# Tagged Questions

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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### What happens to the visible light that a substance does not reflect on an atomic level?

Let's say we have a blue, opaque material. If white light was incident on that material, the blue light would be absorbed by electrons and the electrons would transition to a higher energy state, and ...
29 views

### Wave nature of light [on hold]

I'm currently working on a project to demonstrate the wave nature of light. Now, my question in specific is: How could the phenomenon of interference be used to MATHEMATICALLY explain the wave nature ...
3k views

### Why don't loop currents produce light?

If a charge travels in a circle it must accelerate, thereby producing EM. However, a wire in a circular loop is analogous to many charges moving in a circle. So, why don't circular currents produce ...
15 views

### Finding the index of refraction [on hold]

I'm given the angle of incidence which is 60 degrees and the angle of refraction 27 degrees. And two possible indexes of refraction for the second medium which are 1.923 or 2.417. How do I figure out ...
49 views

### Are RGB LED's only capable of producing three colors of light? What about all the other colors?

I had a light sensor hooked up to an LED and noticed that it only read a constant lux value when I had any of the RGB values at 255. Anything less made the lux value fluctuate quite rapidly. I get ...
2k views

### How many photons are needed to make a light wave?

What is the smallest number of photons needed to make a "light wave"? In other words, how many (coherent?) photons start to exhibit classical behavior? For example, how many photons are needed to get ...
41 views

### How Light interacts with Atoms? [on hold]

I think I have confused myself about how light interacts with matter, would somebody be able to clear these questions up for me? How does an atom reflect light? Can an electron just essentially '...
410 views

### trouble reconciling doppler shift for energy and momentum of light

[See late addition at bottom for what currently remains unresolved in this question.] I have seen the assertion in a couple of places that if you trap light in a reflective resonator, or any other ...
23 views

### How does infrared light behave through water?

I have a project involving the detection of the presence of water in a cylindrical transparent plastic container (diameter=25cm) And I cannot put any device inside, so I was thinking of using and IR ...
50 views

### What is the actual mechanism behind the reflection of various colored light by material of different color?

When light (composed of photons with different frequency and wavelength) falls on a material, some of it gets absorbed and some get reflected depending upon the color of the material on which it ...
1k views

### The speed of light upon reflection [duplicate]

(NOTE: I am an 8th grader, so I may not be capable of perfectly elaborating my point by scientific measures. Also, English is not my first language) I have read that the speed of light is constant, ...
1k views

### How do color filters placed in front of a light source,change the color of light that passes through?

For instance, placing a blue transparent folder in front of a desk lamp, makes the light that shines through it appear blue. From what I understand some red car lights do not have red light sources, ...
43 views

### Density of photons at distance, r, from point light source [on hold]

I am trying to derive an expression for calculating photon density $\left(\frac{Photons}{M^3}\right)$ at any given distance from a point monochromatic light source for fun (assuming previous knowledge ...
31 views

### How thin can a mirror be? [duplicate]

What affects the reflective properties of a material? From everyday experience, metals are good at reflecting light. How thick should be a mirror to reflect most sun light energy? For the case with ...
109 views

### How is light related to photons? [duplicate]

This may seem like a duplicate but I do not understand other explanations. I have read that light is an electromagnetic wave (a fluctuation or disturbance in the electric and magnetic field). How ...
31 views

### Refractive index calculation without using snell's law [on hold]

If I have a ray of light travelling from a transparent medium to the air in 70 degrees in the medium and 52 degrees in the air, how can I calculate the refractive index of the transparent medium ...
44 views

### Which materials redshift light?

I'm currently researching ways to get the human perceived colors red, green and blue when you only have a mostly blue spectrum available at your source (using electroluminescence). The solution should ...
6k views

### Relativistic Doppler effect derivation

This is about a step in a derivation of the expression for the relativistic Doppler effect. Consider a source receding from an observer at a velocity $v$ along the line joining the two. Light is ...
538 views

### Finding the illuminance from a triangular light source

Since most light sources in games are point-like, it's pretty difficult to approximate area light sources with point sources. As triangles are a universal form to represent 3D models (thus area light ...
131 views

### Calculating the color temperature and intensity of skylight with an algorithm

I think it will be possible to calculate the natural sky light intensity and color balance (CCT) based on time of day/year, GPS location. Has anyone seen a formula/algorithm for this? I would like to ...
31 views

### Does the general theory of relativity imply time dilation across the whole universe? [duplicate]

If not, why is my idea below not possible? The general theory of relativity tells us that time is slowed down by gravitational fields. We bounced radar signals off planets further from the sun, and ...
644 views

### How to use a magnifying glass or any lens to enhance small light power?

I am currently trying to amplify a tiny amount of light so that a photodiode will recognize it. In order to amplify the light I thought about using a magnifying glass since I got to know that such ...
35 views

### How was light used to detect targets in pre-radar times? [on hold]

I read that light (optical systems) was used to detect targets in the air and sea before radar was used. I saw photos of light being focused in the skies etc. But I want to know more about this - How ...
28 views

### Despersion of visible light [on hold]

I was wondering what is the main point for light despersion through a triangular glass prism? Why this dispersion occur?
18 views

