Spectrometer Lab

Seth Dowler

Lab: Spectrometer Lab

Abstract

In this lab students learn to analyze and identity various chemical compositions of gases by observing their spectral emissions.

Introduction

When light passes through a prism, it is split into patterned wavelengths corresponding to its chemical makeup. By analyzing these patterns, it is possible to determine the identity of said compounds. This practice can be performed even with fairly basic technology today; all that’s needed is a spectrometer and the light-emitting gas itself. 

Procedures

Part 1: Observing spectra

A. In part one students learn to look through the spectrometer: “one looks through the diffraction grating at the narrow end of the device…glance to the side of the interior and see the spectrum displayed on the scale.”

B. Students are asked to observe what appears when looking at an incandescent light bulb through the spectrometer. What should appear is a continuous spectrum of light, ranging from purple to red.

C. Students learn how to use the spectrometer. The first step is to find the diffraction grating, which will be on the sloping end of the structure. The second is to aim this at a light source – the hole follows through the structure and out onto the other side, making a flat line along the base. Lastly, simply observe the resulting light spectrum that will appear along the wavelength gradient.

Part 2: Observing spectrum of an incandescent bulb

A. In this portion students observe the emission spectrum of an incandescent light bulb via the spectrometer, paying special attention to “notice that each color falls on a different portion of the wavelength scale.”

 

an example of an incandescent emission spectrum is in the photo above

The colors that correspond to each nm marking are as follows: 400 marks purple, 450 marks dark blue, 500 marks very light blue merging into very light green, 550 marks green, 600 marks yellowish-orange, 650 marks orangeish-red, and 700 marks red.

B. The lowest “limits” the human eye can perceive of the emission spectrum is roughly 400nm; the highest is around 700nm.

Part 3: Observing emission spectra of several elements

In this section students looked at several spectrum tubes, each filled with gas of a specific chemical element; current flows through the gas, thus lighting up the tubes. A hot gas emits only certain colors of light, so students were asked to record their observations of the emission spectra of each tube via their spectrometers, and to take educated guesses as to what elements the tubes held.

This website is very helpful in detailing possibilities for students’ spectra recordings.

Results

The following are the results of my viewings for Part 3.

Gas 1

Continuous spectrum from incandescent bulb, from roughly 400-650nm, made up of the colors of reddish orange to yellow to light green.

Gas 2

Guess: hydrogen

My readings for this gas were red at 670, blue at 470, and purple at 430.

The gas itself appeared to be a flickering, burning blue light, arctic-like.

 

Unfortunately, my photo for this gas did not turn out well, so a stock photo has replaced it.

Gas 3

Guess: neon

My readings for this gas were multiple lines, roughly twenty lines from 525-675nm, with the brightest at 630-660, where roughly 10 red, 3 orange, and 2 yellow lines existed.

The gas itself appeared to be red-orange, and rather than flickering, it burned steadily – which makes more sense of the neon guess, as neon’s use in advertising partly being due to its steady visibility (as well as vivid coloring).

This stock photo corresponds with said description, albeit there being less red lines.

Gas 4

Guess: krypton

My readings for this gas were a bright yellow line at 580, a bright green line at 550, a light blue-green line at 510, and a darkish blue line at 460.

The gas itself appeared as a light blue steady flame, with purplish coloring on either end of the tube.

This stock image corresponds best with these estimates, therefore krypton seems to be the most likely gas.

Gas 5

Guess: helium

My readings for this gas were dark blue at 450nm, light green-blue at 490nm, bright yellow at 570nm, red at 665nm, and faint red at 695nm.

The gas itself appeared to be merely a pale whitish light.

Gas 6

Guess: sodium

My readings for this gas were only 1 light orange-yellow line at 583nm.

The gas itself appeared orange.

Gas 7

Guess: argon

My readings for this gas were 430 purple, 540 green, and 574 orange.

The gas appeared white with slight blue colorings.

This stock photo of argon’s emission spectrum seems to be the best fit for my recordings.

Conclusion

The objective purpose of this lab was to observe the spectral lines of various in-class lights, and then to determine the chemical gases they contain through our recordings. As a class we found this to be a very technical process, whether it be taking photos of the spectra or merely seeing them with our own eyes beforehand. As Dr. Plavchan pointed out, this is indeed in accordance with the practice of science in general, because we don’t know what the answers will be – until we find them…though sometimes not even then.