# Tag Info

90

As mentioned in a number of other answers, there are three different color receptors in a typical person's eye. They respond to different wavelengths of light, as can be seen in the below diagram from wikimedia. The $x$-axis is wavelength in nanometers, and the three curves represent the three receptors' response at those wavelengths. Any incoming light ...

84

Fun question! As you pointed out, $$\theta \approx 1.22\frac{\lambda}{D}$$ For a human-like eye, which has a maximum pupil diameter of about $9\rm mm$ and choosing the shortest wavelength in the visible spectrum of about $390\rm nm$, the angular resolution works out to about $5.3\times10^{-5}$ (radians, of course). At a distance of $24\rm km$, this ...

48

The eye is sensitive to light with a wavelength in the range from about 700nm to 400nm, and for the non-colour blind all wavelengths in this range are detected by one or more of the cone cell types. So there are no hidden colours in this range. Light outside the 700-400nm range can't be seen, so I suppose you could claim these are hidden colours, but then ...

23

The reason "myopic" people see Monroe and others see Einstein is that the high frequency information in the image says Einstein and the low frequency says Monroe. When looking at the image closely, you seen the high frequencies and therefore Einstein. By looking at it out of focus (presumably what is meant by "myopic"), the high frequencies are filtered ...

20

You can't see clearly underwater for a couple of reasons. One is the thickness of your lens, but the main one is the index of refraction of your cornea. For reference, here's the Wikipedia picture of a human eye. According to Wikipedia, two-thirds of the refractive power of your eye is in your cornea, and the cornea's refractive index is about 1.376. ...

19

It really depends on what you mean by colour. If by colour you mean "the human brain's response to a given combination of wavelengths", then by definition there can be no invisible colours; wavelengths combinations that do not stimulate any cones in the eye are just equivalent to black. If by colour you mean "a given combination of wavelengths", then we ...

18

Let's first substitute the numbers to see what is the required diameter of the pupil according to the simple formula: $$\theta = 1.22 \frac{0.4\,\mu{\rm m}}{D} = \frac{2\,{\rm m}}{24\,{\rm km}}$$ I've substituted the minimal (violet...) wavelength because that color allowed me a better resolution i.e. smaller $\theta$. The height of the knights is two ...

10

Take the following idealized situation: the person of interest is standing perfectly still, and is of a fixed homogeneous color the background (grass) is of a fixed homogeneous color (significantly different from the person). Legolas knows the proprotions of people, and the colors of the person of interest and the background Legolas knows the PSF of his ...

10

We have color perception because we are trichromats. In our genes there is code for three slightly different light-sensitive molecules. The light-sensitive cells in the retina are called cones, and neighbouring cones each produce one of the different versions of the light-senstive molecule. So each of the three cone-types responds slightly differently to the ...

9

This amazing image looks like physicist Albert Einstein. However, move a few feet away from the screen and suddenly it'll transform into Marilyn Monroe. The work of Aude Oliva and her colleagues at the Massachusetts Institute of Technology, the illusion was created in three steps. First, the researchers obtained a photograph of Marilyn Monroe and ...

8

The use of anything but properly designed sunglasses is very foolish and poses great risks to your long term sight, and maybe for reasons that many people do not wholly appreciate. First of all, let's write down what unpolarised light is. We choose two basis polarisation states: let's go with left and right polarised light in this case since you say that 3D ...

8

Quickly, try this: Imagine blindingly bright red light! Now blue! Now yellow! You could see stark differences as you shifted from color to color, couldn't you? Yet if you think about what just went on inside of your head, it didn't involve any color photons going into your eyes, did it? So, what you just did must be separate from the light frequencies ...

8

Am I right ? Yes. If so, what lenses should one wear in order to see clearly while under water ? You don't need extra lens you have one in your eyes, just use goggles that makes a layer of air between the water and your eyes. If you decide to put a convergent lens in front of your eyes it won't work because your eye will still not be able to ...

7

Around 555nm wavelength - green color. That's why green lasers are soo cool even at 10 mW :-D

7

Dear Rootosaurus, when you're looking at an image of a chair behind you in a flat mirror, then you're observing the so-called virtual image of the chair. If the mirror's surface is located in the $x=0$ plane and the coordinate of the real chair is at $(x,y,z)$, then the virtual image of the chair is at $(-x,y,z)$. However, the light rays coming from the ...

