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69

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 ...


27

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 ...


24

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

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 ...


21

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. ...


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 ...


19

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. ...


18

The "normal" visual acuity in the Anglosphere is the 20/20 vision which means that on a Snellen chart characters with 8,86 mm height can be read conveniently from a distance of approximately 6 m. Now there are many known people who have very sharp eyes, having the doubled acuity of 20/10. This means they can discern characters of only 4,43 mm height from ...


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 ...


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 ...


14

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 ...


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

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 ...


13

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


13

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 ...


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 ...


11

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 ...


11

I do not know how to explain to a 6 year old how we are able to perceive colour. Does anyone know how this can be explained? Well, depending on the depth you want to introduce her to, it can be difficult to explain to adults - much less children. You explained the basics well enough. Without going to the molecular mechanisms, here's a useful diagram: ...


11

You will be interested in Aphakia, which is the lack of an eye lens usually through surgery but sometimes from birth. These individuals supposedly see UV as a whitish-blue or whitish-violet: This appears to be because the three types of colour receptor (red, green and blue) have similar sensitivity to ultraviolet, so it comes out as a mixture of all ...


11

It's caused by a sudden shift in the pressure needed to circulate blood to your brain which your body fails to respond to sufficiently quickly. This results in a sudden loss of blood pressure termed Orthostatic Hypotension which, in term, results in a transitory reduction in the blood supply necessary for brain function. You experience a momentary loss of ...


11

The general shape of the field of view has been answered in the related question "If human eyes watch an area, what's the shape of its capturing shape? Rectangular? Half spherical?" Regarding the central view: yes, foveal view has a higher acuity than peripheral view. Regarding the numerical dimensions of the monocular and binocular field of view: ...


11

Preamble & Overview. This is a rather unsatisfying answer I'm afraid. I can't seem to find any animal that has exceptional eyesight and sees in monochrome. Baring in mind the context of humans vs machines; machines cope better with shapes rather than colours however it generally appears that those animals which rely on exceptional eyesight to survive ...


10

While the answers to date are correct regarding the wiring of rods and cones in the primate (specifically human) eye, they are also fundamentally wrong. Neither rods nor cones perceive color. The brain does. The rods and cones are just the receptors providing signals. The first answer in fact says this in its very last sentence. As one answer says, during ...


10

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 ...


9

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 ...


9

Here is a comparison of the range of wavelength sensitivities for both rod cells (labelled R) to the 3 subtypes of cones cells (labelled S, M and L) from Wikipedia. If one is exposed to red light (above ~650 nm), it would activate the L-type cones mainly (possibly some M-type activation), but no rod activation. Rods are the low light receptor cells in our ...


9

"To me, it also makes sense that the evolution of sight would have accompanied the evolution of advanced brain functions in almost every case." Not necessarily! For instance, think of phototropism: the plant detects the presence of light and uses it to grow towards the light, but that's very simple process regulated by auxins. Or the light-sensitivity ...


9

No-one can re-implant an entire eye, because the optic nerve has been severed in one who has lost an eye. A cornea can't be grafted to a glass eye. But blindness isn't only caused by loss of the entire orbit. It's also caused by cloudy corneas, which is the purpose of eye-banks. The optic nerve is a cable of nerve fibers that carry visual information from ...



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