blue light is dangerous for the eyes ? can it bring the vision problem ?

Sure, if you shine it really brightly. But I do not think it is any more dangerous than any other optical radiation. We are talking about only about a 75% increase in energy per photon over red light, and only about a 38% increase in energy over green light (the most common frequency we are faced with).

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thanks black cat, if we see constantly towards neon light, later some sort of problem in shifting our eyesights on the other object, like when we see the normal bulb filament and then see towards other objects we found there the shadow image of the filament, is it due to?

sunil

I'm sure TheBlackCat's conclusions are correct, but I note one general issue about the danger that things present based on their energy. Often, damage occurs based on a threshhold of some kind, sort of like the photoelectric effect. Thus UV light, which is also not that much more energetic than blue light, can be damaging where blue light would be far less so. So the issue is not so much how much more energetic is the light, but rather, does that increase take us across some physiological threshhold. I'm sure TheBlackCat is also the person to discuss the biophysics of that particular issue, but from experience with sunglasses, we know that the damaging threshhold appears to be in the break from blue to UV moreso than red to blue. This is also why life on the surface of the Earth must have been pretty harsh prior to the generation of the UV-absorbing ozone layer, even though the Sun is not a particularly strong UV source.

When you get toward the edge of the visible spectrum the eye's ability to detect photons has dropped off quite dramatically, so one also has to consider that you won't have the same reaction to the light, e.g. you won't blink or look away - it won't look bright, even though it has a high intensity - so you might get damage that way.

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Quite so!

I used to think that if something doesn't hurt, it can't do any harm. But then a solar eclipse came along, and I noticed that looking at the last sliver of the Sun didn't hurt like looking at the complete Sun does. But of course, the last sliver is locally as bright as the complete Sun, and will damage the retina just as easily (the intensity of the light striking the retina is just as great, though over a smaller part of the retina).

Now I wonder whether looking at a star might be dangerous, especially if one is looking with a dark-adapted eye through an optical instrument. Some of the more conspicuous stars have greater intrinsic brightness than the Sun, and Sirius, for one, is not all that far away.

I also wonder whether it would be dangerous to look at Venus. Consider how bright the Moon looks at night, because it is lit by sunlight. And most parts of the Moon are actually as black as a blackboard. Venus, on the other hand, is as bright as snow, and the sunlight striking it is twice as intense as the sunlight striking the Moon.

I seem to remember that some astronomers (Lowell, for exampe) went blind in their later years. It might be adviseable to use monitors, photographs or other forms of indirect imaging for intrinsically bright objects, just as one does for the Sun.

You are saved by the diffraction limit of the human eye, which will blur the image of a light source to a spot about one arc-minute across. Since the apparent diameter of all stars is much, much, much less than one arc-minute, their light intensity on the retina is very much reduced by this smearing effect, compared to their surface brightness seen up close.

Reflected light from the sun has a much lower surface brightness than the sun itself, because the sun's light has become less intense as it spreads outwards (the inverse square law). You can look at a piece of paper in white sunlight without damaging your eyes (although it can be a little dazzling).
Recall that Venus is very difficult to pick out against the daylight sky - so its surface brightness on the retina isn't very much brighter than the blue sky, which does your eye no harm.

Grant Hutchison

I think you're describing an "after-image": if you like at a bright source of coloured light, you'll still see its outline in a complementary colour when you look away. So long as the light isn't very bright (in which case you'll get a retinal burn, and the after-image will become permanent!), this is just a temporary effect, with no lasting damage. The receptors and nerves in your eyes have reduced their sensitivity in response to the bright light, and it takes a while for them to recover.

Grant Hutchison

I agree that it will probably do no harm to look at Venus in daylight, at least with the naked eye. But if one looks at Venus during the night, one has a dark-adapted eye, and therefore a larger pupil.

The entrance pupil would be even more enlarged in one were to look at Venus through a telescope. And all the light passing through this entrance pupil will still be concentrated on the same small spot of the retina.

