Hot answers tagged

78

Good question. If you look at the spectral energy distribution in the accepted answer here, we see that photons with wavelengths less than ~300 nm are absorbed by species such as ozone. Much beyond 750 infrared radiation is largely absorbed by species such as water and carbon dioxide. Therefore the vast majority of solar photons reaching the surface have ...


55

Short answer Color-blind subjects are better at detecting color-camouflaged objects. This may give color blinds an advantage in terms of spotting hidden dangers (predators) or finding camouflaged foods. Background There are two types of red-green blindness: protanopia (red-blind) and deuteranopia (green-blind), i.e., these people miss one type of cone, ...


39

For simplicity's sake, let's really reduce this to something like photography. A camera's aperture can stay open indefinitely, allowing the plate (or whatever is receiving and recording light) to "collect and save the effect of photons" over time, if you want to phrase it that way. That allows a camera to make images that our eyes never can, for example, of ...


34

Yes. Far-red vision (>700nm) The ability for retinal-binding proteins to absorb far-red (between 700nm-850nm) light has been experimentally confirmed in this paper. While the authors did not attempt this in vivo in an animal model, they managed to use directed mutagenesis to induce a significant shift in the absorption peak of the chromophore of the ...


32

This is called a phosphene — the experience of perceiving light in the visual cortex without light actually entering the eye. This commonly happens due to stimulation of the retinal ganglion cells by something else. The most frequent source in normal individuals is pressure to the retina (e.g. rubbing a closed eye.) It is also possible for phosphenes ...


27

The field of view is determined by the light-receptive parts of the eye: the retinae (Kolb, 2012). The fovea is the region with the highest resolution. It is located in the central part of the retina and covers only about 2 degrees of the field of view (Lauweryns, 2012). The total field of view is roughly 180o (Fig. 1). Fig. 1. Schematic of the human field ...


26

There is a very different mechanism for generation (and detection) of ultraviolet, visible and infrared light vs radio waves. For the first, it is possible to generate it using chemical reactions (that is, chemiluminescence, bioluminescence) with a typical energy of order of 2 eV (electronovolts). Also, it is easy to detect with similar means - coupling to ...


22

When there is little light, the color-detecting cone cells are not sensitive enough, and all vision is done by rod cells. Cone cells are concentrated in the center of the eye, whereas rod cells are very rare in the center (image source): When you focus on the star, the light is projected close to the center of the retina, where it will hit few rod cells. ...


22

The capture area of the eye is a bit fuzzier and harder to define than that of a camera. A camera captures consistent, fully detailed data right up to the edge of its sensor, and no data at all beyond it. Captured data is clipped by an ideally uniform sensor, augmented a bit by the lens, and is well-defined during design and manufacturing. The eye can ...


20

Like these questions :) Many of these illusions come from Prof. Akiyoshi Kitaoka, a japanese Psychologist and expert for Gestalt Psychology. On his website you'll find some more fascinating illusions and questions to ask here ;) The illusion above is named Cafe Wall illusion and the newest model to explain those illusions is the contrast-polarity model. ...


20

Short answer The eyes need to adapt to the low lighting condition after you switch off the lights, a process called dark adaptation. Background The process behind the reduced visual function when going from bright ambient light to low-lighting conditions is caused by a process called dark adaptation. The visual system works on a huge intensity scale. The ...


19

The spectral sensitivity of photoreceptors expressed is the key to color vision. See figure below for the sensitivity of three-types of cone cells (S, M, L) and rod cell (R, dashed line). From this figure, one can say rod cells provide information about the "blue-greenness" of vision, however, despite their spectral sensitivity, it seems that in human ...


17

If you zoom in on the image, you can see that it is not just composed of black vertical lines, but also has pixels with different gray tones in the white areas. When you move your head sideways, you perceive the gray tones more. If you were to remove the black lines, you could see the face clearly. Initially I thought that by blurring the gray shapes when ...


17

You are asking two questions that you think are connected but are actually not. Question 1 - What is the use of eye banks? Answer: It's to store corneas for transplant for people with cornea damage. Question 2 - What use is cornea transplant to a completely blind person? Answer: It depends. If the blindness is due to clouded cornea (several ...


