Investigating Spectra and the Components of Light
NAME ___________________________________
DATE __________
Purpose: (1) To compare spectra of different light sources and observe the components of visible light.
(2) To begin to understand how astronomers use spectroscopy to study the universe.
Background: The human eye can only see a small portion of electromagnetic energy, the wavelengths known as
“visible light”. Different wavelengths of visible light appear to the eye as different colors. When light passes
through a prism, light rays are refracted (bent). Different wavelengths bend at different angles, causing light to
spread out and appear as a spectrum of colors. Red colors appear at the longest wavelengths; blues and violets have
the shortest wavelengths of visible light. Spectra are also produced when light passes through water droplets
(producing a rainbow) or through a diffraction grating (a thin, plastic sheet with tiny parallel lines etched into the
surface). Astronomers study the spectra of light sources in space (stars, galaxies, etc.) to learn about their
composition, temperature, and relative motion.
All light sources do not produce the same type of spectrum. The type of spectrum depends on the energy produced
by the source. The spectrum may also be affected by the presence of gases or filters between the light source and
the observer. There are four main types of spectra.
CONTINUOUS SPECTRUM – all colors blended evenly with no breaks (a rainbow); produced by an
“incandescent” substance (one that glows when heated and produces light at all visible wavelengths); e.g. an
standard light bulb.
EMISSION (BRIGHT LINE) SPECTRUM – thin, isolated bands or lines of color appear at specific wavelengths
against black background; produced when atoms of an element are “excited” when energized and emit energy. Each
element emits energy at characteristic wavelengths, creating a “fingerprint” spectrum; e.g. a neon light.
ABSORPTION (DARK LINE) SPECTRUM – continuous spectrum with parts (isolated wavelengths or large
sections) missing; produced when a source emits a continuous spectrum but something (e.g. a cool gas or filter)
between observer and source absorbs some wavelengths. The missing wavelengths are used to identify the
substance between the source and observer, like the gases in the outer layers of the star or in Earth’s atmosphere.
CONTINUOUS WITH BRIGHT LINE SPECTRUM – background is a faded continuous spectrum, with distinct
bright lines at certain wavelengths; produced by light source like a fluorescent light bulb.
In this lab, you will use a portable spectroscope to observe spectra from different light sources. A spectroscope uses
a diffraction grating to separate light into its component colors. The spectrum appears near a scale that represents
the range of wavelengths in the visible spectrum. The scale is marked with the numbers 4, 5, 6 and 7, which
represent 4000, 5000, 6000 and 7000 Angstroms, respectively. One Angstrom equals 10-8 centimeters (0.00000001
cm). When you observe a spectrum, the purple end should appear near the “4’ and the red end near the “7” on the
scale.
Procedure
1. Go to one of the light source stations. Record the name of the light source on the Data Chart. It is written on a
label next to the light source.
2. Look through the grating at the narrow end of the spectroscope. Point the open slit on the opposite end towards
a light source. The spectrum will appear along or below the scale to the right of the slit.
3. Carefully and accurately draw the spectrum in the box provided. Draw the colors in the exact order that they
appear and in line with the appropriate wavelengths or ranges of wavelengths on the scale.
4. Use the definitions provided in the Background to identify the type of spectrum (continuous, emission,
absorption, continuous w/ bright line).
5. Repeat the procedure for each light source.
**BEFORE YOU ANSWER THE QUESTIONS, GO BACK AND READ THE BACKGROUND SECTION OF
THE LAB.**
Questions – Please answer in complete sentences.
1.
Explain how you identified the “unknown gas”. Describe the evidence you used for your identification – what
colors or wavelengths were most significant in determining the type of gas in the tube?
2.
List the colors in the spectrum of an incandescent light bulb in order from SHORTEST to LONGEST
wavelength.
3.
Compare the spectrum of the incandescent bulb with the spectrum of the fluorescent bulb. (a) How were they
similar? (b) How were they different? (Didn’t see anything different? Look again!!)
4.
What happens to the spectrum of an incandescent bulb when a red or green filter is placed in front of it?
Explain WHY you saw a different spectrum when a filter was used.
5.
What type of spectrum is produced when a gas or element is energized until it gives off light? Explain WHY
that type of spectrum is produced instead of a continuous spectrum.
6.
How does an astronomer use a star’s spectrum to infer its composition? (Hint: think about how you identified
the unknown gas.)
7.
Draw below what the spectrum of an incandescent light bulb would look like if viewed through an orange filter.
4000
5000
6000
7000
What type of spectrum did you just draw? _____________________________________________
8.
Red light appears at wavelengths between about 6000 and 7000 Angstroms. Convert this value to
(a) centimeters and (b) meters. Show how you set up your calculations. Don’t forget units on your answer.
(If you haven’t read the Background section of the lab, this would be a good time to do it!)
DATA CHART
Light Source (see label): ____________________________________________________
4000
5000
6000
7000
Classification (type of spectrum): _____________________________________________
Light Source (see label): ____________________________________________________
4000
5000
6000
7000
4000
5000
6000
7000
4000
5000
6000
7000
Classification (type of spectrum): _____________________________________________
Light Source (see label): ____________________________________________________
Classification (type of spectrum): _____________________________________________
Light Source (see label): ____________________________________________________
Classification (type of spectrum): _____________________________________________
Light Source (see label): ____________________________________________________
4000
5000
6000
7000
Classification (type of spectrum): _____________________________________________
Light Source (see label): ____________________________________________________
4000
5000
6000
7000
4000
5000
6000
7000
Classification (type of spectrum): _____________________________________________
Light Source (see label): ____________________________________________________
Classification (type of spectrum): _____________________________________________
Light Source (see label): ____________________________________________________
4000
5000
6000
7000
Classification (type of spectrum): _____________________________________________
**Compare the spectrum of the UNKNOWN GAS to the spectra of different elements shown on the Spectra Chart
posted in the classroom. IDENTIFY THE UNKNOWN GAS: _______________________________________
(Look for the best match.)