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Offline samone

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spectroscopy ?
« on: June 05, 2013, 12:40:04 AM »
question 1
I know all of the electromagnetic waves have color in the visible light spectrum.
(i.e we see these with are own built in receiver are eyes)
But is there away of associating a color to different electromagnetic waves other then just the visible light spectrum? ( I would have to say it must exist since infrared and xray images are visible to the eyes) So how are we mapping the visible colors to the other parts of the spectrum when viewing xrays , radio waves , infrared ,...etc)
Hope you understand what I am getting at since even if you have an infrared sensor that converts the different infrared wavelengths emitted by an object to different electrical voltages how does one go about choosing the color or does it even matter.

I am not sure I am even thing about it correctly but I believe I am you observe the emitted wave by converting it into some type of voltage based on the energy it carries then map a color to it. If I am right then is there any standard for choosing the colors associated to the voltage I know this probably doesn't mater that much but I am curious to know if my understanding of how it is done is correct and weather color association matters

question 2
Like for xray is it always black and white based on flashing xray's on the object and the object either emits all the colors (white ) or absorbs all the colors (black)...
I would imagine some material would exist that if you xrayed it you would get something in between like red or blue ,...curious to know because I have only seen black and white....

Maybe I am wrong with this and should have said xray only emit usually to voltage states in most cases that are generally mapped to black or white.
Please correct me and let me know which is the correct way it goes...
Based on the first question it would be more logical to thing it goes with the later

question 3
When measuring the composition of a distant object (star ,...etc) or small chemical compound is there always a certain electromagnetic wave on the spectrum so that if one was to transmit it/bounce it off the object the object would then emit a visible color back to us?


Thanks for any help
I realize night vision now is just amplifying light from the stars and moon and isn't truly using infrared ... sort of like a mini earing aid for your eyes in the dark.  :o
« Last Edit: June 05, 2013, 12:50:13 AM by samone »

Offline Borek

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Re: spectroscopy ?
« Reply #1 on: June 05, 2013, 03:09:47 AM »
is there any standard for choosing the colors

No, it is completely arbitrary. For IR we usually use white/orange/red for the higher energy photons as it more or less follows the way hot objects emits light, but then we use blue/green for lower energies. For other ranges sometimes we use blue for a higher energy photons and red for a lower energy photons, but - again - it is completely arbitrary.

Quote
Like for xray is it always black and white

If I understand you correctly, you think in terms of radiographic image obtained using xrays (say, of a broken bone). On these images gray scale has nothing to do with the wavelength, but with the intensity (brightness) of the rays passing through the xrayed object (and it is a negative picture - white means low intensity, while black means a high one).

Quote
When measuring the composition of a distant object (star ,...etc) or small chemical compound is there always a certain electromagnetic wave on the spectrum so that if one was to transmit it/bounce it off the object the object would then emit a visible color back to us?

No. Besides, that's not how we measure distances on the astronomical scale.
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Offline samone

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Re: spectroscopy ?
« Reply #2 on: June 05, 2013, 03:14:47 PM »
Ok then one other question I had on this
Is for IR , xray , ultraviolet , gamma/cosmic transceivers

How does one detect those electromagnetic waves is it like the way radio and microwaves are detected with a tuning circuit just tuned to a different resonating frequencies in those electromagnetic bands of the spectrum.

Basically I am not trying to understand right know how to build a receiver/transmitter or detector (demodulator) / transmitter (modulator)
for these. But I am curious if it is all the same principle as with radio wave detection
one receives the radio wave on its antenna and the detector circuit converts it into an electrical voltage which then gets amplified and output to a speaker
In the cases of visualizing it it would be output to some lcd or crt monitor.

Or if there was a totally different process of detecting/transmitting these waves without using antennas or satellite dishes or converting these waves to electrical impulses that are then mapped to a certain sound or color...
Or if that is how most of it is done. I know for radio waves that's how it goes from my experiences.

