101
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 ...
45
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 ...
29
Actually, the US Army and Air Force (and I assume the Navy) teach their life support troops about this (as well as the aviators), since so many missions are conducted at night. It is actually some very interesting physiology involved in this. This Army Instructional manual should be of help (PDF File).
On an average it takes 30 to 45 minutes for your ...
21
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. ...
21
Cats and dogs can both view tv screens & monitors ... though their viewing experience is a little different to ours thanks to differences in cone structure leaving them color blind and giving low acuity.
Both species have lower levels of color vision than humans. Cats see slightly more color (in the blue green and yellow end of the spectrum) than dogs ...
21
Short answer, your eyelids does not block all light. Since they are only a thin layer of skin, the light is able to pass through although not completely obviously. Since the eyes are still completely functional when you close them, only covered by the eyelids, you are able to see when light strikes your face.
21
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 ...
20
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 ...
20
Birds have a body part known as the nicitating membrane otherwise known as the "third eyelid". This part has become vestigial in humans, where it remains as the plica semilunaris.
This image of a masked lapwing clearly shows its nicitating membrane in action, where it covers the eye in a horizontal motion. This is analogous to blinking in humans, and the ...
19
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 ...
19
I think you are talking about floaters (a.k.a. eye floaters or flying flies). You may want to have a look at this english.SE post in case you were not talking about floaters.
Floaters are deposits of various size, shape, consistency, refractive index, and motility within the eye's vitreous humour, which is normally transparent.
You can read the wikipedia ...
18
Possible, yes but probable, no. (I assume that the history of the parental family as you mention is important)
Let us first consider HERC2 (H=brown, h=blue) and OCA2 (O=brown, o=blue) as per the names mentioned in the reference you cite, to be completely unlinked, such that they are independently assorted. Since the father's lineage has no history of the ...
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)...
16
"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 a very simple process regulated by auxins. Or the light-sensitivity (...
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
There are two reasons for light to appear red through the eyelids.
Eyelids get a rich supply of blood which contains iron (in hemoglobin). The iron in blood absorbs all colors of light but reflects red light. (reference)
Our tissue transmits red wavelengths of light very well, but it doesn't transmit blue or wavelengths of light towards the blue end of the ...
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
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 ...
14
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 ...
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 ...
14
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
Overview
Laser eye surgery works by altering the shape of the cornea. The cornea works together with the lens to focus rays of light onto the retina. The cornea accounts for two-thirds of the optical power of the eye (1) (i.e. the eyes capability to focus light), however unlike the lens is of fixed power. It is the lens that changes is shape by the ...
13
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 ...
13
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 ...
12
Circular pupils are always functionally superior to vertical pupils; a slit does not correctly focus light from all directions whereas a circular pupil does. If you observe cats when they're hunting at dawn and dusk*, they have big, circular pupils; it's only when they're in bright light that the pupil shrinks to a slit. So why have vertical pupils at all? ...
12
Infrared wavelengths are EM waves of longer wavelengths than visible light. Also, visible waves are more energetic than IR waves.
Because of this, Visible light has the ability to excite organic receptor molecules in our eyes called Rods and Cones. These molecules stimulate cell responses and in turn excite cell responses in our nerves called an Action ...
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