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I saw this symbol like:

$$\lambda=3000\overset{\circ}{\text{A}}$$

and I don't know what this means. Is it a frequency? (since $\lambda$ is usually used for frequency)

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Note that $\lambda$ is usually wavelength not frequency, though the two concepts are related by the wave speed. –  dmckee May 13 at 3:09
    
I've edited the tags accordingly, but I left frequency because it would be kind of odd to have a question that reads as if it is about frequency without it. –  David Z May 13 at 3:20
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In case you ever need to look up a letter/symbol, search it in Wikipedia, and usually they list the applications in mathematics and physics. –  JamalS May 13 at 6:54
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2 Answers 2

up vote 21 down vote accepted

It is an ångström, a unit of length commonly used in chemistry to measure things like atomic radii and bond lengths. Although not an official SI unit, it has a simple relationship to the metric units of length:

$$1\:\mathrm{ångström} = 1\:\mathrm{Å} = 10^{−10}\:\mathrm{m} = 0.1\:\mathrm{nm} = 100\:\mathrm{pm}.$$

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The angstrom was intended to be a tenth-metre, but because the definition being used was more exact than the metre at the time, and based on a mis-estimation of the Stockholm prototype, a different name is used. In a similar vein, the X-Unit is defined as a thirteenth-metre, but also is out by some factor, so is not so called.

It was first used to measure spectral lines of the visible spectrum, but the length is convenient size for atoms and molecules. A hydrogen atom has a diameter of 1.058 A.

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What are you babbling about? Decimetres and thirteenth-metres? Go away. –  Phil Perry May 13 at 14:30
    
a "tenth-meter" would be a decimeter, not an angstrom... You're making no sense whatsoever. –  jwenting May 13 at 14:34
    
I suspect this is a translation issue, as "tenth-meter" likely corresponds to $10^{-10}$ here. See here for a dictionary example of the usage (though apparently the hyphen is omitted by convention.) –  Dan Bryant May 13 at 15:24
    
If the answer was all garbled due to Wendy's poor English skills, I apologize for being harsh, but her answer made absolutely no sense as written. 10^-10 m would make some sense, but what is "thirteenth-metre"? 10^-13 m? –  Phil Perry May 13 at 17:12
    
@Phil, see the X unit, which is indeed approximately $10^{-13}m$. Wendy has pointed out some subtleties here in that the angstrom was at one time only approximately a tenthmeter ($10^{-10}m$). It is no longer approximate, but the historical fact of it having been so is a justification for why it has a different name. –  Dan Bryant May 13 at 20:37
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