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52

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 ...


19

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. ...


18

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 ...


18

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 ...


18

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. ...


13

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 ...


12

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 ...


12

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


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

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 ...


10

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 ...


10

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 ...


9

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 ...


8

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 ...


8

It is a very interesting question and I did some efforts to investigate the literature on this topic, but yet I don't have a definitive answer for you. But let's start from the beginning. First of all, the reason for color deficiency can be not only lack (rare) or impairment (more often) of certain types of color-perceiving cells (cones) in retina, but also ...


8

Your retina has two kinds of light-sensing nerves: cones and rods. Cones are responsible for color vision, i.e. hue, while your rods handle differences in the value, as in how bright it is. Humans have adapted to have cones mostly in the center of the eye (which corresponds to what you're looking directly at), where the most distinguishing color information ...


8

OK, I'll field this one. I'll ignore any of the tell-tale signs of hokum such as writing in ALL CAPS. Nevertheless, it's a lot of hokum. It's true that he goes into a lot of detail and I'm sure his math looks nice but the fact is that it's not grounded in reality. I would consider myself to be something of an expert (in training) in the field of ...


7

"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 ...


6

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 ...


6

All of the above answers are great, and very informative. But they are also technically wrong, in certain conditions. Once you understand them, you'll be able to understand this explanation of why. The canonical answer is that cones are used for color perception in bright light and rods are used in low light. But rods have a peak color sensitivity that is ...


6

The sense of depth is required to us to orient ourselves in a 3D world. Insects do orient themselves in the exterior 3D world, thanks to the ability to detect the plane of sunlight polarization, that is used as a navigation compass in foraging expeditions and when coming back home. You can find a good review on Current Biology (Krapp 2007) and a lecture ...


5

This is an optical pinhole effect. If the amount of light passed through the lens decreases, the sharper the image is at the focal point. For a camera this is accomplished by increasing the f stop (f22 is a smaller aperture size than f6 — f-stop is an inverse number). For the eye the iris will dilate (get larger) or contract to be smaller. In very ...


5

"1001 questions answered about insects" by Alexander Barrett and Elsie Broughton Klots includes the following passage: Do insects have depth perception? Depth perception of some sort is important to an animal who has to catch its prey; fortunately most insects have it to a degree. Although they do not have binocular vision that can be compared with ...


5

I've read somewhere that humans are most capable of distinguishing shades of green, that is a human can differentiate more shades of green than any other color. Is it true? I'd say it's plausible, depending on your definition of "Green." While your eyes do, indeed, have RGB receptors (cones) and Rods for grayscale - they don't perceive the Visible ...


5

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? ...


4

This paper suggests that bats have a comparatively high depth of focus. It's believed that butterflies (depending on species) have the widest visual spectrum.


4

Yes! And no. But first, some background. Your central nervous system has (or is believed to have, as do other primates) a number of different organizational structures. Cells within a column (the cortex is composed of about 7 layers, and a column would be a vertical group of neurons from these layers) respond to stimuli with similar features. There are ...


4

Another reason why red lights are now sponsorized for night illumination is because they are supposed to be safer in terms of interference on the circadian cycle. This is not related to better vision, but better health. The mammalian eye senses the light by the conventional rode and cone cells. However, a third light-sensing cell type has been recently ...


4

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 and 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 ...


4

"seeing stars" or phosphene is caused by stimulation of the visual cortex or retina by causes other then light hitting the retina. http://www.oubliette.org.uk/Three.html



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