Wikipedia:Reference desk/Archives/Science/2014 September 24

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

September 24

Colour in electron microscopy

Asking from a lay point of view - I understand that everything is splattered in gold, so a true-colour image would be pretty much monochrome anyway - but don't electron beams have wavelengths as photon beams do? - if so, why don't they translate to different colours on the monitor?

Thanks Adambrowne666 (talk) 00:27, 24 September 2014 (UTC)

...Because such an image is not generally useful, scientists rarely build or use a machine that would create such an image.

If you're willing to stretch your definition of "imaging," you can find machines that do actually distinguish different type of electrons: electron spectroscopy. For example, a long time ago, I studied results from the electron spectrometer on the DEMETER spacecraft. The signals we processed were not images - we processed the data into spectrograms. If you wanted to build an imager in the sense of an electron microscope, you'd have to do one of several things: you'd need to turn your electron microscope's imager into a spectrometer, either by adding a filter pattern (analogous to a color filter array on a digital camera); or you'd need to combine multiple exposures with different settings (e.g., hyperspectral imaging). Alternately, you'd need to take an existing spectrometer and turn it into an imager, by adding directionality and some kind of focusing plane. Or you could come up with something else new and creative that hasn't been invented yet! (Again, most scientists haven't invented such a device because it'd be really hard to make, and probably wouldn't provide much new useful information). But if it did work, it would provide an image based on the electron's energy - not its "color." As you already know, the perceptual relationship between color- and wavelength- is only relevant for photons in the visible spectrum!

Our article on SEM covers the use of false-color, and also describes some extant techniques used to collect multiple-data-values per pixel, which can be rendered in post-processing into colorful images.

Nimur (talk) 01:58, 24 September 2014 (UTC)

Excellent answer, thanks, Nimur. Adambrowne666 (talk) 02:27, 24 September 2014 (UTC)

Whether it's coated in gold depends on the sample. That's only done for non-conductive samples. It's unnecessary for conductive samples and it can be avoided in other situations by using a low accelerating voltage and/or low vacuum.

And the interaction between the electrons and the sample isn't reflection like with photons. Most of what is used to form the image in the SEM is secondary electrons produced by inelastic scattering events. They fall into a fairly small, low energy range and aren't very dependent on things like atomic number, so even if you could distinguish between electrons of different energy (wavelenth), I'm not sure it would give you any useful contrast. But the process used to collect them would make it basically impossible anyway. The electrons can be emitted in any direction, so the detector works by using a positively biased grid to accelerate them toward the detector, which means any of the original energy information would be lost.

The other main imaging mode uses backscattered electrons formed by elastic interactions with the nucleus. The energy is high enough and the angular distribution small enough that they're generally measured directly. But since the electrons can undergo multiple scattering events I don't know that the energy value would really be that useful.

There is a similar, but much less common technique that does use the energy of the electrons to form the image called scanning Auger microscopy, based on Auger electron spectroscopy. [1] has some example images. Because Auger electrons are emitted from the top few nanometers of the sample, the preparation requirements are much more stringent. Obviously coating would be impossible and even small amounts of surface contamination could make imaging impossible. Systems all have to use ultra-high vacuum. But like the more common SEM technique of energy-dispersive X-ray spectroscopy, which can generate similar images, the colors are assigned arbitrarily. Mr.Z-man 03:31, 24 September 2014 (UTC)

Oh thanks for that. I was just thinking about how one could get a colour and the Auger electron spectroscopy is basically what I was thinking of. So I'm just shy of being 100 years behind the times. Not too bad by my usual standards. ;-) Dmcq (talk) 08:45, 24 September 2014 (UTC)

Thanks, even more interesting. On another topic, from my reading of the WP article, I see no suggestion that samples might be altered or explode from within in the necessary vacuum. Also, where do I get hold of a goldbug after it's been used? Adambrowne666 (talk) 03:56, 24 September 2014 (UTC)

Except for things like certain polymers and biological materials (anything with a liquid in it), most things are pretty stable at room temperature under vacuum. Some materials can be damaged by the electron beam though. This is especially a concern in transmission electron microscopy where the samples are extremely thin and the accelerating voltages are ~10x higher than in SEM (or in some specialty cases, 100x higher). I'm not sure where you'd be able to get a used insect sample. Maybe contact your local university's entomology department. Mr.Z-man 17:33, 24 September 2014 (UTC)

Thanks again - and I think I'm going to nag a friend at uni for a used insect sample... Adambrowne666 (talk) 02:53, 25 September 2014 (UTC)

Moon in collision course with Earth

As shown in the TV miniseries Impact, is it possible that the moon might ever be on a collision course with Earth? If not, please explain the scientific reason behind it. Thanks. --EditorMakingEdits (talk) 01:55, 24 September 2014 (UTC)

