ASTRO 110, Section 4 Light and Atoms. CH 5

• What is the relation between wavelength of light and color? Between photon energy and color? How are the various types of electromagnetic radiation ordered by increasing wavelength? By increasing photon energy? Familiarize yourself with typical wavelengths for visible light, and the different forms of light (X-ray, ultraviolet, infrared, etc.) in the electromagnetic spectrum.

• What is a spectrum? (Think of the definition to also use frequency, color, or photon energy as well as wavelength when speaking about light).

• What parts of the electromagnetic spectrum can penetrate our atmosphere for ground-based obser- vations? What wavelengths must be studied from satellites in space?

• Will a hot body appear bluer or redder than a cold body (Wien’s Law, Appendix A-3)? What is continuum (thermal, or ‘black-body’)) radiation? How does the intensity of radiation increase with the temperature of an object [Stefan-Boltzmann law, Appendix A-3]?

• Describe the structure of an atom. What is the most abundant element (what is the composition of the Sun)?

• How does an atom produce emission lines? Absorption lines? (see also below) What can you learn from absorption or emission lines about a star’s temperature and chemical composition? Under what conditions will you see an emission line spectrum? An absorption line spectrum? A continuous spectrum (or continuum)?

• Some additional notes summarizing the text material: Electrons in an atom can absorb and emit energy in discrete packets, called photons. The energy of a photon is inversely proportional to the wavelength (long wavelength photons are less energetic, short wavelength photons are more energetic). The spacings of the energy levels for the electrons determine the energies (and thus the wavelengths) of the photons a given atom can absorb and emit. Each atom thus has a characteristic pattern of lines in the spectrum that can be used to identify the element. Absorption lines are formed when atoms absorb photons and their electrons move from lower to higher energy levels. Emission lines are formed when atoms emit photons and their electrons move from higher to lower energy levels. The energy of the photon depends only on the difference between the starting and ending energy levels of the electron. For cold gas most of the atoms are in the ground state, or lowest energy level. If very energetic photons are absorbed by the atom, or if the atom is embedded in very hot gas, the atom may lose electrons (become ionized). [The resulting positively charged ions have different spacings of energy levels, and a different spectral signature.] In stars, absorption lines are formed in the continuous spectrum when radiation from the hotter interior passes through the cooler atmosphere of the star. Which of the possible lines of each element are present, and whether these are due to atoms or ions, depends on the temperature of the star.

• What is the Doppler Effect? How would you describe the Doppler effect using sound waves from the siren of a moving ambulance? How much does the position of a spectral line change in wavelength, if the source is moving?