Offline Borek

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Re: spectroscopy ?
« Reply #3 on: June 05, 2013, 04:03:09 PM »
No, they use different principles. I suppose by googling for different combinations of words like electromagnetic, wave, detection, detector you should be able to find more.
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Offline samone

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Re: spectroscopy ?
« Reply #4 on: June 06, 2013, 01:36:16 AM »
So they are not all necessarily  detected by electronic circuitry ? Find that hard to believe. At least it would have to be some sensor (probably chemical electric based)
hooked up to an electric circuit.

Basically electrical waves I would think would at least have to be detected thru chemical-electric based (sensor that react to the waves and give off a voltage for them ) or electrical circuit based like the way radio waves are detected I.e with a tuning circuit/demodulator , envelope detector ,...etc.

What other methods could there be at the very least the detecting object used would have to be an object that reacts to a wave and gives off a particular voltage/current for it. And the transmission would have to be an object that oscillated at a certain wave / also be able to modulate on it in the frequency range of the electromagnetic spectrum. There maybe many different machines to do this stuff or sensors to do this stuff but essentially is just that in bold

Offline samone

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Re: spectroscopy ?
« Reply #5 on: June 06, 2013, 01:45:59 AM »
As for

Quote
Quote

When measuring the composition of a distant object (star ,...etc) or small chemical compound is there always a certain electromagnetic wave on the spectrum so that if one was to transmit it/bounce it off the object the object would then emit a visible color back to us?



No. Besides, that's not how we measure distances on the astronomical scale.

I understand that for far away objects for space but what about objects on earth
Is there not always a wave we could emit on an object so that object emits some wave in the visible light spectrum back ?
Maybe your right with the black body objects but even those I would image emit something

Anyway thanks I think I got me reasoning sound now.

Though curious how many transceivers, receivers , transmitters could be handmade from scratch by a home user. I only ever made those devices for radio waves from scratch by buying the individual cap , resistors , inductor , making the circuit ...etc . Though I am curious if it is possible to make the other electromagnetic spectrum of transceivers, receivers , transmitters  with only home tools and local equipment/limited money. I would imagine the answer is no but curious if anybody out there has maybe IR or microwave I would imagine though the other ones like above visible light I would have to say probably to difficult with limited money

Offline Borek

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Re: spectroscopy ?
« Reply #6 on: June 06, 2013, 03:27:44 AM »
You are right there is electronic circuitry involved, but the sensors are different from those used for radio waves. Googling for "IR detector construction" already gives links to pages explaining how IR is detected. Have you tried to visit them?

Quote
Is there not always a wave we could emit on an object so that object emits some wave in the visible light spectrum back ?

No. Even assuming just heated black body - to observe it in the visible light you would need to heat it up substantially, not every substance will retain its properties. For an exaggerated example - think about solidified hydrogen.
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Offline Enthalpy

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Re: spectroscopy ?
« Reply #7 on: June 06, 2013, 02:15:12 PM »
Detectors and selectivity:

Up to at least 300GHz (probably a lot more these days) receivers can be built like at 1MHz or 1GHz, with preamps, mixers, filters... Which does not mean that they're as sensitive. Active amplifiers, including low-noise, are presently common at 100GHz; well above that, it's more a domain for varactor mixers and harmonic generators, which only lose power, with all the consequences on the noise figure.

So-called terahertz waves (they use to be the upper GHz... marketing) have no active amplifiers, no continuous-wave transmitters... Nothing accurate is done with them, just a few images and vague spectra at metre range.
Some day better components may emerge from my suggestions there
http://www.physforum.com/index.php?showtopic=15617&st=30
but I must first make drawings and understandable explanations. Presently, it's a holy mess, sorry.

Things improve again around 10┬Ám, for which bolometers exist, as well as quantum detectors like HgTe. These are not selective, they just give one electron per photon.

