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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2answers
173 views
Is a white object always white?
Is a object that’s white with the lights on, still white although it looks different under a different light or in total darkness?
2
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1answer
103 views
Why is the shadow of earth red?
Reading this article about the forthcoming eclipse on December 10th, I noticed mention that the moon passes through the red shadow of Earth; I'd always held the impression that a shadow is universally ...
2
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1answer
152 views
If we were to travel through space (sci-fi style), how close to the false-color images would the galaxies we see be?
I understand that the black-and-white images you see looking through a household telescope are only like that due to the intensity of the light that reaches us, and that most of the astronomy images ...
5
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2answers
183 views
Does light escape when I open my blinds?
When I open my blinds, there is light enough to read a book outside while the lamp is on inside.
So when I close my blinds, does the light that otherwise will escape, stay inside and thus being ...
4
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2answers
1k views
What's the difference between dim and bright light?
When comparing two light sources, for example, a light bulb at 20W and a light bulb at 100W, what is it about the incoming light that makes the latter look brighter than the former? Are there ...
2
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3answers
272 views
What was the motivation behind suggesting the trichromatic theory of vision?
Background
In this thread, I asked whether it is true that the colors red, green and blue, through additive mixture, can make up any color. Turns out they can't.
However, when reading about the ...
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5answers
1k views
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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8answers
1k views
Can there be black light ? I mean is it possible to devise a machine that outputs darkness ?
I understand there are various colours that light can have. But i was wondering why there is no 'black' light. What is the logical explanation for this ? I mean I am expecting an answer that goes ...
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4answers
1k views
Is it only red, green and blue that can make up any color through additive mixture?
I'm reading about color vision and have some trouble understanding the motivation for why the trichromatic theory was suggested in the first place. The book I'm reading ("Psychgology: The science of ...
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0answers
318 views
How to turn water opaque by pouring the smallest quantity of matter into it?
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 ...
3
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1answer
38 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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0answers
330 views
How to use the Coddington shape factor?
We are given $\sigma=\frac{r2+r1}{r2-r1}$, with $\sigma$ being the Coddington shape factor. What I am having problems with is solving for $r1$ and $r2$ when I am just given a focal length and a ...
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5answers
970 views
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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4answers
2k views
Can someone explain the color Pink to me?
I just finished watching this interesting video:
http://youtu.be/S9dqJRyk0YM
It does a very quick explanation of how pink light doesn't exist, and that the concept of pink is our brain's attempt at ...
1
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3answers
457 views
Conversion formula from spectrophotometer readings to any standard color space profile?
I have readings of colors from a spectrophotometer that records across the human visible color spectrum 390nm to 790nm intervals. I'd like to convert this into any color space, could be CIE XYZ, HSL, ...
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4answers
269 views
How are we able to view an object in a room with bulb..?
This is a very basic question on optics. How are we able to view an object kept in a room with a bulb?
From what I understand, light rays from bulb will hit the object and some colour will be ...
2
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1answer
293 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 ...
0
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1answer
79 views
Explain moving lightbulb [closed]
An acquaintance of mine, while being home alone, saw that the light bulb in the room which was hanging from the ceiling with wires having a pendulum motion which was more than noticeable. He says that ...
2
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1answer
321 views
Snell's Law of Refraction
I was told that "Snell's law of refraction implies that a light ray in an isotropic medium travels from point a to point b in stationary time." Why is this true?
Thanks.
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1answer
66 views
energy field, sum of charges [closed]
Let's say I have a linear field with one "point source" of "power" $B$. At a certain distance, I have $A = \frac{B}{d^2}$.
Now I have a grid of size $X \times Y$, with $N$ sources at random ...
2
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1answer
505 views
Effects of high frequency lighting on human vision?
I have a couple of different LED flashlights. One of them has three different "modes" of brightness, and the way it controls it is via pulse width modulation (PWM). Here is a picture that illustrates ...
3
votes
1answer
191 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 ...
3
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3answers
784 views
Conservation of energy and Doppler effect?
From what I understand, the frequency of light coming from the source moving towards an observer increases. From $ E=hv $ , this implies increase in energy of photon.
What really is confusing, is ...
2
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1answer
327 views
Is traveling at the speed of light the same thing as teleportation?
If I were on one side of the room and moved at the speed of light to the other side of the room, to an observer it would appear that I teleported. If time stops at that speed, it would be ...
