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Earth Science
Astronomy Unit
Line Spectra of Elements and Excited Electrons Lab
Introduction:
Energy comes to us from our sun and other stars in the form of electromagnetic energy. The visible
portion of the total spectrum of energy received can be analyzed by using a spectroscope. Dark absorption
lines can be observed which can be used to identify a star’s properties such as its composition, velocity, and
direction of movement.
Using ground and space-based telescopes, along with innovative technology and interdisciplinary
research, NASA’s Astronomical Search of Origins mission seeks to understand how today’s universe came
to be and what its future may be. Powerful new telescopes will allow us to detect faint infrared energy from
the earliest stars and galaxies in the universe. These telescopes will probe the skies collecting evidence
about the environments where new stars and planets form. Characteristic features in the spectrum of light
emitted by atoms and molecules (spectral fingerprints) will allow us to identify the chemical elements and
molecules in the nearby and distant universe. We can use this information to search for indicators of life on
other planets, such as oxygen, ozone, and methane. The goals is to search for terrestrial planets beyond our
solar system by measuring the light from stars with high precision.
Why do different elements emit different spectra? Well it’s all due to the movement of the
electrons and energy exchange. As electrons absorb energy they become excited and move to higher energy
levels. As the electrons fall back to lower energy levels they release the energy they absorbed in set
amounts called quanta. The energy that is released as electrons fall from higher to lower energy levels has a
characteristic wavelength and frequency that corresponds to a particular type of electromagnetic radiation.
For example, when electrons fall from a higher energy level down to the 2nd energy level, the wavelength and
frequency of the energy produced correspond to that of visible light. The color of the visible light will
depend on the wavelength of the visible light produced.
Electrons of atoms can be excited in various ways including heat, electricity and friction. For
example, a solution of sodium chloride placed on a platinum wire and held in a flame emits a bright, yellow
light that is characteristic of the metallic element sodium. Another method of spectrum analysis involves the
application of high voltage across a gas-filled glass tube. Gases under low pressure and excited by an
electrical discharge give off light in characteristic wavelengths. The emitted light is passed through a
spectroscope, which breaks light into its components for analysis. A gas viewed through a spectroscope, such
as the one shown in Figure 1, forms a series of bright lines known as a bright-line or emission spectrum. Since
each element produces a unique bright-line spectrum or pattern, spectroscopy is a valuable branch of science
for determining what elements are present.
The composition of stars and other objects in outer space is determined using this technique of line
spectra analysis. Unlike gases, heated solids produce a continuous spectrum. Notice when you view
incandescent bulb that a continuous spectrum is produced. A gas is identified by comparing the wavelengths
of its emission (bright line) spectrum to the spectrum produced by a known gas. Scientists have observed
that each different element emits a unique pattern of electromagnetic energy when the element is heated to
glowing. Since stars are very hot, each element in the star glows, emitting it’s signature spectrum. It is in
this way that scientists have been able to identify the composition of stars form a few light year away to
billions of light years away. In this experiment, you will use a spectroscope to determine the bright-line
spectra characteristic of different elements.
Purpose:
To observe the characteristic bright line spectra (signature spectra) produced by applying high voltage
across a sample of a gas at very low pressure and to determine the identity of an unknown gas.
Vocabulary: Spectroscope, Signature Spectrum, Stars, Spectra Analysis, Galaxy, Universe
Materials/Equipment:
High voltage power supplies Spectral tubes: Hydrogen, Helium, Oxygen, Mercury, Nitrogen, Neon
Spectroscopes
Fluorescent bulb/socket
Colored Pencils
Incandescent light bulb
Safety Considerations:
DO NOT TOUCH the spectrum-tube power supply or spectrum tubes when power is applied. Several thousand
volts exist at the power supply and spectrum tubes.
Procedure:
Obtain a spectroscope and start by look through it at the incandescent light bulb. The spectrum should
appear when the slit in the spectroscope is pointed just off center of the glowing filament. Practice moving
the spectroscope until you see a bright, clear image of a rainbow underneath the white number scale. Adjust
the spectroscope until the brightest image is oriented on your scale.
Record in Table 1 the five brightest lines of the observed spectrum. Some of the spectrum tubes produce
light so dim that you must be very close to them to get good observations of the spectral lines.
Repeat for each of the other spectrum tubes.
Collecting Data:
Using the templates on the next page, draw in lines at the proper distances from the slit on the scale. Draw
the correct width and intensity to reflect what you have observed. In the box at the left of each scale,
write the name of the element you are observing.
Questions:
1. Which type of spectrum is produced by an incandescent bulb?
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2. Observe a fluorescent light bulb with the spectroscope. While a continuous spectrum will be visible, you
will also see a bright-line spectrum. Compare this spectrum with those of the gases observed in this activity.
Which is most similar to the bright line spectrum from the fluorescent bulb?
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3. Compare the color of light emitted by the spectrum tube to that observed through the spectroscope. Can
you predict the spectral pattern by looking at the tube with just your naked eye?
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4. Are all lines of the spectral patterns of the same intensity?
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5. Are they always in the same order: ROYGBIV ?
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6. How do electrons produce emission spectra?
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7. Why are the lines different colors?
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Applications:
8. Astronomers can identify the chemical composition of distant stars. Explain how the methods you used in
this laboratory could be applied to do this.
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9. Why is it easy to identify the composition of a star that is many light years away from us?
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10. What does a “red shift” of a star’s spectrum tell us about that star?
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11. The movement of a galaxy can also be figured out by looking at the red- or blue shifts of a the light
coming from stars in that galaxy. What may we infer from the fact that there are very few blue- shifted
galaxies (from our point of view)?
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12.
Blue
Red
The above image shows the line spectra for gas tube of hydrogen, at rest in a
laboratory. Examine the image below, which shows the line spectra of a distant star,
toward which direction do the lines shift, red or blue? What does this indicate?
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Spectrum Line Templates: Record your drawings of the observed spectrum lines here!
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