At maximum elongation, Venus can still be above the horizon while it is already quite dark.

As to the blurring effect: this will only begin to help when the blurred image is actually covering more cells than an unblurred image would do. If the blurred image has an extent of one minute of arc, and the eye is a few centimeters in diameter, the blurred image will have a diameter of only a few microns. Which means that only a few cells may have to cope with all the light from the star. And though they might not actually cook, thanks to the blurring, light-sensitive cells may be more easily damaged by light than non-light-sensitive cells.

We should be careful with this one, though. The characteristics and extent of the reflecting surface plays a huge role. Snow reflects UV very well and you can damage your eyes by skiing or ice-fishing on a sunny day, especially if you wear cheap sunglasses. If your sunglasses are dark but do not block UV, you can be in real trouble because your pupils will dilate to give you as much visible light as you need and WAY more UV than you should get. You'd be better off to wear no sunglasses and let your pupils contract naturally, cutting back the amount of UV getting to your retina. Snow-blindness is not fun and can be as painful as the damage caused by arc-welding with insufficient shielding.

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The ether of general relativity therefore differs from that of classical mechanics or the special theory of relativity respectively, in so far as it is not 'absolute', but is determined in its locally variable properties by ponderable matter.

Albert Einstein, "On the Ether", 1924

The difference made by a dilated pupil turns out not to be significant. The luminance of the solar disc is about 2,000,000,000 cd/m². The luminance of Venus' clouds in reflected sunlight, at Venus' distance from the sun, is about 60,000 cd/m² - it's orders of magnitude too low to cause concern, even with a dilated pupil.
(I don't know if you've ever had your pupils dilated for an eye examination. They used to stay dilated for hours afterwards, when I was a kid. I can vouch for the fact that something as bright as the blue sky does not injure your eyes, even when viewed through perfectly dilated pupils.)

But it's concentrated into a larger area, because of the telescope magnification: the surface brightness is unchanged.

That doesn't work out. A very very tiny hot image would burn a tiny hole in the cell membrane, killing the cell. A blurred image covering the whole cell would be insufficient to heat it. So the blurring serves to save individual cells.
(An analogy would be using a magnifying glass to project an image of the sun on to your hand. Blur the image, your hand is uninjured; concentrate the light, you damage your hand.)

Grant Hutchison

OK, that's a good thing to point out - I didn't intend to actually encourage people to stare at large white surfaces in broad daylight.
However, snow-blindness is caused by a UV burn to your cornea: it's sun-burn of the eye surface, not retinal damage, and so it happens in situations where UV light bathes your eye from all directions, as happens when there's good snow cover, especially at high altitude. Since the burn happens to the outside surface of your eye, the state of dilation of your pupils makes no difference. I think the reason "no sunglasses" is better than "cheap sunglasses" is because it forces you to screw up your eyes and to keep looking away from bright reflecting surfaces, so your corneas take less of a UV hit. Whereas cheap sunglasses let you look directly at the snow for long periods with your eyes wide open, so that UV can bathe your corneas continuously.
(Your eye lenses are actually opaque to UV, so none of it gets to your retinas.)

Anyway: with reference to light reflecting from Venus, although I've no idea how much UV is reflected, it's going to be a very small dose coming from a very small area of the sky, so there won't be a risk of "Venus-blindness".

Grant Hutchison

You are right about the snow-blindness - I dashed off that answer without thinking all that through. It is the UVB that causes the corneal damage characterized by snow-blindness. Cheap sunglasses will expose you to more of this because you will not avert your gaze, as you mentioned. UVA will cause retinal damage, however, and that's the one (I should have remembered) that gets lots worse if you wear dark sunglasses with no UV protection, because your pupils will dilate and let more in. I was a dispensing optician years ago, and sometimes you forget the physiology if you haven't dealt with it for a long time. Thanks.

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The ether of general relativity therefore differs from that of classical mechanics or the special theory of relativity respectively, in so far as it is not 'absolute', but is determined in its locally variable properties by ponderable matter.

