Wikipedia:Reference desk/Archives/Science/2014 September 28

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September 28

Trees shedding leaves

When deciduous trees shed their leaves in the autumn, are the outermost leaves the last leaves to fall from most species of trees?
—Wavelength (talk) 02:30, 28 September 2014 (UTC)

I'd expect the reverse, since the outermost leaves will get coldest first, which is what I think triggers them to change colors and then fall. StuRat (talk) 05:56, 28 September 2014 (UTC)

Going on simple observation (looking out of the window) it seems to be random. Alansplodge (talk) 08:05, 28 September 2014 (UTC)

It could depend on the species. I think the birch, for example, retains some of its leaves all winter. ←Baseball Bugs ^{What's up, Doc?} carrots→ 10:13, 28 September 2014 (UTC)

The shedding is a form of abscission. Our article suggests that leaf abscission is indirectly due to changes in sunlight, but it's as unreferenced as StuRat's opinions above. This suggests that ethylene plays a role. According to that site, each leaf produces ethylene and, as that builds up, it eventually triggers the abscission response. Neither of those things would be influenced much by placement on the branch - in fact, if sunlight hours are the trigger and each leaf is triggered individually, it would seem that the outer leaves would be the last to go. Of course, it could also be that the plant triggers the abscission en masse and the fall of individual leaves is mostly due to chance and/or wind. An image search of autumn tree doesn't show any particular pattern to the sequence of leaf fall, though it does seem that some branches empty ahead of others. Long story short, the facts of the matter are either poorly understood or at least are not well explained on the web. If you find something good, our article could really use an expansion. Matt Deres (talk) 12:09, 28 September 2014 (UTC)

Whenever a tree loses it's leaves as Matt Deres said, it is a form of abscission. Another article states that the sunlight has no factor in the tree's abscission (http://www.npr.org/templates/story/story.php?storyId=114288700). Rather, the colder temperatures trigger a hormone to be released that begins the process of shedding. Abscission cells will begin to form around the stem close to the branch that the leaves are connect to. These begin working at the leaves a slowly but surely making the connection the leaf has with the tree weaker. It is a common misconception that the trees will shed their leaves due to wind, which definitely helps to get them off the tree but they wouldn't even come off if it were not for the abscission cells. Going to your original question about if the outermost leaves are the last to fall, it is merely a matter of which ones mature first. As stated here (http://www.usna.usda.gov/PhotoGallery/FallFoliage/ScienceFallColor.html) the leaves will start the abscission process and those that manage to loosen and separate are not determined by the position on tree. Very interesting though, as stated here (http://dnr.wi.gov/eek/veg/trees/treestruecolor.htm) Oak trees do not lose their leaves through the winter. Oak trees cannot fully shed their leaves, however they do die in the winter. Kevinhaney17 (talk) 22:00, 3 October 2014 (UTC)

optics

The color of an object is said to depend on the wave length. the object reflects. So if you viewed colored objects under water in which the wave length of light is different, does the color change? — Preceding unsigned comment added by Janfred keno (talk • contribs) 10:45, 28 September 2014 (UTC)

How would being under water change the wavelength? ←Baseball Bugs ^{What's up, Doc?} carrots→ 11:59, 28 September 2014 (UTC)

Going under water changes the spectral distribution of the light, because longer wavelengths are more strongly absorbed. That does indeed change the perception of coloured objects. During my Advanced Open Water Diver deep dive, the instructor (very dedicated and not afraid of sharks) pricked his finger to show us that blood looks black at a depth of 90 feet. --Stephan Schulz (talk) 13:47, 28 September 2014 (UTC)

Then the general answer to the OP's question is "Yes." ←Baseball Bugs ^{What's up, Doc?} carrots→ 14:10, 28 September 2014 (UTC)

There is probably some confusion here. The speed of light changes in water, so the wavelength for a given frequency changes correspondingly. However our eyes, if I remember correctly, don't detect the wavelength of light, they detect its frequency, which does not change. Therefore this effect does not cause a perceptual color shift. There is, as others have already said, a color shift caused by the fact that water transmits some colors better than others. Looie496 (talk) 14:58, 28 September 2014 (UTC)

It's been a long time since Physics 101, but I thought wavelength and frequency were effectively inverses of each other. ←Baseball Bugs ^{What's up, Doc?} carrots→ 15:07, 28 September 2014 (UTC)

