Assign Ch. 3 (1),2,4-6,8-13,15 (Note #10 and 11 are a bit redundent, next time assign: 2,4-6,8-10,12-14.) Ch. 4 1,2,5,8-12,15 Ch. 3 1. (Bad question "What is motion wave?") 2. What is the relationship between wavelength, wave frequency, and wave velocity? The wave velocity is equal to the product of wavelength and freq., i.e., v = w*f, where w=wavelength. There is an inverse relationship between wavelength and frequency, an increase in w means a decrease in f. 4. Why is light referred to as an electromagnetic wave? Because the quantities that oscillate as the wave passes are electric and magnetic fields. 5. What effect does a positive charge have on a nearby neagatively charged particle? What effect does a positive charge have on a nearby positively charged particled? A positive charge exerts a force of attraction on a negatively charged particle ("opposites attract"). A + charge would repel another + charge ("like charges repel"). 6. What's so special about c? "c" stands for the speed of light in a vacuum. This speed is special because all types of E-M waves travel at this same speed through a vacuum (they will slow down in a transparent medium). This is also nature's speed limit - nothing can go faster. Also, in special relativity, light travels at the same speed to any observer in an inertial reference frame (e.g., in a spaceship moving at a constant speed). 8. Explain how light allows us to learn about distant astronomical objects. Light can travel great distances through the vacuum of space with little alteration (it just decreases in intensity with distance). Light can approach the Earth from a distant star or galaxy and then it runs into Earth's atmosphere. If there are no clouds, visible wavelengths can make it all the way down to telescopes on the Earth's surface. The light refracts a little bit in the atmosphere, but we can figure out which way the light came from and thus map out the stars in the sky. 9. If Earth were comletely blanketed with clouds and we couldn't see the sky, could we learn about the realm beyond the clouds? What forms of radiation might be received? Yes, we could learn about thing beyond the clouds but only in the radio part of the E-M spectrum. Wavelengths between about 10m and 1 mm get through. 10. Name the colors that make up white light. What properties of these colors are different, and what do they have in common? In order of in creasing frequency, the colors are Red, Orange, Yellow, Green, Blue, Violet. The wavelengths, frequencies and energy per photo vary with color. They will also refract at slightly different angles when passing from one medium into another. They have in common a speed of propogation of 3x10^8 m/s and they are all electromagnetic waves. 11. What do radio waves, infrared radiation, visible light, UV, X-rays, and gamma rays have in common? How do they differ? These are all composed of electric and magnetic fields, i.e., electro- magnetic radiation. They all travel at c. They differ in wavelength, frequency, and energy per photon. They also differ in the extent to which they are absorbed and scattered by various media. 12. What is a blackbody? What are the main characteristics of the radiation it emits? A blackbody is a hypothetical object that absorbs 100% of the light hitting it and emits light with a particular spectrum (the Planck spectrum). The radiation emitted has a particular mathematical formula describing its spectrum (a plot of intensity vs frequency). The spectrum is smooth, featureless except for a single peak. The peak is inversely proportional to the temperature, and the total energy leaving the BB (the area under the spectrum) is proportional to temperature raised to the fourth power. 13. In terms of its blackbody curve, describe what happens as a red-hot glowing coal cools. The peak of the spectrum will shift to longer wavelengths as the temperature drops. [14. What does Wien's law reveal about stars in the sky? That blue stars have a hotter surface than red stars. ] 15. How do astronomers use the Doppler effect to determine the velocities of astronomical objects? What are some possible limitations to this approach? We have to measure the wavelength of spectral features (see Ch. 4) like emission and absorption lines and compare the observed wavelength (Wobs) with the rest wavelength measured in the lab (Wres). Then we define redshift, z=(Wobs-Wres)/Wres. Finally, the velocity of recession is given by v=cz, where c is the speed of light. The limitation is that we can only measure velocity along the line of sight (radial velocity) not across the line of sight (tangential velocity). ------------------------------------------------------------------ Ch. 4 Discussion 1,2,5,8-12,15 1. What is an absorption spectrum? An emission spectrum? How are they related? An absorption spectrum shows dips in light intensity at certain wavelengths (absorption lines). An emission spectrum shows peaks in light intensity at certain wavelengths (emission lines). For a given gas, like hydrogen, the wavelengths of the absorption lines are the same as the wavelength of the emission lines. 