### Jones formalism for calculating quarter waveplate angle for circular polarized light

I am planning to change the polarization of a vertically linear polarized laser to circular polarized light with the help of a quarter waveplate. I know the final result: I have to rotate the fast ...
15 views

### Increasing brightness of light source by reducing apature and reflecting the remainder?

Can the amount of light per area be increased by reflecting all but a small area of the lamp back on the light sources rear reflector or would the reflected light just end up as heat? I attempted a ...
277 views

### Why is there less UV light on earth in winter

So I have often read that, at least in e.g. northern Europe, in the colder seasons, there is not enough UV (-B) light arriving from the sun, so many people have not enough vitamin-D from that. At ...
39 views

### Is light spectra different for all living bodies? [closed]

As we know that honey bees can see lots of different colours, is it possible that the light spectra is different for them? My main question is that is light spectra different for all living organisms?...
53 views

### How does a screen (computer moniter movie theater et cetera) produce the color black?

This might seem a dumb question, but here goes: I can't help but notice that a black screen on my computer looks more like black than the screen when the computer is turned off. Similarly a black ...
75 views

### What do we mean by speed of light dependent on direction?

I have a statement in textbook saying: When the speed of light is independent of direction, the secondary waves are spherical. When is it dependent on direction and how will the secondary ...
2k views

### Is there a theoretical lower limit to watts needed for lumens?

For example 100 watt incandescent bulb produces 1600 lumens. Led 23 watt can produce 1600 lumens. Is there minimum amount of watts needed ?
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### Traveling at speed of light - Seeing light [closed]

Assuming mankind was able to travel faster than the speed of light: Imagine someone floating in space with no object in front of himself, he is attached to a huge rocket with a fuel tank with a ...
46 views

### Change in Speed of Light [duplicate]

Whenever light enters from air to glass its speed decreases but when it cross the glass and again come in air its speed increases. What is the reason behind this property? What is slowing it and what ...
255 views

### Dispersion in a rectangular prism

Why are the colours visible only at the fringes of the light refracted through a rectangular prism?
51 views

### Can light change the shape of a black hole?

If light orbits a black hole without ever getting closer or further from it, and the black hole pulls on the light from all directions nearly simultaneously, does the black hole extend at its equator? ...
43 views

### Why LiFi can't penetrate like WiFi? [closed]

My common sense says that Lifi signals can't penetrate through walls (since visible light cannot) but other part of the electromagnetic spectrum can, I mean radio signals which power WiFi. So is ...
56 views

### Why is glass transparent to visible light and opaque to ultraviolet amd infrared light

I was thinking that UL and IL are being absorbed when striking glass which makes no light passing through. need clarification.
76 views

### Expanding Universe [closed]

So an expanding universe has the metric $$ds^2 = g_{\mu \nu} dx^\mu dx^\nu = -dt^2 + a(t) ( dr^2+r^2 d\Omega^2)$$ where $d\Omega^2 = d\theta^2 + \sin^2\theta d\varphi^2$ as the usual spherical ...
60 views

### A concept of primary colors [closed]

It's a common conception that there are at least three different colors which produce most of the visible spectrum. I am wondering if there is a unique set of these "primary" colors which could be ...
43 views

### Time dilation along a curved path

I was thinking about Einstein's elevator, more specifically, a box, say with one side labeled A and the opposite labeled B. On side A, a laser sends pulses of light to side B at equal time intervals. ...
25 views

### Mueller matrix and Lorentz group

I have just learned about Stokes vectors and Mueller matrices for description of polarized light. In the text I studied there is clear restriction for the Stokes vector $\vec S$ that \$S^2_0=S^2_1+S^...
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### Can Light Really be Matter? [closed]

I remember during my early education being taught that light is a form of matter. This has always seemed counter-intuitive to me.
16k views

### How do electromagnetic waves travel in a vacuum?

This is perhaps a total newbie question, and I will try to formulate it the best I can, so here it goes. How does an electromagnetic wave travel through for example, the vacuum of space? I usually ...
24 views

### Light scattering: Why is the natural frequency in air higher than the light frequencies?

In the Feynman Lectures on Physics Vol. I Ch. 32 (Radiation Damping. Light Scattering) it says: On the other hand, if we take the case of light in the air, we remember that for air the natural ...
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### Is it possible to create a Faraday cage transparent to only violet/blue light from natural white light?

If a Faraday cage’s hole size determines the wavelength of light which can penetrate it then I would imagine this would be possible.
5k views

### Why does the sun have to be nearly fully covered to notice any darkening in an eclipse?

I was looking at eclipse footage and I noticed that it doesn't get any noticeably darker until the very end when it suddenly all the light is gone. As the moon blocks out the Sun, I would expect that ...
12 views

### Projection of 3d object to an optical 2d matrix

I want this projection explained. I can explain it like you have a rectangular "screen" of paper that you translate along its normal towards the 3-d line. If screen is parallel to the line, the hole ...
34 views

### A logic question on binary variables

I recently purchased some nightlights with sensors. The sensor will make sure the nightlight will only be tuned on in a dark environment. Let's assume the sensor is binary, ON or OFF. Here is the ...