Same in the near-IR, visible, UV : selectivity and spectra are made by other means. Check for diffraction grating, Fourier transform spectroscopy, interference filters... plus in some future my evanescent wave filter, there:
http://www.scienceforums.net/topic/74445-evanescent-wave-optical-filter/

The usual means is to have very wide-band quantum detectors. Though, optronics also uses non-linear crystals the same way as electronics takes non-linear components to make down-mixers, the output at radio frequency being then processed as in a radio receiver, with great selectivity. It relies on big power density at a stable frequency from the local oscillator, hence works only at the available laser frequencies. Typical for Lidar.

At X and gamma rays, detectors have little selectivity. They give several electrons per incoming photon; the number of electrons in a pulse tells the approximate energy, provided the photons come isolated enough and the detector is proportional. For some X-ray energies, atoms exist that absorb more strongly just above some energy threshold (=when the 1s ionization energy is exceeded, also the 2s...), so removable filters can be made, to compare the picture with/without the filter(s) and deduce how much energy arrives in that (many % wide) energy band. This allows to know what atoms compose an observed material.

Offline Wastrel

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Re: spectroscopy ?
« Reply #8 on: June 08, 2013, 09:07:08 AM »
question 2
Like for xray is it always black and white based on flashing xray's on the object and the object either emits all the colors (white ) or absorbs all the colors (black)...
I would imagine some material would exist that if you xrayed it you would get something in between like red or blue ,...curious to know because I have only seen black and white....

The world is not just in colour, it's actually much more rich in information.  Colour is the very crude way our eyes captures some of this information from the limited range we call visible.  It's not useful to split X rays into 3 bands, generally we either want intensity for a whole area (an image) or full spectroscopic information about a single point for analysis.

question 3
When measuring the composition of a distant object (star ,...etc) or small chemical compound is there always a certain electromagnetic wave on the spectrum so that if one was to transmit it/bounce it off the object the object would then emit a visible color back to us?
This is not impossible but there are big problems.  Something that is 10 light years away will take 10 years for our EM pulse to arrive and another 10 to get the reply, which will be unimaginably faint.  Short of being able to lase a star this will never be a useful method.  Instead it makes more sense to look for known IR and microwave frequencies in existing sources.  In light reflected from a star or when a planet passes in front it's star.

At X and gamma rays, detectors have little selectivity.
This simply isn't true.  There are a wide variety of detectors with different strengths, some of the semiconductor detectors have very high spectral resolution.

Offline curiouscat

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Re: spectroscopy ?
« Reply #9 on: June 08, 2013, 09:55:09 AM »
[
At X and gamma rays, detectors have little selectivity.
This simply isn't true.  There are a wide variety of detectors with different strengths, some of the semiconductor detectors have very high spectral resolution.

+1 There seem to be XRay and Gamma detectors with fairly good resolution.

http://www.canberra.com/literature/fundamental-principles/pdf/Gamma-Xray-Detection.pdf


Offline Enthalpy

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Re: spectroscopy ?
« Reply #10 on: June 10, 2013, 05:48:25 PM »
There are a wide variety of [X and gamma rays] detectors [...], some [...] have very high spectral resolution.

We were comparing among others with radio-waves and their heterodyne receivers, which have typically 10kHz selectivity or better at 1GHz.
Quoting Sameone: "...those electromagnetic waves - is it like the way radio and microwaves are detected with a tuned circuit"
Semiconductor particle detectors are very far from that, despite their resolution is amazing, yes. I had the ppm selectivity from radio in mind when writing "little selectivity for X and gamma".

DWDM on fibre optics isn't too bad neither
http://en.wikipedia.org/wiki/DWDM#Dense_WDM
100GHz channels around 1550nm or 194THz, this makes 500ppm channel width.

Offline Wastrel

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Re: spectroscopy ?
« Reply #11 on: June 11, 2013, 01:31:48 PM »
Selectivity is usually measured in decibels, I assume you mean resolution.  Since a radio channel is not an event the resolution is limited mainly by the quality of the local oscillator and the length of time being sampled.

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