2. What is spectroscopy? How can spectroscopy be used to infer the composition and temperature of a star? Spectroscopy is the observation and study of light by spreading out (dispersing) the light into a spectrum. This reveals emission and/or absorption lines. The strength of those lines depends on the temperature and abundance of the species of gas which produces them. Thus line strengths can tell us about the temperature and composition. [3. Describe the basic components of a simple spectroscope and the additional components in spectrographs used by astronomers for modern observations. The most essential component is a dispersive element - this can be used with the eye to make a simple spectroscope. A spectrograph involves a telescope for gathering and focusing light on a slit, a dispersive element (grating or prism) for spreading out the light after the slit, and a collimator to focus the spectrum on a screen or a camera. ] 5. In the particle description of light, what is color? Color corresponds to different photon energies. Blue is higher energy per photon than red. [7. Give a brief description of a hydrogen atom. It consists of a positively charged proton for a nucleus and a single electron. A rare isotope is deuterium which has a neutron joined to the proton. ] When a physical quantity is quantized, it means that it can take on only specific values rather than a continuous range of values. We think light is quantized because of experiments like the photo- electric effect where light acts like a particle. 9. What is the normal condition for atoms? What is an excited atom? What are orbitals? Atoms are normally in the "ground" state, meaning that the electron is in the lowest energy state (n=1). An excited atom has its electron in a higher orbital (n>1) which means that the atom has more energy than when in the ground state. Orbitals are the different quantized energy levels of the atom. The electrons are not actually confined to narrow paths like a planet's orbit, but that name "orbital" stuck from an early "planetary" model of the atom. Electrons are confined to precise energies (this is why we say "quantized"). 10. Why do excited atoms absorb and reemit radiation at characteristic In order for a photon to be absorbed, it has to have an energy that is precisely equal to the energy difference between two energy levels, the lower level of which is occupied by an electron. The electron absorbs the photon and moves to the higher energy level. Very quickly thereafter the electron moves back down to the lower energy level emitting a photon of equal energy to the energy difference between the two levels. 11. How are absorption and emission lines produced in a stellar spectrum? What information might absorption lines in the spectrum of a star reveal about the cloud of cool gas lying between us and the star? A star's interior produces a continuous spectrum. However, this light passes through a cooler layer of gas that surrounds the star. Specific wavelengths are absorbed by this gas and the resulting spectrum appears as an absorption spectrum, a continuous spectrum with specific wavelengths missing. Emission lines are not normally found in a stellar spectrum because they are produced in a hot, low density gas. Most stars have a layer of cool, low density gas forming an absorption spectrum. However, in some cases, a hot low density layer can form or can be found in clouds of gas between stars and an emission spectrum is seen. Information about the composition and temperature of the cool gas, along with its motions, can be determined from the absorption lines. 12. Why might spectral lines of an element in a star's spectrum be weak, even though that element is abundant in the star? Not only must the element be present, but its electron(s) must be in the proper state (energy levels) for the transition. For example, the H-alpha absorption line of hydrogen results from electrons jumping from the second to the third atomic orbital. Because the Sun's lower atmosphere is rather cool, relatively few atoms have electrons in the second orbital; most are in the ground state. Hence, in sunlight, the H-alpha line is weak. 15. List three properties of stars that can be determined from the observations of their spectra. 1. Composition 2. surface temperature 3. rotation rate 4. turbulence, 5. surface gravity 6. magnetic field strength 7. radial velocity