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3answers
1k views
Why is moonlight less colourful than yellow light?
Just what the title states; and this isn't an original question ...
On full-moon, moonlight is almost as bright as mid-dawn ; yet there is hardly any colour visible. This is not the case with ...
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2answers
325 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 ...
7
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4answers
1k views
Why is the speed-of-light “the upper limit” rather than the speed of “particle type X”?
Basically, I can't stop wondering why light (the photon) is so special, compared to all the other particles known (and unknown) to modern day physics.
Could it be that there exists an upper limit on ...
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0answers
47 views
Light vs. X-ray interactions with Phosphor
Looking for some information on how light and xrays interact with phosphorous, or what might cause the following:
Scenario
A focus beam of light is directed at a phosphor plate at the same time as ...
0
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1answer
112 views
Name for phasor model of light
I'm looking for the name of the model of light "exploring" every path to a given point, and reaching that point with a probability proportional to the square of the resultant phasor's amplitude. (Yes, ...
5
votes
2answers
254 views
Why does a firebrand glow brightest before it burns out?
I've noticed this in match-sticks, and mango wood. After the flames die out, the brand glows red for a while. The glow is brightest right before it dies out.
My guess is that immediately before the ...
2
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4answers
468 views
Why are color values stored as Red, Green, Blue?
I learned in elementary school that you could get green by mixing blue with yellow.
...
3
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1answer
401 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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3answers
865 views
Is visible light and radio waves made of the same thing?
I understand that there is such a thing as the electro magnetic spectrum, and that light and RF are both on it, so dose that mean that they are made of the same thing? Just at different frequencies.
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5answers
2k views
Focal Point vs where you see the images
I am trying to figure out where is the focal point and where is the image. I read some information online about the point where you see the image is the focal point, but however, my supervisor ...
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2answers
295 views
On Einstein's original paper: speed of light in different reference frames
On Einstein original paper "On the electrodynamics of moving bodies", on section 2 of the first part (Kinematics), the following thought experiment is described: a rod is imparted constant speed $v$ ...
3
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1answer
285 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 ...
5
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4answers
1k views
Focusing light into an optical fiber cable
I am trying to focus light from a flashing light lamp (a bulb and then covered by a glass of 7.5 x 7cm), throught two lens (Fresnel Lens 310 x 310mm, Thickness: 2mm, Focal length: 220mm) into a ...
2
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3answers
466 views
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 ...
0
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1answer
330 views
total intensity measurement (of the whole visible light spectrum; from 400 - 800 nm) with a powermeter PM100?
I was using a power meter PM100, with a Si-photodiode sensor head, to measure the intensity of the white light source (250 W quartz halogen lamp). I have recorded the power, shown by the sensor, by ...
2
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2answers
793 views
Wave / particle duality question for sound and light
Sound is usually referred to as just "sound waves" - we do not talk about a "sound particle" and only as a wave or "matter wave."
Could something similar apply to light i.e. that there really is no ...
3
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4answers
144 views
Rainbow around Sun
From the perspective of a person, a rainbow is formed when the Sun is behind the person, and there is a critical angle made by the rainbow.
However, on several occasions, usually at noon when the Sun ...
7
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2answers
476 views
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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2answers
2k views
What happens to the absorbed light energy?
When light comes across with a solid material, some of it is reflected, some of it passes through and some of it is absorbed. I understand the reflection and passing through, but I don't understand ...
3
votes
1answer
31 views
Assuming an observer is 50 light years away, in the plane of the solar system and observing earth, what is the light flux of earth he would see?
As in, from what the planet emits and re-radiates out into space (this is going to be important if we are to image any Earth-like planet)
1
vote
1answer
128 views
How colored light can be explained if light is considered as emission of photon?
If light is considered as wave, then different colored light can be explained as waves of different wavelength/frequency. How colored light can be explained if light is considered as emission of ...
0
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1answer
1k views
What color a transparent object reflect?
We say an object's color is blue if the object is opaque, reflects blue color and absorbs waves of other color. What color a transparent object actually reflects?
3
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2answers
534 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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3answers
307 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 ...
6
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6answers
2k views
Why does light change direction when it travels through glass?
This was explained to me many years ago, by a physics teacher, with the following analogy:
"If someone on the beach wants to reach someone else that is in the water, they will try to travel as much ...
3
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2answers
2k views
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 ...
