Physical processes involved when seeing, and comparisons between with other light detection systems. Includes questions about the eye, optical nerve, brain, corrective lenses, etc.
37
votes
10answers
3k views
Is it possible that there is a color our human eye can't see?
Is it possible that there's a color that our eye couldn't see? Like all of us are color blind to it.
If there is, is it possible to detect/identify it?
13
votes
1answer
2k views
Why can you see virtual images?
In optics it is widely mentioned real images are projectable onto screens whereas virtual ones can only be seen by a person. Isn't that contradictory? I mean in order to see the virtual image it has ...
12
votes
3answers
12k views
Why does the moon sometimes appear giant and a orange red color near the horizon?
I've read various ideas about why the moon looks larger on the horizon. The most reasonable one in my opinion is that it is due to how our brain calculates (perceives) distance, with objects high ...
10
votes
0answers
475 views
Why does the moon look bigger at the horizon? [duplicate]
Possible Duplicate:
Why does the moon sometimes appear giant and a orange red color near the horizon?
Why does the moon look bigger at horizon or skyline than at other times e.g. at ...
7
votes
5answers
756 views
Why is Light invisible?
Why can't we see light? The thing which makes everything visible is itself invisible. Why is it so?
6
votes
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 ...
6
votes
2answers
163 views
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 ...
6
votes
1answer
130 views
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. ...
6
votes
2answers
891 views
Are regular light bulbs better for the eyes than CFLs or “tube lights”?
I've heard that regular light bulbs with a filament are better for the eyes. Is the spectrum of one worse than the other? If so, are there any regulations for their use in industrial settings for ...
6
votes
1answer
30 views
Refraction and scattered light for NLCs
For helping with judging NLC candidates (are they NLC or not) I have a set of formulas to calculate the minimum altitude (in km) of the candidate given an observed altitude (in degrees) of the ...
5
votes
5answers
2k views
Eyes open under water
Yesterday I looked underwater with my eyes open (and no goggles) and I realized I can't see anything clearly. Everything looks very, very blurry. My guess is that the eye needs direct contact with air ...
5
votes
2answers
503 views
Infrared remote flashes blue light in camera
I know that if you held an infrared remote in front of a digital camera, it'll flash a blue/purplish light when you press the buttons. Why?
5
votes
2answers
371 views
Why do green lasers appear brighter and stronger than red and blue lasers?
This is mostly for my own personal illumination, and isn't directly related to any school or work projects. I just picked up a trio of laser pointers (red, green, and blue), and I notice that when I ...
5
votes
2answers
123 views
What is the minimum optical power detectable by human eye?
If one is in complete darkness, what is the minimum optical power that the eye can "see" (let's say in 500-600 nm range).
I found that for 510 nm, 90 photons can be detected ...
5
votes
1answer
1k views
How much sky do we see at any one moment?
When we look at any particular point the sky, what percentage of the celestial sphere do we see?
This question arises from the notion that on average there passes one meteor per hour overhead. So ...
4
votes
2answers
67 views
Is there a good explanation for the observation of Martian canals?
Martian "canals" have been observed by independent observers after their first description. Now, they are attributed to "optical illusion", but I think that this is not a good choice of word, because ...
4
votes
1answer
72 views
Human eyes vs aberrations
There are no perfect lenses in nature. Aberrations of some kind will always be there. But why healthy human eyes circumvent this issue? Or, are there any aberrations we don't "see?"
4
votes
2answers
164 views
How photons represent colors that you see?
Right now, my understanding is that, a mixture of photons of many different frequencies is perceived as white by your eye. While no photons at all, is perceived as black. And photons with the blue ...
4
votes
1answer
31 views
Vision vs. limiting magnitude
Does anyone know how the acuity of your vision translates to a difference in limiting magnitude?
e.g., the kind of answer I'm looking for would be
"For each factor of 2 improvement in your vision ...
3
votes
4answers
204 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...
3
votes
1answer
718 views
Purple doesn't occur in rainbow - or does it?
Usually, when asked whether the purple color exists rainbows, an answer similar to this is given:
The purple color is perceived by human eyes via the activation of both red-sensitive and ...
3
votes
1answer
548 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 ...
2
votes
4answers
450 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.
...
2
votes
3answers
161 views
Why we see upright images?
Since it's convex lens there in our eyes so image formed on our retina is inverted, so how come that we see upright images?
2
votes
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 ...
2
votes
6answers
2k views
Limit of human eye flicker perception?
I am designing a LED dimmer using software-controlled Pulse Width Modulation, and want to know the minimum PWM frequency that I must reach to make that LED dimming method indistinguishable from ...
