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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Redshift of a reflected photon from a solar sail

Can someone help me understand my mistake here? I'm thinking of a photon travelling with momentum $p=h/\lambda$ which reflects off a solar sail of mass $m$ travelling with momentum $p_0$ in the same ...
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1answer
128 views

Relation between intensity of light and refractive index

The intensity of light (as calculated from time average of the poynting vector) is given by $I = (1/2) \epsilon v E_0^2$. Here the intensity is dependent on the velocity of light in the medium. The ...
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2answers
87 views

What is the physical structure of light?

Conceptually, I understand what light is, but I don't know what a photon would "look like" if it could be frozen in space/time. For instance, the notion of amplitude seems to be absent when discussing ...
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2answers
189 views

Does the formula $ \theta = \frac{v}{c} $ to find out deflection of light make sense?

I read in reliable sites that GR and classical physics calculate the angle of deflection in the same manner. The formula is almost identical: $$\theta = \frac{4GM}{c^2*r} \rightarrow \frac{4GM}{c*r} = ...
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2answers
3k views

Why is the sun brighter in Australia compared to parts of Asia?

Background: I've lived in Philippines for several years, and visited other parts of Asia occasionally (Singapore, Indonesia, Hongkong). I just moved to Western Australia a few months ago and I ...
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1answer
46 views

Optical fibre modes and intensity pattern

I shine laser line in front of one side of multimode optical fibre.Light coming out of other side is projected into wall.Whether the intensity pattern seen on wall is same as the mode of propagation ...
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0answers
30 views

Cause and properties of northern lights [duplicate]

If outside on a dark night in the middle to high latitudes, one might be able to see an aurora. It may be several hundred kilometers high and several thousand kilometers long, stretching around Earth ...
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2answers
113 views

How can heat turn into light

I am confused about how hot surfaces can radiate light to their surroundings. When I shine a light on a surface the light turns to heat spontaneously, and when I leave that hot surface it radiates ...
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1answer
45 views

Is this “cloaking in time” serious and what is really meant?

I just found this news article: http://www.nature.com/nature/journal/v481/n7379/full/nature10695.html What did those researchers actually do? The article itself doesn't sound to me like it can be ...
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3answers
116 views

How to generate Red light from a Blue-Dominated Spectrum [closed]

I have a white LED light source, the spectrum of which looks as follows --- I want to filter out red light from this white LED source. But as you can see the spectrum dwindles in the red zone. And ...
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4answers
3k views

How much red, blue and green does white light have?

Different kinds of white light have a different spectrum. Light from a white LED will have blue at the peak intensity while white light from a CFL or something else will have a different looking ...
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3answers
81 views

Double rainbows

In my garden, when I'm watering the plants I sometimes see a rainbow or two. How did two rainbows appear? Why can't I see three rainbows then, or how can I see three rainbows?
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0answers
25 views

Quantum Efficiency Estimation

Might there be a way to do a rough estimate of the quantum efficiency of a photo-detector like a CCD or CMOS sensor based only on a picture taken with it? I've read papers and guides (like this one: ...
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2answers
75 views

Where does wave frequency come from

I am trying to wrap my head around where do oscillations in electromagnetic waves come from. As an example if I would take a string of guitar and ring it, it would produce a certain sound based on the ...
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2answers
118 views

How can space be euclidean when light bends?

I have read people arguing that tridimensional space sections of space time continuum (whatever its number of dimensions) appears to be euclidean from empirical evidence. I cannot reconcile it with my ...
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2answers
287 views

Is there a substance that doesn't reflect OR absorb light from the visible light spectrum?

Is there a substance that doesn't reflect or absorb visible light but may reflect light from another spectrum? Is there a theoretical substance that would have these properties? EDIT: Sorry I wasn't ...
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1answer
94 views

Single slit diffraction pattern in 2D

I was looking at previous exams and I saw a question with single slit diffraction. Please look at picture on the website: http://www.physicsforums.com/showthread.php?p=4807732#post4807732 So, this ...
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1answer
114 views

Is that a result of thin-film interference?

Several years ago, I was laying on my bed and had a CD shaped transparent plastic disk (which was covering a 100 CD stack), basically a transparent CD. I don't know why but I took my phone and took a ...
2
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5answers
76 views

Is the plasma state always visible?

All the examples of plasma I have come across are visible. Is there any plasma which is not visible? For example, during a dark lightening we don't see the radiation because its gamma radiation. Is ...
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2answers
125 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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4answers
2k views

Is the frequency of light restricted?

What are the factors that limit the frequency of light? Can it have wavelengths ranging between zero and infinity?
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1answer
33 views

Color in Different media

When a light wave passes from one medium to other, the wavelength of the light wave changes and the frequency remains constant. If then color of the light waves depends on changing wavelength or ...
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1answer
167 views

Why do objects of a given color appear white under light of that color?

Under light from the sun, a red object will scatter the red component of light, and absorb all others. Hence, the human eye perceives it as "red". White objects scatter all wavelengths of light. ...
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2answers
93 views

Faraday Effect does light bend or lose energy?

I was reading upon Faraday effect when it said Faraday effect causes a rotation of the plane of polarization That in mind, does this mean the light can be bent around or does the light loose ...
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1answer
101 views

If I'm running away from chasing gamma photons, I can see them, right?

Suppose Bruce Banner goes back in time, convinced that the Hulk is a stupid menace. Let's say he also borrows Quicksilver's speed abilities (maybe the Flash is more appropriate, but based on the ...
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1answer
22 views

Do the characteristic vibrational modes of a material define how much will a material heat up upon radiation excitation?

