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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

Very nice question! First of all, the 'smallest size' that a human eye can perceive is called visual acuity, and can be expressed in various ways. It cannot simply be expressed by means of size measures, as objects with a fixed size are perceived as smaller when viewed from a distance (perspective). A familiar example is the train track: Hence, visual ...


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 ...


20

Short answer The visible spectrum has the highest energy in sunlight at the earth's surface, explaining the gross location of the visible spectrum in life on earth. The specific frequency range varies across species and can be explained by species-specific survival strategies. Background When you look at the solar light spectrum at the earth's surface the ...


19

Depth perception consists of what are called monocular cues and binocular cues. As you mention, binocular vision has a lot of advantages for depth perception, but it is not completely necessary. Many animals, particularly those that don't need especially precise vision, have little to no binocular vision, opting instead for visual coverage in more ...


18

You can't catch conjunctivitis by looking at someone, but you can spread it around by touching your eye and touching something else, which then can pass the virus to someone else (viral conjunctivitis can be highly contagious). There are no hard facts, but one estimate is that people touch their faces an average of 3.6 times per hour, more if your eyes are ...


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 diseases ...


17

Short answer In humans it is basically the red choroid plexus in the back of the eye you are seeing on a flashed photo, while it is the green-reflecting tapetum lucidum in dogs. Background The red-eye effect in humans was explained nicely by Yale Scientific Magazine, and I adapted the following text from that source: The human eye can effectively adjust to ...


17

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 scale)...


15

The basis of this question is a common misconception, and unfortunately the accepted answer by @CHM is also based on this common misconception. The misconception is based on the homunculus falacy: the tendency for people to think that the image that lands on the retina is somehow 'assembled' and presented for something (the 'consciousness') to view. This is ...


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

This is because of the soaps chemical nature. They are alkaline agents and human eyes have a very low tolerance for alkalinity. Human eyes are roughly neutral with a pH of around 7, and soaps are around 7.5 to 9. The eye can't handle the alkaline object as it is potentially harmful to the eye so it will start watering to get rid of the alien substance. ...


14

Each eye is controlled separately. Three cranial nerves emerge at the brain for each eye to control the so called extraocular muscles. That we usually move both eyes in one direction is due to that our brain is trained that way. Nonetheless, we sometimes move each eye individually, for example when looking at an object that is very close to our face (...


13

You have misinterpreted the nature of the light rays shown in the diagram in the question. The two rays are representing light coming from the same (distant) point in the environment, that is, some small feature on an object is reflecting (or emitting) light in slightly different directions and reach the eye separated by a small amount (less than the width ...


13

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: ...


12

No not white or grey generally, it's a mix of other colors, they often have low resolution of a particular color. Here's a page where you can mouse over a color wheel and see a version in color blind mode: http://www.archimedes-lab.org/colorblindnesstest.html The first type of cone is primarily sensitive to short wavelengths (blue), another to medium ...


10

Most of the light from the sun doesn't actually reach the earth's surface due to the atmosphere. [source] So the light reaching earth includes near-UV, visible, near-IR and a band of radio waves. Seeing any other part of the spectrum would be impossible since it doesn't reach earth. You asked why we only see in the visible light range; this is due to ...


10

This is an interesting phenomenon which I have been aware of for several years, but I haven't yet come across any official description of it or explanation for it. Nonetheless, the phenomenon is easy to explain. It is a result of light adaptation in your retina responding to different levels of light being received by each eye. Background The level of ...


10

This depends to some extent on how you define "resting state" (it matters). Innervation of the eye occurs in the brainstem and upper spinal column, so is, like most brainstem functions, on autopilot (like breathing.) If you define resting state as that which the lids would assume without any innervation, then the resting state of the eyelids is semi-open. ...


10

Blindness can be due to a damage of the lens, retina, optic nerve or the visual area at the occipital lobe of the brain, for example. Sensitivity of the eye cornea is enabled by a different nerve - the ophthalmic nerve, which is a branch of the trigeminal nerve (Yale Journal of Biology and Medicine). As long as this nerve, somatosensory cortex at the ...


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 ...


9

What is myopia? Myopia, a.k.a. near-sightedness, is a refractory error. Refraction is the process by which the optical system focuses images of objects on the retina, which are then transmitted to the visual cortex of the brain to be interpreted as vision. The major components of refraction are the cornea, lens, and axial eye length (distance from the ...


9

Short answer: The genes that encode eye color do not change, but the pigments in the eye can change due to external factors like diseases of medication. Long answer: Yes, it is possible that the eye color of adults can change, and it can also only happen to one eye. It is then called Heterochromia. There are two possibilities for different colored eyes, ...


8

Short answer The total number of photoreceptors ('pixels') in the human retina is 63 million. Approximately 3 million of them transmit focused, color-coded information. Background In essence, each photoreceptor on the retina acts as a photosensitive sensor. We have rods and cones in our retinae. Rods confer gray scale vision, while cones are responsible ...


8

Short answer Smooth pursuit is driven by retinal slip, which is determined by external input registered peripherally in the retina. Background The smooth pursuit system is a system designed to minimize retinal slip, i.e., the movement of an image across the retina. Saccades are rapid eye movements meant to fixate quickly on a new target (Purves et al., 2001)...


8

A "touch" or "haptic" sensation will be much faster due to several reasons: Haptic feedback can be processed without the presence of any higher-order cognitive processing, therefore meaning that the signals are being processed via a monosynaptic route. There are short reflex arcs between the spinal cord and the limb (meaning that you can react before you ...


8

The motion of the eyelid is driven by the levator palpebrae superioris, i.e. elevating muscle of upper eyelid, and it can be positioned to the intermediate, half-closed positions. At least I can do it. Yes, this muscle tends to shake or oscillate simply because it's a very weak muscle. It's similar with the muscles controlling the motion of the little ...


8

You seem to have many misconceptions about how we see and how that relates to light. I'll address them one sentence at a time: Why are we able to differentiate between colored objects without the presence of light? Answer: the premise of the question is wrong: we aren't able to differentiate between colors without the presence of light, because we aren't ...


7

The same thing happens in photography when you see an image with colorful shadows of objects on it - this is called chromatic aberration. Check this wiki page if you're not familiar with. Now this happens because the lens in the objective has different refractive properties for different wavelengths or if go the other way around different wavelengths refract ...


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