7

This is what I think the first bit of the calculation does. Suppose you start with a spherical eye with a hole in it (e.g. the pupil in the human eye): The radius of the eye is $ER$ and the radius of the hole is $AR$, and with the length $DA$ these form a right angled triangle. Pythagoras' theorem tells us: $$DA^2 + AR^2 = ER^2$$ so:  DA = ...

7

Compared to naked eye view, a telescope image never increases surface brightness. This fact is related to the concept 'etendue'. However, although the image formed on your retina is never brighter than the corresponding naked eye image, the image through a telescope is magnified. This means that looking through a telescope at the sun can expose your whole ...

6

I believe you should have googled first: google hits Especially the second link very clearly explains the main reason: The primary reason why the color red is used for danger signals is that red light is scattered the least by air molecules. The effect of scattering is inversely related to the fourth power of the wavelength of a color. Therefore blue ...

6

One thing that you failed to take into account. The curve of the planet (Middle Earth is similar in size and curvature to Earth). You can only see 3 miles to the horizon of the ocean at 6 feet tall. To see 24 km, you would need to be almost 100m above the objects being viewed. So unless Legolas was atop a very (very) tall hill or mountain, he would not have ...

5

It's because our eyes have a 2D array of photo-sensitive receptors. Using two eyes you can 'detect' the third (depth) component. If you consider each eye to be a single point (for simplicity), the distance between the eyes themselves is a given 'constant'. A distant object in your vision, seen by both eyes, will be in a different position for each eye. You ...

5

One can give a highly qualified, but definite "yes" in answer to your question. Contrary to popular belief, if it weren't for the UV, then staring at the Sun would not be a particularly hazardous thing to do for the majority of people. This is why I said "highly qualified" - for people with certain conditions, simple staring at the Sun may be hazardous, ...

5

Yes, It's true... We know that our eyes have three types of cone cells - S (short), M (medium) & L (large). The naming is done in order to differentiate the cells from "which cell absorbs which color". S to Blue, M to Green and L to red. The peak wavelength of L is 564 nm, yellowish-green. The peak of M is 534 nm, bluish-green. The peak of S is 420 nm, ...

5

Deconvolution can work but it only works well in case of point sources as e.g. pointed out here. The principle is simple; the blurring due to the finite aperture is a known mathematical mapping that maps a hypothetically infinite resolution image to one with finite resolution. Given the blurred image, you can then attempt to invert this mapping. The blurred ...

5

The Wikipedia article Aberrations of the eye discusses this. The article is really about non-normal aberration i.e. the sort of thing that makes us have to wear spectacles, but it also mentions the aberrations in normal eyes. The conclusion is that normal eyes are actually pretty good with just a small amount of spherical aberration. The lens and cornea form ...

5

The answer is that your eyes would be focusing not at the concrete distance where the mechanism is placed, but at a virtual image which appears farther away. This is just basic optics. It is (one aspect of) what happens when you use a magnifying glass, for instance. The virtual image of some point viewed through an optical instrument is that location in ...

4

due to the curvature of the earth, you can only see flat land for about 5km give or take a few hundred meters. from a sky object you are limited by the photon flux emitted or reflected by that object (50-150 photons in a 1ms burst, and all landing within a 10 arcminute spot on your eye) the wavelength of those photons affect your sensitivity (green photons ...

4

Unless you deal with holograms, all displays we are observing follow the laws of ray optics. So the display has some specific location – for example, rectangle between four points with certain coordinates – and all the light rays leaving from the points of the display carry the information about their origin because they are diverging from that point, and if ...

4

This is more of a psychology question. After you start seeing things, you notice that a certain side of your vision is the part that will see your finger if you touch your forehead, and the other side will see your finger if you touch your lips. We designate the first as "up" and the second as "down". The brain just gets a bunch of signals. "up" and "down" ...

4

In the spirit of your question, having two eyes and assuming you can use them as an array (which requires measuring the phase of the light-something eyes don't do) allows you to use the distance between them for $D$ in the resolution equation. I don't know the spacing of an elf's eyes, so will use $6 cm$ for convenience. With violet light of \$\lambda = ...

4

No. The world we observe with our five senses is three dimensional. Two independent measurements are enough to calculate the three dimensional position of everything, which is what our brain does with the input of two eyes. More eyes would only over constrain the solution, and might help in low lighting or long distance estimates when the errors are large. ...

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