In looking at spectroscopes and spectrum tubes, it seems like there are two different approaches to having a narrow line to view with some dispersing aid. So is it the job of the spectroscope to make the light source a narrow line, or is it the job of the light source to be a narrow line?
Is there a name for the distinction?
I got some nifty spectrum tubes for Christmas, to add to my element collection (hydrogen, argon, neon and xenon). These are glass tubes about 10" long, with electrodes in the end, filled with the gas of interest at low pressure. The middle third of the tube is much narrower, a capillary of maybe a millimeter or two inside diameter. The neon one is, in effect, a neon sign that is just a single bright straight short line segment. The other ones are analogs of the same. I think they are narrow and straight so that you can look at them through anything that disperses the light by wavelength, such as a diffraction grating. You see nice neat crisp narrow lines because the light source itself is a nice neat narrow crisp lamp. In this case, it’s the job of the lamp to create a narrow line, and the spectroscope is only a dispersing means (I think).
But I also have a spectroscope at work which is a handheld tube that fits into a box about the size of an office stapler. It’s a cylinder with a lens at one end and an adjustable slit at the other. You turn a knurled ring to make the slit wider or narrower. You can shine any light on the slit, for example from a diffuse LED, and see a spectrum. In this case it’s the job of the spectroscope to create a narrow line.
So if I wanted to use the work spectroscope (or one like it) to see my elemental line spectra, I’m not going to image the beautiful straight line spectrum tube, right? I’m only going to get the spectroscope very close to the capillary to brightly sample as much of the emitted light as possible. In this case I have two different places that are creating a narrow line, which is actually a disadvantage because I can’t couple the two together efficiently (unless I mess with some kind of a relay lens).
How is this supposed to work? Are there two different kinds of spectroscope in this special sense?
I believe it’s the spectroscope’s job to create a slit to view any random light source you select, since not every light source is a linear source to begin. The version I recall from Physic lab had a slit and some lenses to produce a nice, narrow light source. That fed onto a plate with a prism on it, then an eyepiece. You could rotate the eyepiece and see the relative angles of the spectral lines by centering them in the eyepiece reticule (IIRC). The thinner the slit, the thinner the resulting lines.
Are you sure the tubes you got are not designed to be put in a special fixture which has a slit?
As I understand the term, “spectroscope” is synonymous with spectrometer, which is a device to measure the property of light. For a spectrometer to produce a narrow line, the light source needs to have a narrow line length, and the spectrometer needs to have a high resolution. If you have a perfectly monochromatic (infinitely narrow line width) light source but an imperfect spectrometer, you’ll end up with a broad line. If you have a light source that produces a broad line or a continuum, then even a perfect spectrometer will show a broad spectrum.
If you are wanting to create a narrow line from a broad/continuum light source, you can use a narrow bandpass filter (such as an interference filter), or better yet, a monochromator, which is basically a spectrometer with an exit slit.
Ah, that makes sense! I suspect you have got to the heart of the matter.Thank you, Bee-wolf!
They are designed to be put in a special fixture, which is a tidy black box that provides a dark and nonreflective background behind the tube. But it doesn’t enclose the tube. The design of the tube, particularly the capillary section, creates a narrow light source just as a slit might have done.
The black box also has spring loaded contacts to hold the tube and deliver 5000 volts to it. Memo to self, when plugging this in the first time, check the status of the on/off switch before probing the spring loaded contacts with a finger…
where we read “The narrow middle section eliminates the need for a slit-collimator when examining the light output with a simple prism or grating spectroscope (it also ensures high brightness when a slit is used).”
It al depends on what you want to do.
As noted above, a monochromator is the device you use to take a light source and select from it a tunable single-color light. Invariably, there is going to be quite a bit of spread in wavelengths. You can narrow this by making your slits narrower, but there’s a limit.
A Spectroscope is a device for viewing spectral lines. There are various designs, and they can be built using prisms or gratings (or even other devices), but the idea is to set out the wwavelength makeup so you can see it. Hence “scope” part.
A spectrometer is a measuring device that gives you the relative strength of the light as a function of wavelength. They used to be basically monochromators with a detector at the output and a drive mechanism that varied the wavelength, but nowadays they’re more like spectroscopes with an array of detectors spanning the displayed spectrum, so they give much faster results.
It’s not the “job” of a source to do anything. A source is what it is. You can select a narrow-line source if that’s what you need. Discharge tubes are not simply teaching devices – they can be extremely useful in calibrating spectrometers and spectroscopes, for instance. But there are other ways of produced narrow lines besides discharge tubes. Arc spectra, for instance, or flame spectra (the two very narrow lines from sodium salts in a hot flame, for instance). And, of course, laser light, however produced. If you don’t need particularly sharp narrow-band output, many fluorescent emission lines are relatively narrow. You can tailor-make these now with quantum dots. And there’s always the light from LEDs
There are many other ways to separate light besides prisms and gratings. There is the Fabry-Perot interferometer, and the Fourier TRansform spectrometer based on it.
I once built a FT spectrometer that had no moving parts, but it required extremely sharp and narrow line sources. So I had to use those gas discharge tubes mentioned above. A laser was sharp enough, as was the light from sodium salt in a flame. But the outrput of an LED or from a monochromator was hopelessly broad, despite the rerlatively narrow output.