Astronomy 320 HOMEWORK #1 Spring ’04

Due date: Wed, Feb. 11

1. Visible light with a wavelength λ = 550 nm appears green.

(i) Express this wavelength in Angstroms (Å) and in meters (m).

(ii) Compute the frequency n that corresponds to this wavelength of light.

Express your answer in Hz and in GHz.

(iii) Compute the energy of a photon of this wavelength of green light.

Express your answer in Joules (J) and in electronvolts (eV).

(iv) Would this photon have enough energy to ionize a hydrogen atom with an

electron in the ground state (n=1)? With an electron in n=2? n=3?

Hint: Appendices 6 and 7 in Zeilik & Gregory contain many useful unit conversions

and prefix definitions.

2. The rest wavelength of the Ha line is at λ₀ = 6562.8 Å. Suppose you observe this

line in the spectrum of the Andromeda galaxy but find that it is at λ = 6560.2 Å.

(i) What can you conclude about the relative motion of the Milky Way and the

Andromeda galaxy? Are they moving together or apart? At what speed?

(ii) At this rate, how about long might it be until the two galaxies collide? Express

your answer in seconds and in years.

(iii) Compare your results in (i) and (ii) to the statements on p.8 of Croswell.

(iv) Can you think of any reason the two galaxies might not collide? Explain.

(v) To check your result, you decide to look at the Hb line, for which λ₀ = 6562.8 Å.

At what wavelength do you expect to observe this line in the spectrum of Andromeda?

3. Consider a hydrogen atom.

(i) Compute the energy of the photon that would be emitted if an electron dropped

from the n=6 to n=3 level.

(ii) What wavelength would this photon have?

(iii) In what part of the electromagnetic spectrum does this photon lie?