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97

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 ...

55

Is blurred effect due to turbulence? No, it is not. The turbulence has a little effect here. Even if there is no turbulence, one see everything blurred underwater. The reason is explained below. An eye is a natural lens. A clear shot of something you see depends on how well the image is focused on your eye. The most of the refraction in the eye occurs ...

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 ...

39

This is from the Physics FAQ article that I wrote 15 years ago: If shorter wavelengths are scattered most strongly, then there is a puzzle as to why the sky does not appear violet, the colour with the shortest visible wavelength. The spectrum of light emission from the sun is not constant at all wavelengths, and additionally is absorbed by the high ...

36

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. ...

29

Let me start by clarifying that I assume the question is whether a superhuman or any object of human size can render itself invisible through speed alone. And that the speed of said object must be $v\ll c$. From this, I assume that the object or person being viewed must spend a reasonably long amount of time within the observer's field of view such that ...

28

This is actually a really good question. (And I'm not one of these people who insists that there's no such thing as a dumb question; I just think we shouldn't be embarrassed to ask dumb questions. Anyway, this isn't a dumb question.) As you may know, collisions between two protons (like those the LHC usually does) can produce many different types of ...

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 ...

22

Your eye is a second optical system. It re-focuses the diverging rays to produce a real image on the retina. This process is exactly the same thing it does when looking at a nearby (i.e. not at effective infinity) object.

22

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 ...

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

As previous answers have stated, the wavelength (or frequency) and intensity of the beam are important, as well as the type and amount of impurities in the air. The beam must be of a wavelength that is visible to humans, and fog or dust scatters the light very strongly so that you can see it. However, even in pure, clean air, you will be able to see a laser ...

19

The two effects are not related. The size appearing larger is a matter of some speculation to this day, but it is purely a psychological effect. If you want to prove this, take a look a the moon while standing up and looking between your legs. It won't look nearly as large. The red/orange color is related to the sunset being red. In fact, it's the same ...

19

(Source, Wikipedia Commons) The moon is generally called a "Harvest Moon" when it appears that way (i.e. large and red) in autumn, amongst a few other names. There are other names that are associated with specific timeframes as well. The colour is due to atmospheric scattering (Also known as Rayleigh scattering): may have noticed that they always ...

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 ...

19

I would think an object can be invisible to a human if it moves so fast that, within the time it passes the field of view of the human, it reflects too little light to be detected visually (human vision has very high, but still limited sensitivity to light). On the other hand, if the object moves so fast in air, it will produce a lot of noise and probably ...

19

If you consider a rotating propeller, it has the following properties: you can see that something is there you cannot see what it is; you just happen to know it you cannot count the blades or really distinguish the features at all you cannot even tell the distance to the blurry "thing" in front of you people are known to walk into running propellers ...

18

Because Maxwell's equations are linear. Equivalently there is no elementary photon-photon interaction. If there were, say, a quartic photon interaction then you would be able to see a beam of light directly instead of seeing its interaction with dust particles.

17

A mirror, or a perfect mirror at least, is the same colour as a perfectly white sheet of paper. Both a perfect mirror and a perfectly white sheet of paper reflect all the light that hits them. The difference is that the paper scatters the light so what reaches your eye is a mixture of all the light hitting the paper, while the mirror reflects the light ...

16

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.

15

In the 19th century, the physicists Young and Helmholtz proposed a trichromatic theory of color, in which the eye was modeled as three filters with overlapping ranges. This is essentially a physical model of the pigments in the eye, and it predicts the response of the nerve cells at the retina. Helmholtz did related work on sound and timbre. Ca. 1950, ...

14

This happens because the spectral response for each red/green/blue pixel in cameras don't exactly match the spectral response of the receptors of your eyes. For example, check here and here, and compare them to the human eye (and read the whole wiki article for interesting details on human color perception). What digital cameras do, is to try to mimic your ...

13

Your eye has three types of receptor cells, the sensitivity of each type peaking in different spectral regions. Roughly speaking, there's one that peaks in red, one blue, one green. (It's not quite so clear cut, but your brain is really good at sorting out messes like this!) When you look at a fire engine (assuming it's red) It's mostly the red receptors ...

13

Human color vision is based on four types of receptors in the retina: rods, and three types of cones. Their response to different wavelengths is shown in this graph: . It shows clearly how certain wavelenghts, mostly around the yellow-green portion of the spectrum, are absorbed more strongly, and by more types of cells, than the rest. So it is normal ...

13

In bright sunny conditions, the eye's pupil shrinks to about 1mm diameter. If you look straight at the Sun with the pupil in this state at noon, when intensity is of the order of $1000{\rm W\,m^{-2}}$, your eye will therefore focus about $0.7{\rm mW}$ onto the retina. This is actually considerably below the amount of heat the superbly densely envasculated ...

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 ...

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