Albert Einstein, "On the Ether", 1924

It is not clear to me that a light must be as bright as the Sun before it can be harmful. People have gone blind from looking at sunlit snow.

If that is true, the telescope will not make it more dangerous. Except insofar that small movements of the eye will be less likely to move a cell out of harms way. But we are always being warned not to look at the Sun through a telescope. If the telescope would merely enlarge the image without improving its brightness, the Sun through a telescope should not be (much) more dangerous than the Sun seen with the naked eye.

Normal cells, certainly. But a light-sensitive cell is sensitive to light. In response to light, chemical reactions occur, reactants are depleted and reaction products accumulate. In order to remain functional, the cell must be able to replace the reactants and remove the reaction products. If the total amount of light entering the cell (whether in a tiny bundle or as an all-encompassing glare) is more than the cell can cope with, chances are that it will at least temporarily go off-line. I am no ophthalmologist, so I don't know whether this would be enough to kill it outright.

What I do know, is that if am sitting in a pitch-dark room, and then suddenly turn on the lights, the indirect light from the walls will hurt my eyes so much that I have to close them. (Normally, of course, I close my eyes before I turn on the light.) Pain is an alarm-signal, so if something hurts that much, there must be some danger. Perhaps the dark-adapted eye is made more vulnerable not merely by having a larger pupil, but by other kinds of adaptation as well.

So the idea of sitting in just such a pitch-dark room, and looking at the brightly lit clouds of Venus, or indeed at an object which, though extremely tiny, is considerably brighter than a welding torch, does not feel completely safe.

It's not simply the brightness of the image, but the total flux. Someone told me about a guy at Stellafane years ago that popped a cheap eyepiece into his scope, removed the solar filter and lit his pipe with the beam emitted from the eyepiece. That must have been a pretty impressive demonstration to people who think that looking at the sun is safe.

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The ether of general relativity therefore differs from that of classical mechanics or the special theory of relativity respectively, in so far as it is not 'absolute', but is determined in its locally variable properties by ponderable matter.

Albert Einstein, "On the Ether", 1924

Would all the light that reached the tobacco have entered the eye of someone unlucky enought to look through the eypiece?

Anyway, if your telescope were pure (aberration-free) enough, and if you would hang a tiny piece of tobacco from a non-thermally-conductive thread in the focus from the eyepiece, you might conceivably use the light of Sirius to ignite that tiny piece of tobacco.

Fair enough. I was using a figure I happened to know by heart, but I agree it's not a watertight argument. But maybe you'd agree that the light used by ophthalmologists to examine your retina after they have dilated your eyes:
a) Is uncomfortably brighter than Venus
b) Produces a nasty afterimage, which Venus does not
c) Is probably designed not to injure your eyes, even with pupils dilated ?

See above. They got a sun-burn to their corneas from a source of reflected UV that filled a large area of their field of view for several hours.

I agree. But it's simple physics that a telescope can't increase the surface brightness of extended objects.
Let's say your pupil, dilated, is 7mm wide. A telescope with an aperture of 14mm is twice as broad, and will gather four times the light. For you to get the benefit of all that light, it must be compressed down to an "exit pupil" 7mm wide. But the ratio of the aperture to the exit pupil is simply the magnification of our scope (um, sorry, I think I'd need a diagram to explain why that is). So that's 2x magnification, which will spread the light over four times the area. So four times the light is spread over four times the area: it's the same surface brightness.

So an arc-lamp bright light bathing the whole cell might interfere with its function. But a tiny needle of arc-lamp bright light, much narrower than the cell, would interact with only a few molecules, and therefore have little impact on the consumption of reactants or production of metabolites. Similarly, if that light is spread out to illuminate the whole cell (as it is, by diffraction) it will have a minimal effect on the cell.

Well, the star's image is much less bright than a welding torch, for the reasons I've given. But I guess I'm not going to convince you about Venus without digging out some figures, unless my argument from ophthalmological experience has worked.

