ASTR 503 Homework 2 : Wavelengths and Doppler

Always use scientific notation, always give units, and show your steps!

1. The space station a while back launched a "suit-sat" satellite, which is an empty space suit, that broadcasts a message, using radio waves at a frequency of 145.990 MHz. One Hertz = Hz = one cycle per second. Calculate to 3 significant digits the wavelength of that radio emission. (wavelength = c / f)

2. What is the frequency (Hz) of a photon whose wavelength is 1 angstrom = 0.1 nm? (the size of a small atomic nucleus).

3. What is the energy (J) of the photon in the previous question? Use E (J) = hf where h = 6.6 E-34 (J-s).

4. A Joule is a huge unit for a single particle (or photon) = one watt of energy flow times one second, or the force of one Newton moving one meter. So the unit is equal to one kg - m*m/s*s. Or one Coulomb times a Volt (a current of one Amp flowing through a battery of one Volt for one second). A more convenient unit for the energy of a particle or a photon is an "electron volt" - the energy gained by a particle with a single electric charge falling through a electric potential of one volt. Since the charge of one electron = 1.6 E-19 V, then one electron volt = eV = 1.6 E-19 J. What would be the energy of that same photon (question 2) in electron volts?

5. The bright red line of Hydrogen is called H-alpha. We frequently use a filter at that wavelength to see the prominences on the Sun. Its wavelength is 656.28 nm. What is its frequency?

6. If a prominence is "erupting", it is being blasted off the surface of the sun at speeds of 1500 km/s or more. If that prominence is on the limb on the sun, and is traveling perpendicular to our line of sight at 1500 km/s, what wavelength will we observe the H-alpha?

7. If the prominence is near the center of the Sun, and is heading towards us at 1500 km/s, what wavelength will be observe the H-alpha? Is that a red shift or blue shift?

8 (2 pts). The speed of sound in dry air at room temperature is about 770 mph. What is that in meters per second?
If a train horn is at middle "A" (440 Hz) and is approaching us at 60 mph, what frequency will that horn sound to us?
What frequency if it is receding at that same speed?
What speed would it need to be approaching us to have its sound exactly one octave higher (doubled in frequency?) (for a good chart of frequencies of notes, see

Last updated 1/18/2018