They are related that way as long as the speed is constant. When light enters a different medium (like going from air to water) its speed changes, so the wavelength is different. But as Looie says, what really matters is the frequency and that doesn't change. It's just that, because we usually think of light moving in a vacuum (at a truly constant speed) or in air (at very nearly the same speed), each frequency has a specific wavelength associated with it when the light is in air or vacuum and we happen to use that wavelength as the way of describing colors. --65.94.51.64 (talk) 15:23, 28 September 2014 (UTC)

Speed of light in water is about 25% less than in vacuum. So for the same frequency, wavelength is reduced accordingly. --Stephan Schulz (talk) 15:29, 28 September 2014 (UTC)

Something seems wrong there, since objects don't change color when submerged in shallow water. Only deep water appears to have this effect, where enough water is present to absorb the reddish end of the spectrum. StuRat (talk) 18:50, 28 September 2014 (UTC)

What's wrong is that you're missing the point: the wavelength changes, but the color doesn't change because color depends on frequency, not wavelength. We just usually describe it in terms of wavelength. Of course it's different if the depth is sufficient for the fact that water is slightly blue to be noticeable, but that's because the color distribution of the incoming light is changed, not the color of a particular frequency of light. --65.94.51.64 (talk) 06:38, 29 September 2014 (UTC)

All of the above discussion is entirely irrelevant. Your retina is what detects the color - and that lives behind the aqueous humor and various other tissues making up your eyeballs. So the speed of light in the aqueous humor (or some other tissue of the eye) is all that matters - and that doesn't change when your eyes are in the air or in the water, so no - the color doesn't change and it has nothing to do with wavelengths or frequencies or any of that other stuff. SteveBaker (talk) 13:25, 29 September 2014 (UTC)

A photoreceptor cell detects light when a photon hits a protein in the retina and excites it to a higher state. This excitation depends on the energy of the photon, which is proportional to frequency: E=hf. That's why changing the wavelength of light doesn't change our perception of color.

By the way, color has a lot more to do with biology than it does with physics, and depends on a lot more than the wavelength of light. See this entertaining video for how human vision works. The part about color begins at 7:00. --Bowlhover (talk) 18:13, 29 September 2014 (UTC)

optics 2

how is it possible that the complete circle of a rainbow can sometimes be seen from an airplane? — Preceding unsigned comment added by Janfred keno (talk • contribs) 10:50, 28 September 2014 (UTC)

I would say it's because there is no ground to get in the way. Read Rainbow and it should answer your questions. ←Baseball Bugs ^{What's up, Doc?} carrots→ 11:57, 28 September 2014 (UTC)

You see part of a rainbow wherever the rain or mist, the sun, and your eye make a certain specific angle. So any time you are looking away from the sun, there is a cone shape with its vertex at your eye (and the line from the sun to your eye as its axis of symmetry), and if any part of that cone has rain or mist in it then you see part of a rainbow there. (If the sunlight is intense enough to make a dobule rainbow, the geometry is the same but the specific angle for the second rainbow is different.) When you're seeing a rainbow because you're seeing the sun shining on distant rain, it cuts off at ground level because that's as far as the rain falls. But if you're above ground, you can see the rain falling below you, and yes, that means it's possible to see a full-circle rainbow like this one. You can get similar effects with the rainbow is formed by the mist from a waterfall, because you can be much closer to the mist and can see down into the gorge, as in this example and also this example. It's all a matter of how much of that cone shape the mist is in. --65.94.51.64 (talk) 15:35, 28 September 2014 (UTC)

Should there be maximum speed limit for the perception of human vision?

Human eye can’t see an object if move beyond certain speed such as

1- Spinning of spooks of wheel 2- Spinning of propellers or blades of fan 3- Firing of bullet from firearm 4- Spinning of the coloured desk which turns into white

Similarly vision becomes out of sight upon moving the cursor on the recorded events (red in color) in the selected timeline faster than the normal rate (play) of any video camera e.g. surveillance

Although all above are either spinning or travel with high speed but untraceable by the human eyes. No idea if size of an object would affect that limit or not but should there be a maximum speed limit for the perception of human eye beyond which it can’t see things in motion (just like a sound for ears) if not then how light/ photon is perceptible with its high speed (299,792,458 m/s) when our eyes simply can’t notice aforementioned motion?162.157.248.16 (talk) 05:43, 28 September 2014 (UTC)EEK

Are the spinning spooks for Halloween ? StuRat (talk) 05:51, 28 September 2014 (UTC)

