Recent comments in /f/askscience

Indemnity4 t1_j2usq4x wrote

Reply to comment by graebot in Molecularly, what make pine pitch sticky? by orangegore

The properties of tar will not change in a vacuum (depending on how strong is the vacuum).

Some glues are air drying, but pine tar is not. The adhesion and cohesion properties do not change with oxygen or gravity.

However, pine tar is a mixture of hydrocarbons, acids and bases. At low vapour pressure some of those will evaporate / outgas. All the volatile stuff is acting as a solvent for the non-volatile stuff. Too strong a vacuum or too long in vacuum and the pine tar will become hard and brittle.

Problems exist in space because of the vacuum but also it's really cold. Many adhesives are only active above the glass transition temperature or melting point.

There are test standards for minimal outgassing required for adhesives used in space craft. It must not lose too much mass, and that lost mass should not recondense in a harmful way.

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Indemnity4 t1_j2us3j3 wrote

Reply to Molecularly, what make pine pitch sticky? by orangegore

> what makes it adhere and resist pulling your fingers apart?

We call that effect "tackiness". In the link a bunch of scientists (including one from 3M) discuss it better than I can.

> Also, is the pitch molecularly homogenous?

Not quite.

It is a mixture of different molecules of varying length. It's roughly the equivalent of your junk drawer full of loose cables. Yes, they are all cables, but different connectors, sometimes more than two ends, different lengths, etc.

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Prestigious_Carpet29 t1_j2ups0a wrote

Reply to Does cold temperature make vistas more 'crisp' looking? by colorado_hick

Plenty of other good answers.

In addition to the comments about the air effectively being "clearer" at low temperatures etc, there is a known visual or psychovisual phenomena where higher-contrast images or scenes are perceived as being sharper (this is exploited by people trying to sell you new TVs etc).

"Mucky" air will decrease the contrast (as well as perhaps physically blurring) which will make the scene "pop" less. As others have said, the angle of the sun can also dramatically affect the scene contrast.

Related to this, I find general urban street scenes "pop" in the sunshine shortly after rain - the rain clears the air, washes away dust, and if surfaces are wet and shiny the contrast is much higher - it can look "hyper-real".

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Prestigious_Carpet29 t1_j2uolrk wrote

Reply to How close does one need to bring two coloured lights together to perceive a compound colour effect? by romxza

I was going to say, an LCD (or other computer/TV) display screen is the classic example of where colours in close proximity (RGB sub-pixels) are perceived as the compound colour.

Basically I second kilotesla's answer, but will add some additional clarification and related ideas.

The "resolution" of the human eye is highest in the very central few degrees of vision, the "fovea", which is populated with "cone" cells which are colour-sensitive. The rest of the visual field is mostly populated by rod cells, which are only brightness-sensitive - but are sensitive to lower light-levels. See https://www.cis.rit.edu/people/faculty/montag/vandplite/pages/chap_9/ch9p1.html#:~:text=Rods%20are%20responsible%20for%20vision,is%20populated%20exclusively%20by%20cones.

Empirically (presumably in the fovea) the human visual system has a higher "resolution" for brightness or "luminance" than for colour - this has been exploited for decades in the way analogue colour television or JPEG images are encoded - with the colour being coded ("sub-sampled") at a lower resolution than the brightness (to reduce the information), with little visual perceptual loss.

Empirically the black-and-white resolution of the eye is of the order of 300dpi at about 14 inches for someone with good vision who can focus properly, in high-ambient light levels. You could probably find a reference to the definition of "20/20 vision" and get a comparable angle subtended. In very low light levels, the effective resolving power will be lower.

Combining those two observations, I would expect the colour resolution to be something like 80-150dpi at 14 inches. This is equivalent to a subtended angle of around 1/(100*14) radians, so (180/pi)/(100*14) = 0.04 degrees, give or take.

If the coloured lines or stripes are closer than that sort of subtended angle, the colours are likely to merge into one - they will not be "resolved". The merging will work a bit better where the pattern is alternating (like in a TV or computer screen), rather than just a single source of each colour - in the latter case you may still perceive a coloured "fringe" on each side, even when you can't properly resolve the two colours.

In the early days of LCD computer screens, in the early 2000's, when they were only 1024x768 resolution, and before the days of sub-pixel font rendering, if you had white text on a black background, where the letters were only 1 pixel wide, the text often appeared to have some chromatic aberration, an orange-tinge on the left of the letters and a bluey tinge on the right, just because of the subpixel layout. As displays became higher resolution, and font often rendered more than 1 pixel wide, and/or they used more-clever sub-pixel rendering techniques (such as Microsoft's ClearType) these effects largely became consigned to history.
See also https://en.wikipedia.org/wiki/Subpixel_rendering#:~:text=Subpixel%20rendering%20is%20a%20way,the%20screen%20type's%20physical%20properties.

Don't take my word for it... you could print a piece of paper with fine alternating black and white lines and establish at what distance the lines cease to be resolved and "go shimmery" and then merge into grey - to get a gauge of your own personal black and white resolution.
If you can find an old Trinitron cathode-ray tube (this uses RGB lines of phosphor) - which is lower resolution than modern displays, you could try the same thing - look closely, then move back until the colours merge. If you can determine the pitch of the stripes and the distance, you can work out the subtended-angle when the colours merge.

Perhaps more easily, you could create a graphic of red/green/blue stripes on your computer (make each line several pixels wide) then see how far away you need to be for the lines to merge and it looks white.

The results are likely to be slightly different (fuse at a slightly closer distance) if you match the luminance of the coloured stripes (have blue at full brightness, red somewhat less, and green lower still), making probably a bluey-lilac colour when merged.

If you do some Google searches (other search engines are available) relating to measuring the resolving power of optical systems, contrast ratio etc, this will get you a sense of the underlying physics, which is then largely applicable to the eye - for the purposes of the question.

(I'm a physicist/electronics engineer, who has also spent several years of my professional life in optics, imaging systems, colour-reproduction and display-screen technology.)

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FMM_UV-32 t1_j2uin7i wrote

Reply to comment by Alittlebitmorbid in Are there any examples of natural hybridization, or any possibilities of this occurring in nature? by Owlsthirdeye

I think that genetic sequencing established that przewalski's horse might be wild,since genetic evidence shows that they are on a separate branch, meaning they are most likely wild:https://www.science.org/content/article/ancient-dna-upends-horse-family-tree

It still isn't clear what the genetic ancestry of domestic horses is though.

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Triairius t1_j2ugj9b wrote

Reply to comment by graebot in Molecularly, what make pine pitch sticky? by orangegore

I am actually unsure, but I suspect it might be the same amount of sticky, as there is still no air getting in to regulate the pressure. However, the tar itself might react to a vacuum somehow.

These are just my thoughts, since your question intrigued me. Someone else will hopefully provide an informed answer.

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