Hot answers tagged

96

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 ...


89

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 ...


49

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 ...


41

The eyes are measuring the number of photons of each color that are hitting a given point of the retina – that are coming from some direction. This is a function of time, $f(t)$, for each point. However, when this function is changing too quickly, the eye can't see the changes. Effectively, the eye may also see the average of $f(t)$ in each period of time ...


28

From the wiki article on color vision as an illustration of how photons are absorbed: Perception of color begins with specialized retinal cells containing pigments with different spectral sensitivities, known as cone cells. In humans, there are three types of cones sensitive to three different spectra, resulting in trichromatic color vision. Each ...


24

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 ...


23

A quick footnote to Nathaniel's answer: If an image looks blurred to you it's because you are viewing it in a plane that isn't the focal plane. If you put a screen where I've drawn the red dotted line then the image on the screen will look blurred. If you measure the light in the red dotted plane then at every point in that plane the light wave will ...


22

The others have already provided good explanations, but since it sounded like an interesting question and I already sketched up a diagram, I thought I would show it, too. As already mentioned, if you have an object that is to be shown as the exact same distance as the distance between you and the screen, it's very easy to represent that: It's just a single ...


22

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. ...


20

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 ...


19

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 ...


18

Photons can be created and destroyed freely, since they don't have charge or mass. Turn on a light, and you create many photons. Any body (made of atoms) not at absolute zero temperature will spontaneously emit photons. They are consumed just as easily. Most any bit of bulk matter will absorb a photon in the electrons on the surface, transforming the energy ...


15

Let's take a simple original picture to look at - just two nearby dots on a white background. If you have bad vision, the dots look blurred. The way good vision works is to ensure that all the light hitting any particular small area of your retina comes from the same direction in front of you. Conversely, all the light coming from one direction hits one ...


15

Not on regular monitor screen. The technology necessary to achieve such effect would be holographic display, holographic in the sense of wavefront synthesis. Although this device would be a 3D display, not all 3D display are holographic. You would need technologies such as spatial light modulator. Which only exists as low specs laboratory devices.


13

Different parts of the eye have different response speed. The corner of your eye doesn't see color, but is fast; the center sees color, and is slower. This means that when you look at a 60 Hz monitor straight-on, the image is perfectly steady; but when you look at it from the corner of your eye, it is flickering. As you go to even higher frequencies of ...


13

No, it's not possible, sorry. This is because blurring (or more generally, convolution) is a lossy operation, meaning that information is lost when an image is blurred, such that it can never be completely retrieved. While there are ways to sharpen a blurred image, these are either very non-trivial or else they're only approximations - there's no way to ...


11

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 ...


11

Yes, it does. We don't see it because our brain automagically 'correct it' because it always see the same aberration from the childhood. Our eye focuses on 'green' wavelength as it's its peak sensitivity, so red and especially violet lines are usually slightly out-of-focus.


10

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 ...


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 ...


10

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, ...


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

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

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 ...


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

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 ...


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

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 ...



Only top voted, non community-wiki answers of a minimum length are eligible