What do you mean by what "should" be the limit ? From an evolution POV, there would be no point in being able to spot things faster than would occur, or could be reacted to, in our evolutionary past. Much smaller animals, like insects or hummingbirds, might well benefit from faster vision, since they have the ability to move out of the way of fast predators or capture fast prey. StuRat (talk) 05:51, 28 September 2014 (UTC)

... and to answer the last part of the question ... photons are invisible as they pass by (at any speed) ... they are detected only when they enter the eye and cause a stimulus to the rods and cones of the retina. Dbfirs 06:49, 28 September 2014 (UTC)

I can see the moon, it is travelling at 2000 mph. Greglocock (talk) 07:39, 28 September 2014 (UTC)

Yes, it's not absolute speed alone that matters, but how quickly a given point in your visual field moves from not having the object in it to having the object in it, to not having it there again. The direction of movement is important, and even a bullet in motion can be visible if it's moving straight away from your eye or towards your eye (duck !). The size of the object and distance also matters, and the Moon is large enough and far enough away that it takes quite some time to move noticeably. StuRat (talk) 18:56, 28 September 2014 (UTC)

Obviously...it's the speed that the object tracks across the retina...which is the angular velocity at the eye, not the linear velocity - so it doesn't matter how fast the moon is moving in linear velocity terms - it's the rate that it moves across the retina - which is incredibly slow. The size of the object (on the retina) also matters. In the end, it's the amount of time that the focussed light from the object remains on a particular cell in the retina that matters. When you hit the cell with light, it takes time for enough light to accumulate to cause the cell to fire - and after it's fired, it takes a while for it to recover enough to fire again. However, how much time that takes depends on the brightness of the light. A very brief (but intense) flash (like a lighting strike) is enough to trigger the perception of light - but the light reflected off of a passing bullet isn't. Notice that you can easily see 'tracer rounds' fired at the same speed as a normal bullet - that's because they are so bright that the cells in your eye get enough light to cause them to 'fire' in a very short amount of time. A regular bullet at the exact same speed and distance is invisible. SteveBaker (talk) 13:42, 29 September 2014 (UTC)

Re: "it doesn't matter how fast the moon is moving in linear velocity terms". That seems a bit too strong, since it's linear speed, distance, direction, relative luminosity, and size all play a part. StuRat (talk) 17:29, 3 October 2014 (UTC)

Motion perception is the relevant article. Tevildo (talk) 09:28, 28 September 2014 (UTC)

Is there any phenomenon in physics which can explain why 1- above mentioned spinning blades or spooks become out of sight 2- we can see through them.162.157.248.16 (talk) 22:48, 28 September 2014 (UTC)EEK

1+2) The brain averages out the spokes and the image behind them, and you see a slight graying of the image through the spinning wheel. There's a toy with a spinning wheel like that, and if half the image is blue and half is yellow, it all averages out in the brain and you see green. StuRat (talk) 23:05, 28 September 2014 (UTC)

The fact that motion does eventually blur into a smear means that there are limits. I do know that this limit varies significantly from person to person - and also across the retina. When a sequence of still images fuses into smooth motion is one indication of those limits. I used to work in flight simulation - and having flickery displays proved to be a serious impediment to people trying to learn how to do air combat in a simulator with a wide field of view display.

With the center of your eye, very few people have trouble seeing flicker in a 60Hz computer screen or TV tube - quite a lot of people see flicker in 24Hz movie theatres (they used to be called "the flicks" because they flicker). I met one guy who continued to see flicker all the way up to 90Hz. He has to use a special computer monitor that refreshes at 120Hz and he's completely unable to enjoy television or movies. I've also done experiments that show that some people still see continuous motion at 12Hz. So the limit is probably somewhere between 10 and 100Hz. However, at the edges of the retina, most people see flicker at 50Hz and quite a few at 60Hz - but very few see it above 70Hz.

SteveBaker (talk) 13:35, 29 September 2014 (UTC)

Movies haven't been 24Hz for a while, the shutter closes either 2 or 3 times a frame while the film only moves once. If someone sees 100Hz flicker, they could be pissed off if they were too poor to leave the 50Hz area (the entire Europe?) and was easily annoyed. (I'm distracted watching '14 World Cup super-slow motion, is it that hard to make 4 phase lights?) Sagittarian Milky Way (talk) 07:28, 30 September 2014 (UTC)