16

Short answer Visual acuity decreases with age. Your son's age is within the age range that visual acuities are best. Acuity starts to decrease from about age 45. Background Visual acuity (visual resolution) first increases from birth up until around 4-6 years. Note that in the following graph better acuities are represented by lower numbers (logMAR ...


16

TL;DR: We have a good physiological understanding of how eyes work, so by examination of other species' eyes, we can tell a lot about what colours they are capable of seeing. First, a little bit about the physics of colour Light is made of photons, and each photon has a wavelength. The distribution of wavelengths coming from sunlight looks like this (...


16

The simple answer is, that eye is not constructed such way. The eye have much more "pixels" than "links" to the brain and sends in "preprocessed" image. Moreover the the eye is constantly moving and scanning the "area of vision" and the body and head are supposedly also moving (willingly or not - nobody can freeze totally) so longer accumulation of data ...


15

The selection you refer in multiple species could be due to a mutual advantage. If fruits absorb visible wavelengths, they can be spotted by other animals and eaten together with the seeds. Seeds can then mature inside the host and, once eliminated with the feces, grow up a new plant in a different place. This is not only valid for light absorption, but for ...


15

Those are floaters. These are objects floating in the vitreous humour that fills the eyeball. They typically look like:


15

I used to work at an eye bank so I have a bit of knowledge about this, though some of it may be out of date. There are several aspects to an eye bank. The corneas are one of the primary things that are kept for transplantation. Of course, this will not repair blindness in someone that has problems in other areas of the eye, but corneal transplants are ...


15

March Ho's answer is quite good. A few extra tidbits: The population of humans contains DNA encoding for two substantially different M receptors. (differing by more than normal variation) See tetrachromacy in humans. (This doesn't produce IR vision except ...) The population of humans contains DNA encoding for several different L receptors. Two well ...


15

There seems to be some evolutionary advantages to red-green colorblindness. The paper in reference 1 (a summary can be found in reference 2) shows that people with red-green color blindness can differentiate between much more shades of khaki than unaffected people. This might help detecting camouflaged food in a green environment. Reference 2 quotes an ...


14

Cone cells are each connected to their own neurone. This allows them a great deal of resolution as the brain can interpret the exact position of the cone cell that was stimulated by a light photon. However in order to improve low light vision, multiple rod cells are connected to a single neurone - this is called summation. Whilst it does allow for an ...


14

Short answer Peripheral vision is more light-sensitive than central vision. Background When you look directly at an object the image is projected onto the fovea. The fovea has maximal visual acuity (high resolution) and a high density of cones, which are specialized photoreceptors to sense colors. However, cones are not very sensitive to light. Here is a ...


14

Your retina contains both rods and cones. Cones are color sensitive, slow, and concentrated near the center of your field of vision. Rods are "light" sensitive, fast, and concentrated near the periphery. You want to be able to respond quickly to a threat "in the corner of your eye" without needing to see the color of the threat. This is nicely explained by ...


13

Short answer Spectral sensitivity of cats indeed ventures into the UV, but not beyond ~320 nm. Their maximum is likely similar to ours, i.e., ~750 nm. Background The spectral sensitivity of blue cones (photoreceptors detecting low wavelengths) of many species, including humans and cats, extends into the UV range (Fig. 1). Fig. 1. Human cone absorption ...


13

The differences at the photoreceptor level have been addressed by others. The mechanical restrictions of the visual system were shortly hinted at by @gilhad et al., but deserve more attention in my opinion. First off, in darkness we cannot focus on an object and our eyes will move. And even when we focus on a specific point there is always movement of the ...


12

This is a very good question. Red light is routinely used by scientific laboratories to do low light dissections of retinas, and of course it is used in other low light contexts such as printing plate development. In both of the above contexts, you have a clear subject: the retina being dissected or the printing plate being developed. In the case of the ...


12

I'll address the question in the title "At which time did sight evolve for the first time?" by assuming that by the evolution of vision, we mean the evolution of the eye. Molluscs are an excellent phylum to investigate this question because they exhibit a wide range of eye designs and levels of complexity. At the most basic level, limpets such as Patella ...



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