Inertia is a real thing that exists! If the moon were on a collision course with Earth, ... we'd see it coming from two hundred thousand miles away! Perhaps you'd like to read about orbit or conservation of angular momentum: the moon won't just magically start coming towards us. Something immensely energetic would have to take place that could transfer energy and momentum to move the moon off its existing course, which is a very stable, nearly circular orbit around our planet. Objects like the Moon are quite large and massive: they do not change their motion with great vitesse. Nimur (talk) 02:00, 24 September 2014 (UTC)

(edit conflict) No. The moon is drifting away from earth. See Orbit of the Moon#Tidal evolution. The article Tidal acceleration explains why. --Jayron32 02:01, 24 September 2014 (UTC)

Right, but that's an incredibly slow change! Nimur (talk) 02:02, 24 September 2014 (UTC)

So an asteroid impact may send the moon on a collision course with Earth? (as trasfer of energy and momentum takes place) --EditorMakingEdits (talk) 02:07, 24 September 2014 (UTC)

It must be a very very big asteroid indeed! (or traveling very fast) 202.177.218.59 (talk) 03:13, 24 September 2014 (UTC)

It would have to be a stupendously huge asteroid. Unimaginably large. Here is a Quora response to exactly this question. Basically, it would take something a bit smaller than 4 Vesta. Huge. Mingmingla (talk) 03:18, 24 September 2014 (UTC)

Hmmmm, given the mass of the Moon and its orbital speed which you want to reduce to zero realtive to Earth in the collision, Vesta won't be enough. You would need approximately 4 times the mass of Vesta coming in at a relative speed of 60 km/s. Count Iblis (talk) 03:36, 24 September 2014 (UTC)

Some of the bits and pieces flying away from the impact even on the other side of the moon would probably come around and cause pretty complete devastation here long before the moon arrived to finish us off. And you would be hitting it on its side to get it to go towards the earth. Dmcq (talk) 08:56, 24 September 2014 (UTC)

If you hit it so as to stop the forward movement in it's orbit, it would then fall directly into the Earth. However, the impact required to do that might destroy the Moon. Perhaps a series of smaller impacts might do it, while leaving the Moon intact. See Comet Shoemaker–Levy 9. StuRat (talk) 17:52, 26 September 2014 (UTC)

Thanks for the explanations. --EditorMakingEdits (talk) 05:07, 24 September 2014 (UTC)

Mosquito clearance without killing other insects

When local government or landowners spray large areas with mosquito poison, are they also wiping out most of the other insects and devastating the local ecosystem? If not, how do they avoid it? 31.108.54.8 (talk) 12:38, 24 September 2014 (UTC)

You may find this interesting. Perhaps this one even moreso. --Jayron32 12:41, 24 September 2014 (UTC)

There are also non-chemical means to reduce mosquito numbers:

1) Drain any sitting water. Of course, this can affect other insects which depend on stagnant water.

2) There's a machine that burns fuel to generate heat and carbon dioxide, both of which attract mosquitoes, which are then killed and collected. The effect on other insects should be minimal, since they aren't attracted to the kill zone. StuRat (talk) 17:57, 26 September 2014 (UTC)

Reducing mosquito populations can be a complicated process, and is described in "Mosquito. A Natural History of our Most Persistent and Deadly Foe" by Andrew Spielman and Michael D'Antonio in chapter 6 "Man against mosquitoes". The techniques are described through several cases, so you pretty much have to read the whole chapter to get all of them. Water drainage appears to be the major portion, but there is a lot more. The chapter also describes eliminating malaria from a region, if that's of interest.--Wikimedes (talk) 04:05, 28 September 2014 (UTC)

Topic of discussion: World Recycle

please keep discussion in one place. Currently at WP:RDH
The following discussion has been closed. Please do not modify it.
Ladies and Gentlemen, Please kindly provide me the links related to the 'world recycle' topic, from science and religion, along with a brief knowledge in simple terms. Basic 'scientific & religious' possession of knowledge: The world tilts upside down apparently every 3,600 years, scientists disputes the idea, mythologies/ancient histories define otherwise. The world apparently does tilt upside down, Scientists say it occurs over many years during its rotation, definitely not every 3,600 years and definitely not in 30 minutes when the so called planet 'nibiru' appears. Note that some say planet 'nibiru' comes near Earth/Mars, some say near Pluto, and this is the only time when all the planet aligns, when truly the planets align every, in between a few years... Prophet Muhammad from Islam religion said, "Judgement Day will be the Day when the Sun seen rising from the opposite way". In ancient Egyptian stories, apparently they've evidence, well according to human knowledge, the world tilted upside down twice/The Sun rose the opposite way twice... I require extensive information and elaboration in simple terms... Kind regards, (Russell.mo (talk) 22:37, 24 September 2014 (UTC)) If the world literally turned upside down it would have catastrophic effects. If this story is not in Fringe science then it should be. ←Baseball Bugs What's up, Doc? carrots→ 00:14, 25 September 2014 (UTC) This speculation is in no way science, but we do have a good article on the Nibiru cataclysm, which should get you started. --Mark viking (talk) 00:26, 25 September 2014 (UTC)