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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Why is getting a tan in the evening less likely than getting a tan in the morning?

When I was young I read one book in which is written that you get more tan on the morning than on evening even light angle from Sun to Earth is the same. Don't remember exact reason, I think because ...
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Slowing down light in an opaque crystal for a whole minute

I just read about a team of physicists at the University of Darmstadt, Germany, that managed to completely slow down a beam of light that traveled through an opaque crystal (article here). How is it ...
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What happens to light that falls into a black hole?

When light enters a black hole, what happens to it? I imagine the photons will either fall into the singularity, or the light will orbit just inside the event horizon indefinitely. (Some background ...
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1answer
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Why does separating plastic from a cough drop create flashes?

I was tearing open individually-wrapped cough drops in the dark, and noticed that when I pulled apart two adhesive-joined pieces of silvery plastic film, small flashes of light were produced. It was ...
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High speed and low speed photons

Looking at the discovery of the neutron, and I came across this page: http://www-outreach.phy.cam.ac.uk/camphy/neutron/neutron3_1.htm The animation on the left, talks about low energy photons and ...
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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. ...
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1answer
363 views

Why are black mosquito nets so much less visible than white ones?

I have just been installing a couple of mosquito nets for windows in our apartment. The material is polyester, fibers are approximately 0.2mm thick, and the hexagonal "holes" apprixmately 1.5mm in ...
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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)?
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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 ...
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How to turn water opaque by pouring the smallest quantity of matter into it? [closed]

Consider a glass of water and a glass of coffee. Their contents differ by no more than a few grams of particles coming from the roasted and ground coffee, yet the former lets almost all visible light ...
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Is it possible to witness a circular rainbow?

What conditions would make it possible to see a naturally occurring fully 360° circular rainbow? Would it even be possible?
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Reflection at intersection of two mirrors

Consider two mirrors, which intersect at an angle. Let us talk about geometric optics. If we draw a light ray intersecting the mirrors at their point of intersection, how will it reflect? What will ...
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How many colors exist?

How many "colors" do exist? Our perception: As far as I know, colors are just different frequencies of light. According to wikipedia, we can see wavelengths from about 380 nm und 740 nm. This means ...
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Why isn't it allowed to use a flash when taking pictures in a certain place?

When I go to, for example, a museum I try to take some pictures. Sometimes the museum staffs forbid me to use a flash. Do you know the reason? I don't think it is related to photo-electric effect, ...
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The Sun as a gravitational lens

Since the Sun is a gravitational lens with as focal length of 550 AU for visible light, with an immense amplification factor, shouldn't it light up objects hanging out there? We should get solar ...
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Why doesn't my pinhole camera work?

We all know that light travels in straight a line, which can be proved by pinhole imaging as in the picture shown : But when I'm doing this little experiment with an apple, no matter how I change ...
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What would we see in the sky if it weren't blue

What would we see during the day when we look into the sky (other than clouds), if it weren't for Rayleigh scattering making the sky blue? Would the sky be dark, like at night?
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Homemade Spectrometer

Recently I have had ideas of how to build a spectroscope, but I'm not sure if it will work. As can be seen in the diagram, the experiment is simple: it consists of a laser that generates the light ...
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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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825 views

Test whether a glass prevents you from getting your Vitamin D portion?

I work in an office with glass, which I believe filters the UV radiation of the sunlight. Is it possible to test if exposure to the light coming through the glass will supply Vitamin D for me as a ...
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Why does burning things make them black?

It's not clear to me how burning process can transform a material which was previously capable both of absorbing and emitting photon to one which can only absorb them. I would like to hear an educated ...
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What is the meaning of this “let there be light” joke?

Someone across the restaurant is wearing this shirt, and I certainly don't get it. Update Related: What does this quote about the four dimensional divergence of an antisymmetric tensor mean?
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What invisible flicker do different types of light bulbs have?

What waveform does the light coming off different types of artificial light sources have and (of course) what are the underlying physical reason for them? I was inspired to ask this question ...
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Trapping a lightray

Given a solid whose interior is a hollow sphere with perfectly reflecting mirrors. A small hole is drilled in the sphere and a photon is sent in at some angle. Will it always eventually exit through ...
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What causes hot things to glow, and at what temperature?

I have an electric stove, and when I turn it on and turn off the lights, I notice the stove glowing. However, as I turn down the temperature, it eventually goes away completely. Is there a cut-off ...
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Can we change a photon's frequency in mid-air?

Can we have a light source emitting photons in the infrared range and after, lets say, 5 meters, these photons become a photon in the x-ray range? The only way I know we can change a photon's ...
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342 views

What exactly are light waves?

