Activity 1: Spectra from various sources
Look through the spectroscopes at each of the light sources. Off to the right you should see a
spectrum superimposed on a scale. Using the colored pencils, record the spectra you see, using
the handout that will be available in lab. (The units are 10-5 cm or 10-7 m).
CAUTION: The emission tubes are powered by a 5000V source, which could provide a nasty
shock to anyone touching the metal supports! DO NOT TOUCH!
Looking at an incandescent light bulb:
4
5
6
1. What range of wavelengths corresponds to:
Blue?
Yellow?
Green?
2.
7
8
Red?
Orange?
Is there a distinct division between the various colors, or do they simply fade into the
next color?
Looking at a Hydrogen (H) emission tube
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5
6
7
8
6
7
8
6
7
8
6
7
8
Looking at a Helium (He) emission tube
4
5
Looking at a Neon (Ne) emission tube
4
5
Looking at a Mercury (Hg) emission tube
4
5
3. Is there a distinct division between the various colors, or do they simply fade into the next color?
4. Are the observed spectra for the different sources the same or different? How does your
answer clarify the saying, "Spectra are like fingerprints".
Activity 2: Energy & Color
The energy of light is inversely proportional to the wavelength: E  1/ . Specifically, for distances
measured in units of 10-5 cm (as recorded on your drawings) the energy should be given as E = 12.4/ .
(The units of energy here turn out to be in "eV". If you know about eV, good for you; if not don’t worry
about it)
For the hydrogen spectrum you observed, fill in the table showing the color, the wavelength, and the
energy for each of the lines you observed. (NOTE: If you did not have a clean hydrogen spectra, that is
a series of lines, the values are: Red 6.55x10-5 cm; Blue 4.85 x 10-5 cm, Violet 1 4.35 x10-5 cm; Violet 2
4.10 x 10-5 cm)
The energy levels for hydrogen can be predicted theoretically. If the lowest orbit is assigned an energy
of 0, the higher orbits will be:
13.22eV
13.06eV
6
5
12.75eV
4
12.09eV
3
10.20eV
2
So I measure an emission line with a wavelength () of 4.85x10-5 cm.
Using the equation, Energy=12.4E-5 /  (10-5 cancels out), I can find my
energy to be 12.4 / 4.85 = 2.55eV.
An electron jumping down from the 4th orbit to the 2nd orbit would release
light with an energy of (12.75 eV – 10.20 eV) = 2.55 eV. See if you can
identify the orbits involved for the other lines.
12.75 – 10.20 = 2.55eV
10.20 – 0.00 = 10.20eV
0.00eV
1
Color
Wavelength (10-5 cm)
Energy (eV)
Red
Blue
Violet 1
Violet 2
Questions:
1. Which has more energy, blue light or red light? How do you know?
2. What energy would you expect for orange light?
Orbit Jump (start-end)
Follow-up Questions
1. When an object is heated and starts glowing, it first appears a dull red. As it gets hotter, it turns orange, then
yellow. Explain why this is logical in terms of your understanding of color and energy as it applies to a candle
or electric stove.
2. Astronomers are interested in learning about the composition of stars and interstellar gas clouds.
a.) Explain how this could be accomplished without having to actually visit the distant stars.
b.) What advantages would there be in doing these experiments on Mars?
3. Ultraviolet light produced by the sun can be dangerous. Fortunately, very little of this UV light reaches the
earth’s surface. What stops the UV light and what would be some problems for astronauts or Mars colonists?