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24 votes
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Can UV radiation be safe for the skin?

You're talking about long-wave UV, or UV-A radiation. In the 80s, experts claimed that this was a safe wavelength. Protection against UV-A was not part of sunscreen in the early days. Consequently, UV-...
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23 votes
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Why is human vision restricted to 400-700 nm?

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 ...
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17 votes
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Why do human eyes glow red on a photo and dog's eyes green?

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-...
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14 votes
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Why can I see a light flicker when it's in my peripheral vision?

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 ...
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13 votes

Are two colors (red + blue) necessary for LED grow lights, or would either color be sufficient?

tl;dr: Sort of? Logically, either red or blue light should be sufficient. Chlorophyll a and chlorophyll b preferentially activate different photosystems, and both photosystems are required in green ...
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12 votes

Why do I still see a bright light after looking directly at it?

Short answer The effect you describe is called a negative after image. It can be explained by adaptation effects of the photoreceptors in the eye. Background source: Dresden University Steadily ...
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11 votes
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Does your iris color have an effect on your vision?

Firstly, note that "eye color" is actually iris color. The person with a lighter iris will have more straylight compared to person with brown iris: The graph represents Intraocular straylight of ...
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10 votes

Why is human vision restricted to 400-700 nm?

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. ...
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10 votes
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Why do dog's eyes glow green on a photo?

Dogs, cats, and many other mammals have a Tapetum lucidum which reflects light back through the retina to help with night vision. Humans don't have this layer. The tapetum is probably reflecting green ...
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10 votes
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Why are fearful stimuli more powerful at night?

Short answer The increased fear responses during the night are believed to be mediated by elevated corticotropin-releasing hormone (CRH) levels in the brain that drive the fear responses in the ...
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9 votes
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Can "red" cone cells actually see much red light?

This is a good question. The first thing to note is that human colour vision is very complex and still poorly understood. If you visit the wikipedia page on RGB, you will find that this correctly ...
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9 votes

How is the extraordinary eyesight of some deep-sea creatures explained?

You will find quite a lengthy and nice discussion and review of current hypotheses (all with some support) in Porter et al. 2020,PNAS. To add even more intrigue, parts of the vision system (e.g.: ...
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8 votes
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Can the retina emit light?

Please allow me to start by saying I know nothing about LEDs, so my answer isn't going to address your analogy. I'm also going to keep it more simple. Can the retina emit light? Yes. The retina is ...
8 votes
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Why are we able to differentiate between colored objects without the presence of light?

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 ...
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8 votes

Cerebral activity during exposure to non - visible light

Vision is based on a protein interaction with a molecule called retinal from vitamin A. Wavelengths of light in the visual range cause a photoisomerization of retinal (a cis- to trans- change), which ...
  • 37.3k
7 votes

Can UV radiation be safe for the skin?

There are (at least) two sides to this story. One is direct DNA damage being caused by UV-B light which happens to have photons with just the right amount of energy to interact with thymine. This has ...
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6 votes
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What happens to plants if they are exposed to "sunlight" 24h a day?

This review (from 2011)1 states that it depends on the plant species and cultivar. Some continuous light (CL)-grown photosynthetic organisms show increased productivity. However, CL also induces ...
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6 votes
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Do color-blind people have more rod cells in their retinae than the normally sighted?

Short answer There are not many studies that have counted rods and cones in the human retina, let alone compared healthy retinas with those of color blind people. Yet direct evidence, as well as ...
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5 votes
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What is the biological potential for vision of wavelengths outside the human visual range?

There are some limits on what light can be detected biologically based on physics and chemistry. Although there are animals that can sense more UV or more infrared than humans, they are still subject ...
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5 votes
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Cerebral activity during exposure to non - visible light

[D]o we fail to see non visible light because our eyes lack the receptors for other frequencies, or does our brain receive signals that it just dumps because it has not developed to process the ...
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4 votes
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Receptors for red and far-red light in plants: Shade avoidance

As far as I know there are 5 receptors for far-red and red light which are the phytochroms(phyA-phyE) Its all about the ratio between red and far-red light. Each phytochrom has an inactive(PR) and ...
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4 votes

What does "chlorophyll photosynthesis peak" mean in relation to photon wavelength?

First thing to note is that the absorption spectrum of chlorophyll A and chlorophyll B is similar but not the same. The reason for the appearance of absorption spectra is because the light is composed ...
4 votes
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What is blue light?

Blue light means wavelengths that appear to the human eye as blue when they are presented alone. This light is important for sleep/wake cycle regulation because this is the wavelength that cells that ...
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4 votes

Would green light look different if blue and red cones were deactivated

The genetic condition you are describing is called green cone monochromacy (GCM). This condition is exceedingly rare because it requires the dual inheritance of tritanopia (absence of blue retinal ...
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3 votes
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Can microorganisms (e.g. bacteria) be damaged or killed by 253,7 nm light?

This is UV-C radiation and light of this wavelength (which is emitted by low pressure mecury lamps) can indeed be used for disinfection purposes. The nucleic acids (DNA and RNA) has an absorption ...
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3 votes
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Why does sunburn cause fever?

Here is a good article on the topic. https://www.nlm.nih.gov/medlineplus/ency/article/003227.htm But it's most likely due to the fact that a sun burn is an actual burn on the skin that can cause ...
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3 votes

Is it safe to look indirectly at the Sun?

Behar-Cohen et al. notes the periphery of the cornea has a focusing effect: UVR incident from the periphery is refracted into the eye, and due to the focusing effect of the cornea, UV radiation is ...
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