irst. Then there’s a head-scratcher. But once the clues are evaluated, the problem solves itself in a flash!
Look up the neutral mercury spectrum with your favorite search engine, and you’ll find the brightest mercury lines are:
Sure enough, all four of these lines are easily visible from the mercury arc. But there’s a light blue line about half-way between the 435 nm line and the 546 nm line, and there’s also some red light there. What’s going on? It isn’t mercury emission. I looked at the NIST wavelength charts. A lot of mercury lamps have an inert gas fill to help the lamp ignite; argon is common, helium, neon, krypton, and xenon less so because they are more expensive. The argon ion laser emits light at 488 nm, and I almost got suckered by that fact. However, I got to thinking,
Upon further reflection, I went back and thought about this a bit more carefully. I interpolated the wavelength of the light blue line. Moving horizontally for + 1st order,
So let’s do linear interpolation.
(754-728)/(778-728) = 0.52
0.52*(546-435) = 58 nm
435 nm + 58 nm = 493 nm
We assume that dispersion is constant everywhere (a poor assumption, even in the best of spectrometers), so if we’re off by a few nanometers, that’s plausible. What common gas emits near 490 nm? And then it hit me. Bohr atom! Hydrogen atomic emission! Blue-green line (H beta) is at 486 nm, and the red (H alpha) line is at 656 nm! 656 nm is red, and lo and behold, there’s red light in this spectrum! While I can’t prove it without a better calibrated instrument and better resolution, it sure is a simple answer to say there’s hydrogen in this mercury lamp.
And that, ladies and gentlemen, is how one uses atomic emission spectroscopy for qualitative analysis.
So now we can label each line. To keep the image clean, use this concordance to link each labeled feature to its corresponding meaning.
|A||Sodium vapor lamp, zero order|
|B||Sodium vapor lamp, 589 nm, first order|
|C||Sodium vapor lamp, 589 nm, second order|
|D||Reflection of sodium vapor lamp, zero order|
|E||589 nm first order image of reflection of sodium vapor lamp|
|F||589 nm second order image of reflection of sodium vapor lamp|
|G||Zero order, incandescent lamp|
|H||Blue end of first order, incandescent lamp|
|I||Red end of first order, incandescent lamp|
|J||Zero order, mercury lamp|
|K||First order 404 nm mercury|
|L||First order 435 nm mercury|
|M||First order 486 nm hydrogen|
|N||First order 546 nm mercury|
|P||First order 656 nm hydrogen|
|Q||Second order 404 nm mercury|
|R||Second order 435 nm mercury|
|S||-1 order 404 nm mercury|
|T||-1 order 435 nm mercury|
|U||-1 order 486 nm hydrogen|
|V||-1 order 546 nm mercury|
|W||-1 order 579 nm mercury|
|X||-1 order 656 nm hydrogen|
|Y||Zero order, reflection of mercury lamp|
|Z||First order 404 nm mercury from reflection of lamp|
|a||First order 435 nm mercury from reflection of lamp|
|b||-1 order 404 nm mercury from reflection of lamp|
|c||-1 order 435 nm mercury from reflection of lamp|
|d||-1 order 486 nm hydrogen from reflection of lamp|
|e||-1 order 546 nm mercury from reflection of lamp|
|f||-1 order 579 nm mercury from reflection of lamp|
Once again, serendipity has reared its head (see my TEDxUIUC talk). I went to the museum to get some material for my blog, but what you’ve read here wasn’t what I was looking for. When I was a postdoctoral fellow in 1978-1979, I visited the same museum, and at that time there was an exhibit with Henry Rowland’s original grating ruling engine – complete with a foot switch to step on so I (or any visitor) could rule the next groove on a piece of speculum metal. I figured description of early grating ruling engines, grating replication and how holographic gratings are now replacing ruled, replica gratings in many cases would make an interesting blog post. But look what happened instead!