Physics 1411/1403
Light from Atoms
You know from lecture that light is an electromagnetic wave, and that the color of the light is
determined by its wavelength: red light has longer wavelength, blue light has shorter wavelength. If the
light source emits light because of its high temperature (a red-hot coal or the filament of an ordinary
light bulb, for instance), it produces a broad, smooth distribution of wavelengths. When such light is
passed through a prism, the familiar rainbow is produced, with red merging smoothly into orange,
orange into yellow, etc. This is also called a spectrum.
Atoms under certain condition atoms also give off light. Their spectrum looks like the diagram below and
contains a pattern of bright lines called the spectral lines. Spectrum of each atom is unique, like the
fingerprints of a person. A spectrum gives important clues about the nature of matter. It is widely used
in today's science and technology.
We believe that an atom is made up of a positively charged nucleus with Negatively charged
electrons moving around it. The electron resides in orbits around the nucleus at different distances.
These distances are calculated from Bohr's theory, details of that are not necessary to know here. Each
of these orbits has energy associated with it.
Under normal conditions the atom is in its natural state however, if it is excited, say by gaining energy
during an interaction (a special light source in this activity does this) then it moves into an excited state.
In an excited atom some of the electron change their orbits and move to ones that have higher energy
associated with it also called energy levels. After a short time interval the atom returns to its natural
state.
In an atom whenever an electron jumps from an orbit with lower energy to the one with higher energy,
it absorbs a specific amount of energy from the process and shows an absorption spectrum that has
dark black lines in place of the bright lines. When the jump is from a higher level to a lower level, than
the electron releases a specific amount of energy to the process and shows an emission spectrum that
has bright lines in it like the ones you will see in this activity. Your objective is to perform a qualitative
study of the continuous spectrum of an incandescent lamp, emission line spectrum of the Helium and
Mercury gas and to compare the spectrum of Helium and Mercury gas.
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TASK 1: Instructions
a. Fill in your name(s), class section on the worksheet.
b. This activity can be done in groups of 3 or 4 students.
c. Each group will be given a hand held plastic spectroscope.
d. Your GSI will setup the equipment for task 2
e. Each student in the group should take turns and point the slit of the spectroscope at the light
source and look through the eyepiece.
f. You may have to point at an angle to see the spectrum.
g. Sitting on a rotating chair can help.
h. After all groups are done, your GSI will next setup the equipment for task 3. Repeat steps e), f) and
g).
i. After all groups are done, your GSI will next setup the equipment for task 4. Repeat steps e), f) and
g).
j. After all groups are done, your GSI will next setup the equipment for task 5. Repeat steps e), f) and
g).
k. Complete task 6 and hand in your worksheet.
Slit
Eyepiece
images are borrowed from other websites
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Name:
Section:
Partner(s)
Worksheet for Light from Atoms
Questions for Task 2: Observer the spectrum of incandescent bulb
1. Use a lead pencil to draw a rough sketch of the spectrum of the incandescent bulb below. Label
the colors.
2. What is the order of colors from left to right?
3. Which color is spread out more than others?
4. Observe the width of the colors, do you notice any decreasing or increasing from left to right?
Questions for Task 3: Observer the spectrum of Hydrogen discharge
1. Sketch the spectrum of helium below (setup has a lamp that looks pink when turned ON). Draw
lines in the middle row. Put b in the last row for bright lines and f for faint lines.
Hydrogen
Violet
Indigo
Blue
Green
2. Observe the line spacing and note your observations here.
3. Which colors have double lines?
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Yellow
Orange
Red
4. If there are double lines of the same color, are they of equal brightness?
5. Which color lines are the brightest?
Questions for Task 4: Observer the spectrum of Helium discharge
1. Sketch the spectrum of helium below (setup has a lamp that looks pink when turned ON). Draw
lines in the middle row. Put b in the last row for bright lines and f for faint lines.
Helium
Violet
Indigo
Blue
Green
Yellow
Orange
Red
2. Observe the line spacing and note your observations here.
3. Which colors have double lines?
4. If there are double lines of the same color, are they of equal brightness?
5. Which color lines are the brightest?
Questions for Task 5: Observer the spectrum of Mercury discharge
1. Sketch the spectrum of helium below (setup has a lamp that looks pink when turned ON). Draw
lines in the middle row. Put b in the last row for bright lines and f for faint lines.
Mercury
Violet
Indigo
Blue
Green
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Yellow
Orange
Red
2. Observe the line spacing and note your observations here.
3. Which colors have double lines?
4. If there are double lines of the same color, are they of equal brightness?
5. Which color lines are the brightest?
Questions for Task 6: Spectral Fingerprinting
1. What difference are apparent in the spectra of Hydrogen and Helium?
2. Which is brighter Hydrogen or Helium?
3. What colors are missing in Helium that are present in Hydrogen?
4. What double lines are not present in Hydrogen or Helium?
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