Download Spectroscopy Lab

Name:
Date submitted:
Accepted / Returned
Self-Paced Instructor: ___________
Spectroscopy Lab
Background work:
This activity should be done after you have read two important parts of your text: The first
should be a section relating to light and what light is. The terminology of wavelength and
frequency will be used regularly and some knowledge of the electromagnetic spectrum is also
expected. The second section should discuss the quantum model of the atom and the idea of
discrete, quantized energy levels.
For Astronomy 1040 the necessary material is all in Chapter 5 in Bennett
For Physics 1000 the necessary material is Chapter 8 and Chapter 13 in Hobson
Introduction:
During this experiment you will make use of a device known as a diffraction grating
spectroscope. Much like a prism, a diffraction grating splits a single beam of light into the many
colors which it contains. The spectroscope is just a device for extracting quantitative information
from the spectrum, such as the wavelength of the various lines in the spectrum. The device uses
the same principles as the experimental apparatus used by scientists for almost two centuries to
study the emission spectra of the elements. These spectra played a crucial role in the
development of quantum science.
When using the spectroscope you look through the small end, and point the small slit on the
large end at the source. You should then see the individual colors spread out inside the
spectroscope to the right of the small slit. The colors are spread across a small scale labeled with
numbers representing the wavelength of the light. These numbers represent hundreds of
nanometers (i.e., a reading of 4 means 400 nanometers). The visible spectrum of light includes
wavelengths from about 400 to 700 nm (nanometers).
Source 1: Incandescent Bulb
When observing the spectrum of filament or bulb you may have to aim your spectroscope
slightly to the side of the actual filament in order to see the best spectrum.
A. What color is the filament when observed with the naked eye?
B. If you look for the color “orange” using your spectroscope, roughly what wavelength would
that be?
C. Sketch and describe the emission spectrum you see from the incandescent source using the
spectroscope. You should probably to get very close to the bulb. Make sure that your words
and your drawing convey what you see. Are there colors missing?
Source 2: Sodium Discharge Tube
This source (labeled “Na”) needs to be plugged in an turned on. You will need to let it warm
up for a couple minutes before it will be ready. Ask your instructor if you are not sure.
Be careful not to touch these dangerous high voltage sources; you might have to lean
close, but just look.
D. What color is the light coming from the sodium discharge tube when observed with the
naked eye?
E. Sketch and describe the emission spectrum you see from the sodium discharge tube using the
spectroscope. Make sure that your words and your drawing convey what you see.
F. Compare and contrast the spectrum you see for the sodium discharge tube to the one you saw
for the incandescent bulb.
G. Compare the spectrum you see for the sodium discharge tube to the one for sodium on the
wall chart. What differences do you notice?
H. We know each element has a fingerprint spectrum, and we know that there is sodium in this
tube. What else can we conclude from comparing the spectrum from the tube with the wall
chart? (You probably can’t be specific, so be general)
Source 3: Mercury Discharge Tube
This source (labeled “Na”) needs to be plugged in an turned on. You will need to let it warm
up for a couple minutes before it will be ready. Ask your instructor if you are not sure.
Be careful not to touch these dangerous high voltage sources; you might have to lean
close, but just look.
I. What color is the light coming from the mercury discharge tube when observed with the
naked eye?
J. Sketch and describe the emission spectrum you see from the mercury discharge tube using
the spectroscope. Make sure that your words and your drawing convey what you see.
K. Compare and contrast the spectrum you see for the mercury discharge tube to the one you
saw for the sodium discharge tube.
L. Consider the yellow and blue lines in the mercury discharge spectrum.
a. Which line represents the longer wavelength? (circle one)
Yellow
Blue
b. Which line represents has the higher frequency? (circle one)
Yellow
Blue
c. Which line represents has higher energy photons? (circle one)
Yellow
Blue
Source 4: Unknown Discharge Tube
Be careful not to touch these dangerous high voltage sources; you might have to lean
close, but just look. This station contains a tube of an unknown gas.
M. Using your spectroscope and the wall chart, determine which gas is contained in the tube.
Describe your process.
Interlude: The Quantum Atom
The physics that explains how each element gets its particular set of colors is quantum
mechanics. In this model we envision the atom as having discrete energy levels that its electrons
can occupy. The diagram shows a hypothetical atom that has four such energy levels. Let’s
consider how these energy levels relate to the emission spectrum we would see if we examined a
discharge tube filled atoms of this hypothetical element.
The energy levels are labeled 1-4, where level 1 is the lowest
energy level (called the ground state).
4
If an electron were in level 3 and then moved down to level 1 this
would be called the “3-to-1 transition”. Since energy has to be
conserved, whatever energy the electron lost gets emitted as a
single photon with that same amount of energy.
3
2
The light from that 3-to-1 transition would show up as one emission
line in this atom’s spectrum with the wavelength determined by the
formula   hc E . In this case the hypothetical line is shown on
the emission spectrum below.
3-to-1
1
N. Working on the energy-level diagram, how many different
transitions could there be in this atom? Draw each transition and
label it just as the 3-to-1 is labeled.
3-to-1
O. Fill in the rest of the hypothetical emission spectrum so that all transitions are included.
Label each one just as the 3-to-1 is labeled. You don’t have to know exactly where each one
should go, but the relative placement of the lines you draw should make sense.
Source 5: Fluorescent Light
For this section, be sure you are looking at the fluorescent overhead lights (long lights), not
the incandescent bulbs. If you are not sure, ask the instructor
P. What color is the light coming from the overhead lights in the lab room when observed with
the naked eye?
Q. Sketch and describe the emission spectrum you see from the overhead lights using the
spectroscope. Make sure that your words and your drawing convey what you see.
R. Compare and contrast the spectrum you see for the overhead lights to both the incandescent
light bulb and the mercury discharge tube.
S. Do you think any of the discharge tubes you looked at would make good overhead lights for
an office or classroom? Why or why not?
T. What kind of spectrum do you think you would see if you used a spectroscope to look at
sunlight? [To be clear you would never point your spectroscope at the sun, just at a patch of
clear sky.]