. My basic ...
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3answers
523 views

Does the wavelength always decrease in a medium?

I was studying a GRE Physics Test problem where optical light with a wavelength of 500 nm travels through a gas with refractive index $n$. If we look at the equations for wave motion and index ...
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2answers
95 views

Physical reason why (hot) objects glow? [duplicate]

Every object at a non-zero temperature radiates light, i.e. it glows. (Is that called blackbody radiation?) What is the physical reason to this? Is it because more heat implies that the atoms ...
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1answer
29 views

How is barometric light porduced?

A shaken sealed glass tube containing liquid mercury in a dark room will glow. This has been know since the 17th century and is called barometric light. How does the addition of potassium ...
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2answers
124 views

Is there any case of total transmission?

When light strikes the interface of two media some light is refracted and some is reflected. But, is there a case where no light is reflected i.e all the light gets passed through the given object. ...
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2answers
271 views

Photons-Wave/particle duality

I know that photons and electrons and such are said to have a wave particle duality, but what does that mean for a photon? When light strikes an object, are many photons emitted, enough to draw ...
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1answer
272 views

Is a 3 LED flashlight brighter than a single LED one? (same LEDs, same power through each LED)

Say I have one LED in a flashlight, it produces some light. Now, if I have 3 of those LEDs in the same flashlight, each LED receiving the same amount of energy than the single LED was receiving in ...
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1answer
139 views

What is the largest diameter an optical fiber can have? - for nonimaging visible light transmission

Is there any theoretical limitation on the size of the diameter of an Plastic optical fiber? I would like to transmit visible sunlight through it. I see bundles of small cores available but wouldn't ...
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1answer
141 views

Can you “fold” EM or light waves? (i.e) long wave that is reflected by mirror in fragments - like in the game “Snake”

So, I was reading about the Casimir effect. Two mirrors facing each other attract to each other in a vacuum. The reason is due to pressure exerted on those mirrors from the multitude of EM waves (like ...
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1answer
94 views

A question about how light hits a surface

my question is about how photons travel from a light source and hit an object. When you look at an object being hit by light the whole surface becomes brighter. What i'm trying to understand is why ...
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1answer
393 views

plotting an sRGB gamut Chromaticity Diagram starting with xy coordinates

Starting with xy coordinates I would like to plot a chromaticity diagram displaying just the sRGB gamut portion of the CIE xy chromaticity diagram. For each ...
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1answer
419 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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1answer
1k views

How do we see different colours?

Why do different wave lengths cause electrons to behave(?) differently, causing us to see different colors? What is happening at the quantum level which causes the colour black to absorb all of the ...
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1answer
297 views

Is there a mathematical way to describe how a flame flickers?

I love the way candles flicker. It's a great effect and I almost want to see it replicated in an actual lightbulb. I was curious if there is any way to express that mathematically? I'm not that ...
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1answer
514 views

Formula for polarized “light” transmission through close filters?

I'm still trying to understand photons (or polarized electromagnetic radiation). This question is similar to one of my previous questions, but different: Consider the famous demonstration of crossed ...
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1answer
20 views

What is the best angle to see water

Sometimes when I clean my house, I miss some of the water since it seems invisible, but when I stand in different angle from the water I can see it clearly, whereas some angles offer partial ...
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0answers
30 views

Does light escape from highly dense bodies [duplicate]

If it were possible to launch a projectile at the speed of light from a body with the mass equal to that of our sun but a density such that its escape velocity was the speed of light how far would it ...
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0answers
53 views

Two Photon Absorption dependence on Semiconductor Band Gap

It's well known that Two Photon Absorption coefficient is scaling with $E_g^{-3}$. Does anyone know what is the physical reason for this scaling? What is the physical theory behind this mathematics?
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0answers
132 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 ...
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1answer
482 views

Dark matter a medium for light propagation

Is dark matter a candidate to fill void left by luminiferous ether as a medium for light travel?
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5answers
2k views

Eye sensitivity & Danger signal

Why are danger signal in red, when the eye is most sensitive to yellow-green? You can check luminosity function for more details...
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4answers
447 views

Is it true that gravitational lensing only occurs for objects made of plasma?

The question concerns this video: It says that the Gravitational Lens effect is an illusion, meaning it's not caused by gravity but by change in density of a plasma atmosphere. It claims further ...
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5answers
368 views

If you shoot a light beam behind the event horizon of a black hole, what happens to the light?

I have a couple of questions about light here, and sorry of they are silly.. So since anything that goes beyond the event horizon can't go out, so what if a light beam was pointed somewhere behind ...