Grant Hutchison

Doesn't UVA primarily damage the lens, because that's where it delivers its energy as it's absorbed? Hence the longterm use of bad sunglasses might increase your cataract risk, in the way you describe.
My recollection (which is admittedly hazy on this one) is that UVA damage to the retina is described only in people who have had their natural lenses removed by cataract surgery, which then allows the UVA all the way through to the back of the eye.

Grant Hutchison

The cell doesn't replace the reactants and remove the reaction products, the reactants are excreted from the cell, taken up by support cells, and recycled there. And this has no toxic effect. In fact, in the rods, the low-light cells that you use, this very thing happens all the time. These cells are completely overwhelmed by even dim light, so in normal day light they are not functioning at all. It is a perfectly natural thing. Once the first stage of light-sensetive pathway shuts down, the cells return to their baseline level of activity.

What is more, the pupil only changes the amount of light seen by about a factor of six. If I recall correctly, the photoreceptors can change their sensetivity by a factor of 10,000 (I may be off by an order of magnitude). The advantage of the pupil is not in that it regulates the amount of light, it is that it responds more quickly than the photoreceptors so it can change the amount of light arriving a little bit to give the photoreceptors a chance to adapt.

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Okay, I guess that looking at Sirius will not be quite as dangerous as I imagined it might be. After all, Sirius A is 700,000 times more distant than the Sun, and only a few times larger than the Sun, so it's image would be at least 100,000 times smaller in diameter. I don't know how many cells would sit side by side on a line crossing the image of the Sun, but as it covers only one degree of arc there would probably be somewhat less than 100,000 of them. And if the image of Sirius is blurred enough to cover, say, five times its ideal diameter, this would already be enough to offset Sirius' greater intrinsic brightness.

Vega and Alpha Centauri would probably be no more dangerous than Sirius, while Procyon and Altair would certainly be less dangerous.

As for Venus: If ultraviolet radiation cannot reach the retina, it cannot be dangerous if reflected by something at Venus' vast distance and relatively small size. It would be dangerous only if refocused. And if snow-blindness is caused by untraviolet radiation only, there can be no analogous Venus-blindness.

Yet it is better to be safe than to be sorry. And I have the impression that several astronomers have gone blind; I could, without consulting books or the internet, name two quite famous ones: Galileo Galilei and Percival Lowell. (In comparison, I could not name mathematicians, painters, philosophers or statesmen, and only one composer.)

I'll have to dig up some literature on this one, but I think that the UVA can get through the natural lens and cause damage of the retina. I remember that macular degeneration can be implicated in this, but I think there were other conditions as well, and they tended to be concentrated in the fovea, which means that the most central, detailed, part of your visual field would be damaged.

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The ether of general relativity therefore differs from that of classical mechanics or the special theory of relativity respectively, in so far as it is not 'absolute', but is determined in its locally variable properties by ponderable matter.

Albert Einstein, "On the Ether", 1924

When i was in college (late 60s, early 70s), I led observatory tours. Our telescope was a 12 inch f/15 Brashear refractor. On clear days, I would show them sunspots using eyepice projection on a large piece of construction paper (about 1mm thick). I would explain that I was using eyepiece projection because looking at the Sun through any telescope is dangerous. I would demonstrate this by moving the sheet up to the focus point of the scope. In less than a second, there would be a puff of smoke and sometimes a flash of flame. I would then show them the charred hole and say "think what it would do to your eye". They were suitably impressed!

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Scroll down to part IV regarding eye protection.

http://www.idph.state.ia.us/eh/commo...anninginfo.pdf

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The ether of general relativity therefore differs from that of classical mechanics or the special theory of relativity respectively, in so far as it is not 'absolute', but is determined in its locally variable properties by ponderable matter.

Albert Einstein, "On the Ether", 1924

Interesting, thanks.
But here is an academic paper which states:Here's the NASA eclipse page, which provides a reference for the shielding effect of the lens, and the problems that can occur when the lens is removed.
But here is an article from Review of Optometry saying that the lens is much more transparent to UV in children (which makes sense when compared against the graphs in my first reference).