2
votes
1answer
209 views
How to determine the required resolution to view an image at a certain size?
If I have several camera specifics, can I determine the required resolution to view an image at the size I want?
For example, let's say I want a picture of a square painting to be at least ...
2
votes
1answer
116 views
Can an invisible light source cast shadows?
Let's assume that we have a mechanism for producing EM radiation suspended in the air, and that that mechanism itself is invisible to the naked eye (e.g. a microscopic light bulb on a microscopic wire ...
2
votes
1answer
517 views
How to determine the field of view for a pinhole camera with given parameters?
I am reviewing some old material I learned years ago and am having trouble figuring this one out. Can somebody confirm that I have done my math correctly, and tell me how I can recover the field of ...
2
votes
1answer
481 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 ...
2
votes
1answer
228 views
Why does light of high frequency appear violet?
When people are asked to match monchromatic violet light with an additive mix of basic colours, they (paradoxically) mix in red. In fact, the CIE 1931 color space chromaticity diagram shows this ...
1
vote
3answers
607 views
Are dangerous rays emitted during Solar Eclipse?
It is said one should avoid staring at Sun as it can damage the eyes, but it is also said that one should not come out in sun during eclipse as it emits dangerous rays. Is that true? If yes, why?
1
vote
4answers
1k views
Why is there a difference between additive and subtractive trichromatic color theories?
Helmholtz distinguished between additive and subtractive trichromatic color theories.
Additive theories concern optical combinations of colored light sources and are usually modelled on RGB while ...
1
vote
1answer
111 views
How does the focal length of the lens change the distance from the camera to an object?
I am reviewing some old image processing notes and have the following problem:
I have a camera with a projection plane of $2cm\ by\ 2cm$ and a $15mm$
lens. If you want an $1.5m$ object to take ...
1
vote
4answers
514 views
Explanation about black color, and hence color
I'm bit confused about 'black' as a color. As per my knowledge, it is not given in visible color spectra like other colors for example red, violet etc. Also I'm confused with definition of color--does ...
0
votes
3answers
369 views
What colour is nothing?
To me this is very confusing, but I hope we can discuss it and find a solid answer to the question.
If you were somewhere where there was absolutely nothing, what colour would your eyes see?
0
votes
2answers
138 views
Is Invisibility possible according to physics?
Is Invisibility possible according to physics? Is there any backing theory to prove it true or false?
0
votes
2answers
81 views
Can I see the light even from 10 lakh (1 million) km?
If light is switched ON, only for a second, and 10 lakh kilometer (which is $10^6$ km) is the distance between the observer and the light source, can I still see the light spark?
For example, let's ...
0
votes
1answer
67 views
How to determine the angle a camera has to be at to image a desired scene, given the camera parameters?
Reviewing old material I learned years ago, having a hard time rewrapping my head around the relationships in capturing images from a camera.
Imagine a scenario with the camera placed on the ground ...
0
votes
1answer
67 views
The color purple in a rainbow
In a rainbow the colors order is red then orange (made from red and yellow, thus making sense that it appears in between them) the yellow followed by green after which comes blue (again green formed ...
0
votes
0answers
9 views
Why are trichromat cone cells unable to sense ultraviolet and infrared radiation? [migrated]
I understand that cone cells vary in the color they sense, is this because of wavelength, frequency, something else, or a combination of the previous? I also understand that tetrachromats can see an ...
0
votes
0answers
74 views
What gives observable light its colors? [migrated]
I know that difference between different colors of light is difference between their wave length but I don't know what gives beautiful colors (like rainbow colors) to different wave length of ...
0
votes
0answers
61 views
How to convert a hologram into an image?
Suppose one knows in full detail the phase and intensity of monochromatic light in a plane. This is basically what a hologram records, at least for some section of a plane. By using this as the ...
0
votes
1answer
69 views
What is the new distance for resolution of the images? [closed]
The taillights of an automobile are $1.25\:\rm{m}$ apart. Assume the pupil of a person's eye has a diameter of $5\:\rm{mm}$ and the light has an average wavelength of $604\:\rm{mm}$. At night, on a ...
-1
votes
1answer
175 views
How can you test what color different people perceive? [closed]
If I would show someone a yellow object and ask them, "is this object yellow?"
That person would say "yes".
But I could never know if my perception of the color yellow is the same as that other ...
-1
votes
1answer
124 views
How do photo mosaic work from eye and image processing perspective? [closed]
Hello fellow investigators
I have two question about optical illusions
1) A photo mosaic is something like this http://i.stack.imgur.com/Pzplp.png
What are the optical principles behind our eye ...