We know a sound wave is a disturbance that moves through a medium when particles of the medium set neighboring particles in motion. And using the pressure variations we can plot a pressure/time graph ...
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Can visible light be emitted from a non-thermal source?

I was reading about thermal and non-thermal radiation and I was wondering if visible light can be emitted from a non-thermal source?
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How is temperature related to color?

I spent some time studying about temperatures and color of objects. It turns out that as we heat something it turns to red, then yellowish white and if we heat it more it turns to bluish-white. Like ...
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Blue-shifting as opposed to violet-shifting

A recent XKCD comic implies that the sky is blue as opposed to violet due to human physiology, and that animals more sensitive to shorter wavelengths will perceive the Earth's sky as the shortest ...
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Is it (theoretically) physically possible to project an image into thin air?

Is there some law of physics that strictly prohibits the projection of 2D or 3D images into thin air (such as holograms in movies) or is a solution to achieve this still up for grabs by an eventual ...
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How to Make RF Waves Visible

I understand RF (Radio Frequency) Waves are electromagnetic waves and a mode of communication for wireless technologies, such as cordless phones, radar, ham radio, GPS, and television broadcasts. Most ...
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394 views

What is changing in latest light bulb technologies?

I'm confused with the latest home lightning bulbs. Understanding filament bulbs was easy. For example take 220V, 100W filament bulb: Power = $V^2/R$ Filament gets heated and emits energy in the form ...
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297 views

Single-mode fibers and ray-theory of light

I have read ray-theory cannot be applied to single-mode fibers. Why? I know rays are kind of an approximation to describe the real behavior of light, and electromagnetic theory and photonics are ...
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976 views

Triple slit experiment

If the experiment is done with single photons and a "detector" at one of the slits the interference pattern breaks down. What happens if three slits are used with single photons and a "detector" at, ...
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Calculating the illumination of the moon

I am trying to find a formula that will enable me to calculate the illumination of the moon down to one thousandth of a percent, given that the Gregorian year, month, day, and hour is known. Can ...
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1answer
206 views

Is Huygen's Principle Axiomatic?

Is Huygens Principle just a fundamental way to understand light? It always seemed to me that it was somehow "derived" or that it should be-but is it simply a well-founded theory?
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How does sun light after it has passed window's pane still heat me up?

I believe it is so because most of photons' energy has successfully passed the glass. But is it so? And how can I roughly estimate part of light's energy which will pass obstacles like glass? And how ...
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Why both yellow and purple light could be made by a mix of red, green and blue?

We see the mix of red light and green light as yellow light (#FFFF00). The wavelength of yellow light lies between red and green. But the wavelength of purple light lies outside of red and blue. ...
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1answer
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Why does the sun “shine brighter” some days?

Today, the sun seems extremely bright; more dazzling than usual, and even the roads seem to be brighter so it's not just when you look up in the sky. Is more light actually getting through (perhaps ...
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1answer
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What common materials absorb most infrared light?

I'm competing in a simple robotics competition where most of the participants use reflected infrared light to detect their opponent. I'd like to make my own robot as difficult to see as possible. What ...
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Why does a glass window reflect white objects white from an atomic scatterers’ viewpoint?

Related: Is a white object always white? If you are standing in front of a glass window during the day, you can see your dim white t-shirt’s reflection in the window. The reflection is dim because ...
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Visible light spectrum to color space

I need to be able to convert an arbitrary emission spectrum in the visible spectrum range (i.e. for every wavelength between 380 and 780, I have a number between 0 and 1 that represents the ...
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Focusing light into an optical fiber cable

I'm trying to focus light from a flashing light lamp throught two lens into a optical fiber cable which is around 0.8cm in diameter. flashing light: a bulb and then covered by a glass of 7.5cm x 7cm ...
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1answer
77 views

Speed of light affecting appearance of galaxies viewed not face on?

So this has been really bugging me over the past few days (and forgive me if the answer is so simple). Let's say we're observing the Sombrero galaxy. It is about 29 million light years away and 50 ...
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Real and imaginary parts of dielectric constant vs refractive index?

So for a complex dielectric constant $\epsilon = \epsilon_a + i\epsilon_b$, the wave vector and index of refraction are related to it through $k = \frac{\omega}{c}n$ and $n = \sqrt{\frac{\mu ...
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1answer
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Why is light produced when an underwater bubble is collapsed with a sound wave?

Why is light produced when an underwater bubble is collapsed with a sound wave? I have come across this fact on a page (similar to this) but can't understand "Why". I'm just curious about this ...
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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 ...
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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.
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Is the sun's solar radiance spectrum matching up with water's absorption spectrum just coincidence?

People frequently point out that water has a pretty narrow range in which it isn't very absorbing of light, reaching a minimum at a wavelength of about 500nm: And that our eyes have made good use ...