Hot answers tagged

196

There is no such upper bound. As a simple counter-example, consider a thin right-angled solid cone of base radius $r$ and height $h$, observed on-axis from some large(ish) distance $z$ away from the cone tip. You then observe the tilted sides, of area $\pi r\sqrt{r^2+h^2}$, and you don't observe the area of the base, $\pi r^2$, so you observe a fraction \...


191

As a completely tangential type of answer. Consider a neutron star; due to the General Relativistic bending of light in curved space we are not bounded by the dull constraints of Euclidean geometry! If the radius falls below 1.76 times the Schwarzschild radius for its mass$^1$, then then all of the surface is visible, when viewed from any direction (e.g. ...


179

Your brain adjusts your perception of color to compensate for lighting that is strongly tinted. This was the reason for the violent conflict some time back about a certain dress. Depending on whether people perceived the dress was being lit by yellow-tinted or blue-tinted light, they saw either a black and blue dress or a white and gold dress. Here's an ...


118

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 $9\ \mathrm{mm}$ and choosing the shortest wavelength in the visible spectrum of about $390\ \mathrm{nm}$, the angular resolution works out to about $5.3\times10^{-5}$ (radians, of course). At a distance of $24\ \...


115

Something special about the visible range is that water has low absorption in this range. It’s a rather sharp dip near the visible region. Since we know that life began in water, the beings that were receptive to these wavelengths had a significant advantage over the others. Thus natural selection would have favoured these life forms over the others. This ...


88

Color perception is entirely a biological (and psychological) response. The combination of red and green light looks indistinguishable, to human eyes, from certain yellow wavelengths of light, but that is because human eyes have the specific types of color photoreceptors that they do. The same won't be true for other species. A reasonable model for colour ...


82

Fermat's principle says that the direction of travel for any light ray can be reversed. Therefore there is always a line of sight between a pair of eyes in both ways. If one person is in the dark, then only one person can see the eyes of the other. So there needs to be enough light reflected from both person's eyes for this to work.


68

There are a couple of issues here. A pink (#FF00FF) object appears pink not because each atom is pink (there is no wavelength of light that is perceived to be the same pink by the ordinary human eye. What is happening is that a pink object is emitting (or reflecting) light of multiple wavelengths that enter the eye and are detected and processed to allow us ...


64

Yes - we are surrounded by a "sea of photons". An individual object that reflects light (let's assume a Lambertian reflector - something that reflects incident photons in all directions) sends some fraction of the incident photons in all directions. "Some fraction" because the surface will absorb some light (there is no such thing as 100% white). The ...


61

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 ...


61

The key is that light must enter the eye for you to see something. You cannot see a beam of light from a low powered laser which is not directed into your eye if the air through which the light is travelling is devoid of dust. Adding dust to the air and you can see the trajectory of the laser beam because of the light being reflected/scattered from the ...


56

You're not seeing the photons in the beam that are traveling from A to B (beam starting point to beam destination), you are seeing photons that are scattering off of dust particles that are in the path of the beam. This is the reason why you see lasers in a night club more clearly when there is a smoke machine, and why cat burglars blow dust onto security ...


54

On the most basic level, the answer is a flat no. The seven primary notes in an octave is specific to the western musical tradition. It's not entirely arbitrary as you say, but there are many other choices that could have been made, and there are other cultures who use fewer notes (e.g. pentatonic scales in blues music) or more (e.g. Indian classical music). ...


48

Yes indeed, infrared light (the wavelengths beyond those of red light) can be very harmful to your eyes even though you don't see them. The same applies for ultraviolet light (the wavelengths beyond those of violet light). You can read more under the topic of laser eye safety. People that work with lasers need to use safety glasses if these lasers fall ...


44

The least distance of distinct vision is the minimum distance your eye lens can focus on an object without any strain. This means the eye is in a relaxed state. But eye is a self adjusting lens. When you try to see an object closer than 25 cm(for a normal eye), your eye automatically adjusts the focal length thus decreasing it. This is why your eye gets ...


44

It's partly about how human colour vision works, partly about avoiding colours you want to keep, such as those of the actors. Colour cameras record concentrations of red, green and blue light to mimic human colour vision. Before digital techniques, blue screens were preferred because, of the three primary colours, that's the one rarest in human skintones. ...


44

The human eye focusing is resolving all the possible detail it can from a scene that is sharp and not distorted. The details of exactly how your brain forms an image from what your eye does is extremely complex, but the basics are : sharp initial image can be focused on to produce sharp image. The blurry photo cannot be sharply resolved in that way because (...


43

The answer of Martin Ueding is correct if there is no intermediate image in the light path. For example, if you use a camera obscura, in general there will in be no way for the observed person to create an image of your eye. So the answer is NO for direct light paths and YES if you allow intermediate images.


42

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 ...


42

That research shows that humans can detect single photons, not that we're particularly good at it. Averaging across subjects’ responses and ratings from a total of 30,767 trials, 2,420 single-photon events passed post-selection and we found the averaged probability of correct response to be 0.516±0.010 (P=0.0545; Fig. 2a), suggesting that subjects could ...


42

The reflected light is moving toward/into your eye, while the light just passing by you isn't. You can see light that's not "reflected", like the light emitted by a light bulb, there's nothing special about reflected light. All that's needed to see light is the light actually hitting your retina.


42

You are correct that almost always it is the UV content of sunlight and not its power that is the main hazard in staring at the Sun. The lighting during a total eclipse is one of those situations outside the "almost always". Eclipses did not weigh heavily on our evolution, so we are ill kitted to deal with them. Moreover, UV sunglasses are not designed to ...


40

Our ability to separate different colors from each others depends crucially on how many different receptors we have for colored light. Humans have three different receptors for light, which means that we can characterize colors by three numbers, just like the RGB-codes of colors on your screen. At the end of the day, what determines with colors we perceive ...


40

Here's an explanation using geometric optics. I'll replace the human eye, which has a quasirandom scattering of light sensors on a curved surface, with a digital camera that has a regular grid of sensors on a flat surface. This doesn't alter the problem in any essential way. Let's also not worry about color. Say the camera is focused on a (monochrome) LCD ...


38

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 ...


36

The damage to your eyes comes from the total energy from the visible and near - infrared region even when you wear a 100% UV blocked sunglasses. When you look at the sun in normal days, the visible light from the sun itself is enough for your eyes to trigger pupillary constriction and blink reflex in order to give you at least partial protection. But when ...


36

Yes, but not with equal amounts of each. In order to answer this, we need to understand the CIE 1931 color space, and think about its algebraic properties. Essentially what the CIE specification says is that, while light comes to us as a spectrum filled with varying amounts of photons in the wavelength range 380-700nm, our eyes are engineered in such a way ...


35

The range of visible light wavelengths has a special property that makes it the commonly used range for all life forms on the Earth: It is the range of electromagnetic wavelengths that are short enough to be conveniently handled by cell sized detectors and that can pass through the atmosphere. The Earth's atmosphere is not transparent at all wavelengths, ...


34

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 individual ...


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