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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-...
S Pr's user avatar
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17 votes
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What is the minimum light intensity that a human eye can detect?

From Hecht, S., Shlaer, S., & Pirenne, M. H. (1942). Energy, quanta, and vision. The Journal of general physiology, 25(6), 819-840.: Direct measurements of the minimum energy required for ...
Bryan Krause's user avatar
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14 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 ...
Resonating's user avatar
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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 ...
AliceD's user avatar
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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 ...
AliceD's user avatar
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10 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 ...
Anon's user avatar
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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.: ...
tsttst's user avatar
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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 ...
Oosaka's user avatar
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8 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 ...
Bryan Krause's user avatar
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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 ...
Bryan Krause's user avatar
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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 ...
Damon's user avatar
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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 ...
Zlira's user avatar
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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 ...
AliceD's user avatar
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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 ...
Remi.b's user avatar
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5 votes
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Are two colors (red + blue) necessary for LED grow lights, or would either color be sufficient?

The plant uses light to produce energy but also as a signal of how and when to grow (phototropism, photoperiodism). In the context of your question I'll first cover light-harvesting in photosynthesis ...
Hachiloni's user avatar
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5 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 ...
acvill's user avatar
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5 votes

What is the minimum light intensity that a human eye can detect?

Apparently that value is coming from a study by Clarke & Denton in 1962 where it is extensively cited in book1 about sound scattering in ocean. The threshold intensity which the human eye can ...
Nilay Ghosh's user avatar
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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 ...
Bryan Krause's user avatar
  • 46.1k
3 votes

Why do stomata close in low intensity light?

The primary function of open stomata is to promote gas exchange in leaf tissue: atmospheric carbon dioxide is a substrate for photosynthesis. A side effect of opening the stomata is an increase of ...
Alan Boyd's user avatar
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3 votes

Myopia & the Blur Effect

You have some misconceptions about visual acuity. You need to understand how lenses work to understand why objects up close are blurry in myopia. Sure, if one elongates the orbit, blurring will occur,...
anongoodnurse's user avatar
3 votes

Do different species of bacteria have distinct absorption bands?

Yes they do, as demonstrated by the fact that bacteria have different colors ! Moreover, some bacteria are photosynthetic and others aren't; the photosynthetic bacteria have a very specific mechanism ...
Oosaka's user avatar
  • 3,253
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 ...
Mr Hons's user avatar
  • 56
3 votes

Why do most plants reflect green and others other colors?

All photosynthetic plants contain chlorophyll, and chlorophyll is green (leaving out various algae). Plant leaves often contain other pigments, which can mask the chlorophyll's green, or be masked by ...
jamesqf's user avatar
  • 3,643
3 votes

Do Vaccinium membranaceum plants need a minimum amount of darkness per 24 hours?

Some plants are long day plants -- they require a certain number of days longer than a certain number of hours. Some plants are long night plants -- they need nights that are longer than a certain ...
Sherwood Botsford's user avatar
3 votes
Accepted

Why is 450 nm monochromatic light perceived as blue or violet depending on its intensity?

This is a general feature of human perception of blue-white mixtures. It's known as Abney effect. It's not limited to highly monochromatic blue colors (not even to blue colors). We can observe this ...
Ruslan's user avatar
  • 425
3 votes

Can mammals and/or birds see / detect infra-red light?

Infrared is a fairly large part of the electromagnetic spectrum, so to get an answer best suited to your specific case, you should specify the wavelength of your LED. There is at least one study ...
TheChymera's user avatar
  • 1,336
3 votes

Can mammals and/or birds see / detect infra-red light?

This is an interesting question, and I think what you really want to ask is How to choose near infrared LEDs that will not be visible to mammals or birds at night? Remember: their eyes will be dark-...
uhoh's user avatar
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