Do, and if yes how, does the intensity and energy of the Raman modes of a material make it to heat more by radiation excitation. I see phonons and vibrations of the lattice as heat, but maybe I'm ...
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0answers
55 views

Mathematical formalism to include wave and particle perspectives of light

Does the exist any mathematical formalism (model) describing the behavior of light and incorporating its particle character (divisibility, quantization) and wave character? (i.e. quantized wave model) ...
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3answers
125 views

Can light be trapped theoretically?

My question is a little silly I know, but I'm curious to know if a particle of light can be theoretically trapped between two reflective screens. For example once the particle of light has left it's ...
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2answers
589 views

Why are most accurate reflective surfaces not white?

Since white is our perception of reflection of light, why are mirrors and other metallic objects that are more grey or silvery capable more reflective than a white object? This is somewhat related: ...
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2answers
164 views

Do the laws of physics that apply to all observers also apply to a non-observer? [closed]

The Timelessness of a photon https://www.youtube.com/watch?v=5ELA3ReWQJY An observer's laws of physics are time based. "When you're traveling at the speed of light, time does not exist" - Neil ...
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3answers
74 views

Unpolarized Light

Suppose I had a ray of unpolarized light, and I was sitting inside the beam and looking at the electric fields oscillating, then , if I am looking at a point how would the oscillations look like? I ...
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5answers
2k views

Why isn't the sunset/sunrise rainbow-colored

When the sun is rising/setting, it goes through a phase where the light is bending from the atmosphere. I believe this image will explain much better than I ever could. Now, if light goes through ...
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2answers
61 views

Is the gradual progression of day(light) into night a clue that the earth is a sphere?

If the earth was flat, would the transition between day and night be sudden?
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2answers
64 views

More about gravitation as the source of redshift of light beams [closed]

In the realm of physics there is no knowledge, the justification of which does not depend either upon experience or upon reason. In the light of the findings of the preconditions of knowledge made in ...
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6answers
939 views

How can molecule of a few angstroms absorb visible light of a few hundred nanometers?

I guess visible light is visible, because it has the right wavelength to be absorbed (or not) and emitted (or not) by many different molecules. Now visible light has a wavelength in the order of a few ...
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2answers
570 views

Do x-rays and gamma rays also contain photons like visible light does?

Do x-rays and gamma rays also contain photons like visible light does? If so, then what makes photons of visible light and other waves different? The rest mass of a photon is zero, but as it moves at ...
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3answers
81 views

Problem on speed of light

As I was reading special relativity, my book says the speed of light is $c$ with respect to any other thing. Does that mean the speed of an individual photon is $c$ even with respect to another ...
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2answers
69 views

What is the maximam distance of light that is released by a flashlight?

Recently I looked at the cloudy night sky and noticed a pale green color light moving around the clouds for about 20 min.After observation I came a conclusion that it is probably because of a light ...
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1answer
132 views

Why do i see bright red hand when i place flashlight behind my hand?

When I place a working flashlight behind my hand, I see my hand bright red because of the light. So my question is, why do I only see red light after transmission even if incoming light from the ...
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1answer
70 views

Idea is to make an equipment like torch, which should emit darkness.. Is it possible? [duplicate]

Can we have any equipment as such? If light can emit form a hand held device, why not darkness
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1answer
88 views

Would white light waves act as same as monochromatic waves in double slit experiment?

Would white light waves act as same as monochromatic waves in double slit experiment? If not, what pattern would be seen on the screen?
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3answers
121 views

How to convert RGB values to physical radiometry and/or photometry quantities and back?

What are RGB values actually mean in physical sense? Are these values mean some amount of light or energy per surface unit or what? Are they logarithmic? Fro example, why do these values are ...
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2answers
374 views

Seeing one's back on the event horizon

If we would hypothetically be exactly on the event horizon, we should see our own back, because of the circular motion of photons on the event horizon, right? But what would be the image size, or $-$ ...
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0answers
29 views

Absorption Spectrum

I just learnt what absorption and emission spectrum are. And our teacher introduced us to what Quantum Dots are. And showed us their absorption and emission spectra. And they look something like this ...
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1answer
136 views

Electromagnetic radiation and black body radiation

I was taught today that the Electromagnetic wave Theory is unable to explain black body radiation. The example that was given to me: When a metal is heated, it emits different frequencies of light as ...
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1answer
93 views

Can horizontally and vertically polarized light combine to become circularly/elliptically polarized light?

Well, we know that circularly/elliptically polarized light is made up from orthogonal components. So is it possible then to create circularly/elliptically polarized light by combining horizontally and ...
103
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8answers
11k views

Could Legolas actually see that far?

The video “How Far Can Legolas See?” by MinutePhysics recently went viral. The video states that although Legolas would in principle be able to count $105$ horsemen $24\text{ km}$ away, he shouldn't ...
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1answer
46 views

Calculate minimum intensity of a point source visible to a camera

I have two cameras and know the specified minimum faceplate sensitivity for both: Camera 1: Minimum faceplate sensitivity 4 x 10-6 Lux Camera 2: Minimum faceplate sensitivity 2 x 10-7 lux I also ...
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2answers
287 views

Why does adding red light with blue light give purple light?

Our eyes contain 3 photoreceptor cells (cones) to perceive three wavelength ranges of light. Here is a visual representation of the wavelengths by these receptors (S, M and L). So if we have light ...
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1answer
40 views

light color and refraction

I'm a bit confused on the topic of refraction, some starting premises: 1) When light refracts from exiting a medium with a low n and entering a medium with high n the light bends. 2) The index of ...