I also found a large number of web-pages suggesting that blue wavelengths are implicated in macular degeneration, which may be what suntrack2 was referring to. All of the ones I looked at came from MD support organizations or people trying to sell yellow-tinted sunglasses / lens implants, and I'm afraid I couldn't find an academic reference.

Grant Hutchison

Please Google on "UVA" and "retina", and follow the links that you find. If you persist in your belief that UV cannot harm your retina, I cannot help you - I can only offer my sympathy.

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The ether of general relativity therefore differs from that of classical mechanics or the special theory of relativity respectively, in so far as it is not 'absolute', but is determined in its locally variable properties by ponderable matter.

Albert Einstein, "On the Ether", 1924

Galileo, solar observing, and eye safety offers a skeptical look at that cause of Galileo's blindness.

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Ummm.
Did I say that?
It seems pretty clear from the links I provided that:
It's a considerable risk for children and those who've had cataract surgery without lens implants, or using older implants.
Young adults' lenses let through a small amount of UVA, older lenses let through progressively less.
At all ages the bulk of the UVA energy is delivered to the eye lens, promoting cataract formation.

The risk of cataract alone should be enough to make people check their sunglasses for UVA shielding; the significant risk to small children's retinas makes it doubly important for them.

Grant Hutchison

Looking at the Moon, Venus, or any extrasolar star through a telescope, no matter how large, will not cause physical damage to the eye. A telescope does not increase the surface brightness of an object. The Sun, of course, is a different story.

The response of a dark adapted eye to bright light (going from a darkened movie theater into bright sunlight, for example, or observing the Full Moon at low magnification) may cause discomfort but is not dangerous.

Here's what Brian Tung, a well-known amateur astronomer, has to say on the subject at http://astro.isi.edu/reference/faq.txt

"Q. Is it true that looking at the Moon through a telescope will harm your eyes?"

"A. You cannot harm your eyes by looking at the Moon through a telescope. It may be uncomfortably bright, and you may can improve the visibility of detail by either adding a neutral density filter (a gray screw-on filter) to the eyepiece, or by increasing the magnification. But there is no safety risk.

You may wonder how this can be, since the telescope gathers so much more light than your eye. However, it also magnifies the Moon, so that the extra light is spread out over a greater area. Each part of the Moon's image is seen by just one portion of your eye, and as far as damage is concerned, the critical factor is the intensity of light falling, per individual portion of your eye. If your eye's pupil is 5 mm across, and your telescope is 100 mm across, then the telescope gathers 20 squared, or 400 times more light than your eye alone. But if you're using a magnification of 20x or greater, then that light is spread out over an image at least 400 times larger, so that the actual brightness seen by any portion of your eye is no greater, and usually less, than when you observe the Moon with the unaided eye.

What if you observe the Moon at less than 20x--say, 10x? Shouldn't the light be spread out over a smaller area, and thus more concentrated? At 10x, the 400 times more light is spread out over an image that is only 100 times larger, so it seems as though each part of the image should be 4 times as bright as when seen by the unaided eye. However, consider that each portion of the Moon can be thought of as pouring down light, out of which only a shaft 100 mm across--as wide as your telescope--actually enters the optics. In the process of magnification, that shaft is reduced to fit into your eye's pupil, and the factor of reduction is equal to the magnification. In other words, if you magnify by only 10x, the 100 mm shaft of light is shrunk down to 10 mm. The result is that only part of the light--a smaller shaft that is 5 mm across--as big as your eye's pupil--actually gets in. The rest of it falls uselessly (at least as far as image brightness is concerned) on the surface of your eyeball. Since a circle 5 mm across has 1/4 the area of a circle 10 mm across, only 1/4 of the light gets into your eye, and this precisely compensates for the extra intensity from lowering the magnification.

