# Tagged Questions

Physical processes involved when seeing, and comparisons between with other light detection systems. Includes questions about the eye, optical nerve, brain, corrective lenses, etc.

2answers
109 views

### Can I see the light from 10 million km away?

If light is switched ON, only for a second, and the distance between the observer and the light source is 10 million kilometers, can I still see the light spark? For example, let's assume that the ...
2answers
47 views

### Hyperopia, Far Sightedness

With hyperopia, the focal point is behind the retina, shouldn't this mean that the image is flipped on the retina itself from what is usual? I must be drawing my ray diagrams wrong. A little ...
2answers
265 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 ...
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 ...
3answers
184 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?
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 ...
2answers
177 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 ...
1answer
128 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 ...
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 ...
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 ...
4answers
228 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...
1answer
72 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 ...
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 ...
3answers
13k 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 ...
2answers
131 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 ...
10answers
4k 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?
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 ...
1answer
155 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. ...
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 ...
1answer
178 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 ...
2answers
140 views

### Is Invisibility possible according to physics?

Is Invisibility possible according to physics? Is there any backing theory to prove it true or false?
2answers
71 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 ...
3answers
638 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?
2answers
176 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 ...
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 ...
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 ...
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?"
1answer
126 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 ...
2answers
444 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 ...
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 ...
4answers
2k 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 ...
5answers
772 views

### Why is Light invisible?

Why can't we see light? The thing which makes everything visible is itself invisible. Why is it so?
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 ...
1answer
791 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 ...
3answers
376 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?
4answers
546 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 ...
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 ...
1answer
567 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 ...
2answers
923 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 ...
1answer
68 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 ...
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 ...
1answer
502 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 ...
1answer
216 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 ...
1answer
115 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 ...
1answer
544 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 ...
2answers
545 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?
4answers
464 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. ...
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