Download 5→2 3→2

Solutions for NATS102 Spectroscopy Laboratory Activity
Questions and Calculations (finish after class )
1. a) Fig. 8 (on p. 6) illustrates electronic transitions from 3 higher levels to a lower one
(5→2, 4→2, or 3 →2). Which transition produces a photon with the highest energy? (1 point)
5→2
b) Which transition produces a photon with the lowest energy? (1 point)
3→2
2. a) What is the visible wavelength range of the spectra you have drawn in the laboratory session? In other words,
what is the shortest wavelength (most blue or violet) and longest wavelength (most red) that you see? (Note that
everyone has slightly different limits on the wavelength range that their eyes are sensitive to. Since wavelengths are
measured here in nanometers (nm), you will need to multiply the number from the spectroscope scale by 100 to get
nm) (1 point)
around 400nm is the shortest
around 700nm is the longest
b) If an electron transition produced a photon at the shortest wavelength, would that photon have a lower or higher
energy than a photon at the longest wavelength? (1 point)
higher
3. a) What type of spectrum (emission, or absorption) does the observer see in Figure 9? Why? (2 points)
Absorption -- the gas is between the
source (star) and observer. Photons
at specific wavelengths (spectral
lines) are removed from the stellar
spectrum and radiated in other
directions.
b) The young, hot star in Figure 10
heats up the gas in the cloud. Assume
Figure 9: The observer can see light from the star through the cloud of
cool hydrogen gas.
that the observer can only see
radiation emitted by the hydrogen
cloud and cannot see the star. What
kind of spectrum (continuous,
emission, or absorption) does the
observer see? Why? ( 2 points)
Emission - The excited atoms that
have absorbed photons from the star
radiate them in all directions.
4. Look at the spectrum of the Sun in the
Figure 10: Starlight from the hot star heats and illuminates the
cloud of hydrogen gas
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Spectroscopy lecture. What type of
spectrum (continous, emission line or
absorption line spectrum) does the Sun
produce? Why? ( 1 point)
Absorption -- cool gas at the top of the
Sun's photosphere and above it absorbs light from below.
5. Compare the spectrum of the fluorescent light bulb to the spectra of the gases in the discharge tube. Note that the
fluorescent light could have several gases in it so look for a gas all of whose lines are seen the fluorescent light's
spectrum. What gas do you think is in the fluorescent light bulb? Why? (2 points )
Mercury has lines of which appear in the fluorescent light. Other elements either have fewer lines or
lines that are missing from the fluorescent light spectrum.
6. For the mercury discharge tube spectrum, record the wavelength of the three most prominent emission lines in the
table below taking the wavelengths from the sketch you made of the spectrum . Then for each wavelength, calculate
how many electron volts (eV), that is, how much energy, are in one photon of light.
To perform this calculation, recall that the formula
E=h×c/λ
gives the energy of a single photon of light in terms of its wavelength . The other symbols are Planck's constant, h,
and the speed of light, c. The lower scale on the spectroscope is in units of nm or nanometers. Since h and c are just
constant values, we can convert them to handy units so that if λ is in units of nanometers (the bottom scale in the
spectroscope), the energy of the photon, E, comes out in units of electron volts, eV. The formula below works only if
λ , the wavelength you have measured, is in units of nm:
E = 1240 / λ
eV
For each line in the mercury spectrum, show your work in the space provided below and be sure to include all units.
As a check of your calculations for each line, compare your calculated result with the energy indicated on the upper
scale shown on the template scale on which you made your sketch (see last page). Give the results in the table below:
(3 points for each calculation)
bright line
wavelength (nm)
calculated energy
(eV)
Energy as indicated
on the top of the
spectrum template
(eV)
Line #1
(2 points)
434
2.85
2.85
Line #2
(2 points)
546
2.27
2.27
Line #3
(2 points)
576
2.15
2.15
Record your spectra here (extra templates in case of mistakes!). Note that the spectroscope scale only shows
wavelengths in 100's of nanometers (nm) -- 6 in the spectroscope = 600 nm and so on.
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