Of course, it *feels* as though the Moon is about to blind us, for two reasons. One is that we typically observe the Moon by night. The same phase by day is just as bright, but it doesn't feel blindingly bright through the telescope because our eyes are then accustomed to daytime light levels. Another reason is that the Moon *is* magnified by the telescope, and at the same intensity throws more total light onto your retina. By way of an analogy, if I shine a flashlight into your eye at a distance of 10 cm, it's uncomfortably bright, whereas if I put a mask on the flashlight that only lets through a tiny spot of light, it's merely annoying. The total light output is much smaller, but the intensity of that tiny spot is just as great as before.

Incidentally, some people may ask, why then is observing the Sun through a telescope so dangerous? After all, although we don't stare at the Sun (at least, we shouldn't), its light still comes through our eye. If looking at the Moon through a telescope is no more dangerous than looking at it without the telescope, why isn't the same true for the Sun?

The answer is that the Sun is so bright that each portion of its image is enough to create some heating in the eye. (So does the Moon, but its light is about 400,000 times less intense and the heating is completely negligible.) If any given part of your eye is subjected to that heating for long enough, permanent damage will result. Your eyes avoid this by moving around, so that the image of the Sun doesn't stay in place, and the part of your eye that is getting heated by the Sun one moment has a chance to cool down the next.

However, if you were to be so foolish as to observe the Sun through a telescope, each portion of your eye gets heated the same amount, but now moving the eye doesn't help, since it is still likely to be heated by the Sun. Moreover, with a small image of the Sun (as when seeing it with the unaided eye), the fraction of your eye being heated is small, and it can dissipate heat rather easily to slow down the damage. With a magnified image, the fraction of your eye being heated is much larger, and there is now nowhere for the heat to go. You can as a result burn out your retina with startling and tragic speed.

Bottom line: DON'T DO IT! DON'T OBSERVE THE SUN THROUGH A TELESCOPE without proper safety precautions, such as an appropriate filter. Do not use solar filters that screw onto the eyepiece. The focused heat at the eyepiece is too intense and will crack the filter, sending all that concentrated light and heat into your eye. The light must be filtered before entering the telescope. (Exception: A Herschel wedge can be safely used. If you don't know what a Herschel wedge is, though, don't guess--just use a proper solar filter.)"

The Solar Observing FAQ at http://jeff.medkeff.com/astro/faq/ provides a lot of useful information regarding observing the Sun safely including the processes involved in solar induced eye damage.

Dave Mitsky

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Thank you for including this website.

Until now I wasn't aware that Galileo might have looked at the Sun through his telescope, because I imagined that nobody in his right mind would ever do such a thing. That was one of my reasons to suppose that looking at planets or stars might have harmed him.

Of course, if he did look at the Sun, we need no longer consider whether he also looked at Venus, Sirius or whichever.

But if his blindness was caused by cataract and/or glaucoma, this doesn't exclude the Sun as a cause (as the website seems to suggest). Cataract, or opaqueness of the eye's lens, can be caused by heat. (Microwave radiation from computer monitors and mobile phones is suspected of causing cataract because heat may be released in the eye's lens.) If sunlight focused through a modern telescope can cause welders goggles to explode (something that everyone should take good note of!), the sunlight focused through Galilei's more primitive telescope might have heated his lens enough to cause (or accelerate the onset of) cataract.

Glaucoma is caused by an obstruction of certain tiny channels which drain fluid from the inner chamber of the eye. One might suspect that localized heat might cause proteins to coagulate, forming particles large enough to obstruct the channels.

Old age may of course be a factor. Not so much because he was 73 at the time, but because he lived for only five years afterwards. (If he had lived to be a hundred, old age would not be such a plausible cause for blindness at the age of 73.)

Personally, I am rather wary of the Sun -- and even of the Sun's reflections. In the hoods of cars, for example, or in the backsides of my spectacles. But after reading the website, I think I will no longer be wary of planets and stars.

Any light is bad for the eyes if it's bright enough. So I guess it depends on how bright the blue light is.