Homework Solutions
Chapter 12
4. In atoms, electrons may only occupy certain orbits, not any possible
orbit. This means that there are only certain allowed energies that the
electrons may absorb or emit, corresponding to the amount of energy
gained or lost when an electron moves from one allowed orbit to
another. By absorbing light of the right energy, an electron moves to a
higher energy orbit. When it reemits that light, the electron moves
back down to a lower energy orbit. Since the energy of light is
determined by its wavelength, the existence of a discrete set of orbits
means that only certain energies may be absorbed or emitted which
means that only certain wavelengths of light may be absorbed or
emitted. The Sun’s spectrum shows a lot of absorption lines, each of
which corresponds to the specific wavelength of light that is absorbed
by a specific atom or molecule.
5. When electrons in the helium atom move between the lowest
energy orbit and higher orbits, the energy changes correspond to
spectral lines that are not in the visible part of the spectrum. To
produce visible absorption lines, helium must first be excited to the
second orbit by some other process and then it may absorb and emit
light in the visible part of the spectrum. It reaches the second orbit by
energy absorbed from collisions with other atoms, but the amount of
energy available in a collision depends on the temperature of the gas.
In the photosphere, the gas is not hot enough to lift a significant
number of helium atoms into the second higher orbit, so there is no
significant absorption in the visible band.
7. If we model the Sun as a black body radiator, its continuous
spectrum must peak (that is, be brightest) at a specific wavelength
(see the picture on pg. 259 of your book). This wavelength is related
to the temperature by Wien’s law. So you measure the intensity of the
light at each wavelength, graph it to find the peak of the curve,
determine what wavelength corresponds to that peak intensity, and
apply Wien’s law to calculate the temperature. This is an estimate
since the Sun is not truly a black body, but it actually comes pretty
close so it is a pretty good estimate.
12. We can see the top of the convection flow in the form of
granulation in the photosphere. We saw a movie of this in class. Hot
material convecting upward reaches the photosphere, cools and sinks
back down. At the highest point of the convection flow, it produces
bumps in the photosphere which can be seen fairly easily.
