RADIATION & THE ELECTROMAGNETIC SPECTRUM ------------------------------------------ General Properties of Light ---------------------------- A charge at rest can create a) electromagnetic radiation b) only electric and magnetic forces, no radiation $c) only electric forces A charge moving uniformly in a straight line can create a) electromagnetic radiation $b) only electric and magnetic forces, no radiation c) only electric forces d) only radiation, but no electric or magnetic forces Electromagnetic radiation will be created by any charged particle that $a) accelerates (changes speed or direction). b) moves in a straight line at constant speed. c) remains at rest. d) is subjected to a gravitational field. Visible light is electromagnetic radiation in the sense that it is a) composed of electrons and protons b) composed only of electrons $c) created only by and interacts only with electric charges d) generated by radioactivity. The wave model of light is better than the particle model for explaining a) shadows b) reflections $c) constructive interference d) the inverse square intensity drop with distance Which of the following characteristics of light are associated with the particle nature of light? a) diffraction, interference, and photoelectric effect b) Doppler effect and formation of spectral lines c) Doppler effect, diffraction and interference $d) formation of spectral lines and photoelectric effect e) photoelectric effect and Doppler effect Comparing water waves and light waves, $a) both spread out after passing through a narrow opening. b) only light waves can reflect off barriers. c) only light waves display Doppler shifts. d) only light waves interfere with each other. e) only water waves interfere with each other. Light waves differ fundamentally from either water waves or sound waves because they a) can have various wavelengths. c) travel from place to place instantaneously. d) have an unchanging amplitude. $e) can travel in a vacuum. If you were going to design a pair of glasses for seeing animals at night, you would want them to convert a) infrared photons to X-ray photons b) X-ray photons to optical photons $c) infrared photons to optical photons d) optical photons to UV photons The observed speed of light is affected by a) the velocity of the source of the light. b) the velocity of the observer. c) the refrence frame of the observer. $d) none of the other answers are correct. Which of the following are believed to be absolute concepts that are the same in any reference frame? a) time and distance $b) the speed of light and the laws of physics c) mass and acceleration d) time, distance, and the laws of physics Which type of electromagnetic radiation has a wavelength adjacent to, but longer than ultraviolet light? a) radio b) infrared $c) visible d) x-ray e) gamma ray Radio waves have a) high energy and long wavelength $b) low energy and long wavelength c) low energy and short wavelength d) high energy and short wavelength. Which of the following is true? Compared to optical photons, a) radio photons have a longer wavelength. b) X-ray photons have a larger frequency. c) infrared photons have a smaller energy. $d) All of the above. e) None of the above. The two main "windows" in the Earth's atmosphere that astronomers can use are in the visual and radio bandpasses. This is mostly because other wavelengths suffer from excessive a) interference b) reflection $c) absorption d) refraction SPECTRAL LINES ---------------- Atoms of different elements have unique spectral lines because each element a) has atoms of a unique color b) has a unique set of neutrons $c) has a unique set of electron orbits d) has unique photons e) none of the above; spectral lines are not unique to each type of atom. The uniqueness of the spectral line pattern of any element is caused by a) the density of the gas in the stellar atmosphere $b) the energy-level structure of the atom c) the temperature of the stellar atmosphere d) the Doppler shift Spectra can be used to measure what properties of a star? a) Radial velocity. b) Chemical composition. c) Surface temperature. $d) All of the above. e) None of the above. In the Sun, the transition from level 4 to level 2 of hydrogen produces photons with a wavelength of 486.1 nm. In a star twice as hot as the Sun, this transition would produce photons with a) half that wavelength $b) the same wavelength c) twice that wavelength d) four times that wavelength Continuous Radiation, Emission and Absorption ---------------------------------------------- When something is what we usually call ``red hot,'' it is hotter than something that is a) blue hot. b) white hot. $c) neither of these. d) both of these. A hot, glowing, opaque solid, liquid or gas emits which type of spectrum? $a) Continuous. b) Emission lines. c) Absorption lines. A hot, glowing, opaque solid, surrounded by a cool gas will show a) Continuous. b) Emission lines. $c) Absorption lines. d) Both emission lines and absorption lines. Assume that a star behaves as a blackbody. If the surface temperature of that star doubles, then the wavelength at the maximum intensity will be ____ times the original wavelength. $a) 1/2 b) 1 c) 2 d) 4 Suppose two observers look at the spectrum of a cloud of gas in a laboratory; the first reports seeing emission lines and the second reports absorption lines. How can this best be explained ? a) the first observer sees the gas against a hot background $b) the second observer sees the gas against a hot background c) one observer is moving rapidly relative to the other d) the atoms in the gas are forming molecules A star with a continuous spectrum shines through a cool interstellar cloud composed primarily of hydrogen. The cloud is falling inward toward the star (and away from Earth). Which best describes the spectrum seen by an Earthbound observer? a) blueshifted hydrogen emission lines b) blueshifted hydrogen absorption lines c) redshifted hydrogen emission lines $d) redshifted hydrogen absorption lines e) a redshifted hydrogen continuum Suppose the atoms in a receding gas cloud have two energy levels separated by an energy corresponding to 4863 angstroms. The observer will see a spectrum with absorption at a wavelength a) less than 4863 angstroms b) equal to 4863 angstroms $c) greater than 4863 angstroms d) 4863 divided by the velocity Now consider a cool cloud of gas between a star and observer to be moving away from the source of continuous radiation (and towards the observer). The atoms in the gas have two energy levels separated by an energy corresponding to 3000 angstroms. If the spectral line is observed to be at 2700 angstroms, what is the relative velocity of the cloud with respect to the light source? a) 5700 km/s b) 300 km/s $c) 30,000 km/s d) 0 (no motion) e) the speed of light divided by 300 km/s You are gradually heating a lump of rock in an oven to an extremely high temperature. As it heats up, the lump emits nearly perfect theoretical blackbody radiation, meaning that it a) is brightest when hottest b) is bluer when hotter $c) Both. d) Neither e) is completely inedible. Doppler Effect ---------------- A star is moving away from the Earth at 200 km/s. What can you say about the wavelength of H_alpha absorption that you would detect in the spectrum of the star? a) It would be less than 656.3 nm. b) It would be 656.3 nm. $c) It would be more than 656.3 nm. d) There is insufficient information to decide. For a sound source passing by at constant speed, the sound a) gets higher and higher in pitch as the source approaches and lower and lower as the source recedes $b) are of constant higher pitch as the source approaches and of constant lower pitch as the source recedes. c) Both A and B above What causes light from a star to be Doppler-shifted? a) the distance between us and the star b) the gas and dust between us and the star $c) the speed of the star toward or away from us d) temperature differences between us and the star We can detect the velocity of a star through the Doppler effect by a) measuring the shift in distance of the star b) taking photographs six months apart c) applying the inverse square law of brightness $d) measuring the shift in wavelength of a spectral line For a sound source passing by at constant speed, the sound a) gets higher and higher in pitch as the source approaches and lower and lower as the source recedes $b) is of constant higher pitch as the source approaches and of constant lower pitch as the source recedes. c) is of constant lower pitch as the source approaches and of constant higher pitch as the source recedes. d) maintains the same pitch except at the moment when the source passes you. Absorption lines due to the same atomic transitions in the interstellar medium are sometimes seen at several slightly different wavelengths in stellar spectra. This is evidence that interstellar gas is composed of a) one huge flowing cloud b) one huge expanding or possibly contracting cloud $c) several separate clouds moving at different speeds d) many clouds in turbulent motion Two stars have the same mass and spectral class. Star A is rotating faster than Star B. The width of the spectral lines of Star A appear to be ____ than the spectral lines of Star B. a) narrower $b) wider c) stronger d) the same Energy Levels -------------- Atoms have particular associated spectral lines because $a) electrons have only certain allowed orbits b) light consists of waves c) light waves can show the Doppler effect d) photons have only certain allowed orbits e) speed of light in a vacuum is a constant If a photon of wavelength less than 912 Angstroms interacts with a hydrogen atom, what happens? a) the atom recombines $b) the atom ionizes c) an electron jumps up one energy level d) an electron jumps down one energy level The number of electrons lost by an atom in a gas (i.e. its ionization state) depends primarily on the a) velocity of the gas b) level of the ground state $c) temperature of the gas d) size of the gas cloud e) energy required to strip away all the atom's electrons An atom has energy levels with energies of 1, 3, 9 and 10. Energy level 10 represents the outermost bound orbit of the atom. Assuming this atom has a single electron in the 3d energy level, which of the following photon energies can the electron absorb? a) 1 b) 2 c) 6 d) 8 $e) two of the above answers Atom X has energy levels of 1 and 10. Atom Y has energy levels of 1 and 12. An electron in each atom moves from the upper energy level to the lower energy level, emitting a single photon in the process. Which of the emitted photons has a longer wavelength? $a) The photon from atom X. b) The photon from atom Y. c) The wavelength is independent of the photon energy. d) Cannot tell without more information.