amp. Around the edges are spectra whose source and identity aren’t immediately obvious.
So I marked off all the lines I could identify. Here they are:
The yellow lines relate to the low-pressure sodium vapor lamp, the light blue bands identify light from the incandescent lamp, and the green lines relate to the mercury lamp. There are additional continua from the incandescent lamp to the right of the field of view. Let’s dissect these spectra.
Low-pressure sodium vapor lamp (identified with yellow lines)
At the left, middle, is zero-order for the sodium vapor lamp. Because the lamp is large, the 588.9 nm/589.5 nm doublet cannot be resolved. From the center of the zero order image to the center of the first order image is about 3/8 of the picture width, and second order is another 3/8 of the way across. As one would expect, the space between orders is about the same. Where’s third order? It’s off to the right of the field of view.
Near the bottom of the picture, there’s a reflection of the light from the lamp. And sure enough, first and (weakly) second order show up as the grating disperses light from the reflection as well as from the original lamp.
The diffraction orders are tilted to make for easier annotation (I was thinking ahead). If I had rotated the grating about 5° clockwise, the orders would have been dispersed parallel to the horizontal edges of the photo, but then distinguishing some orders from each other would have been more difficult.
Incandescent lamp (identified with blue lines)
Zero order shows a highly saturated image of the incandescent lamp. First order (with the visible range marked in the image) comes not only from the filament but also from glints off the glass envelope of the bulb. There is a weak second order spectrum farther to the left but I left it unmarked because the mercury arc lamp and sodium vapor lamp give more prominent features in this region. But can you see the continuous spectrum there?
Mercury arc lamp (identified with green lines)
Analyzing these lines is pretty simple at first. 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