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Astronomy & Astrophysics
Michael Reid

Assignment #6 Transmission:  Visible light meets clear glass  Cell phone signals pass through walls Absorption:  Blue light hits a red sweatshirt  Visible light does not pass through a black wall Reflection or scattering  Red light hits a red sweatshirt  White light hits a white piece of paper Emission  Light comes from a light bulb  Light comes from your computer screen Process of Science: The Solar Spectrum  The prism bends different wavelengths of light by different amounts, causing the white light from the Sun to spread out into a rainbow of colors. Absorption features appear as dark lines against the brighter background of the spectrum.  Wavelength × frequency = speed of light o The key relationship is wavelength × frequency=speed of light. The speed of light through space is always the same, so when wavelength goes up (longer), frequency must go down (lower), and vice versa. o The visible light with the longest wavelength is red.  To remember this, think about the fact that infrared light has longer wavelength than visible light and is called infrared because it is close to red light in the spectrum.  Red light is the longest wavelength visible light, and longer wavelength means lower frequency (because wavelength×frequency=speed of light).  Which of the following best describes why the Sun’s spectrum contains black lines over an underlying rainbow? o The Sun’s hot interior produces a continuous rainbow of color, but cooler gas at the surface absorbs light at particular wavelengths. o The Sun’s spectrum is an absorption line spectrum, which is produced when continuous light from a hot source (the Sun’s interior) passes through a cooler cloud of gas (the gas that makes up the Sun’s visible surface).  Notice that the Sun’s spectrum appears brightest (or most intense) in the yellow-green region. This fact tells us __________. o The approximate temperature of the Sun’s surface o One of the two laws of thermal radiation (Wien’s law) states that the peak wavelength of a spectrum is directly related to an object’s temperature. A peak at yellow-green wavelengths corresponds to a temperature of about 5800 K.  A spectral line is produced when __________. o an electron in an atom, ion, or molecule gains or loses a precise amount of energy o an electron in an atom, ion, or molecule gains or loses a precise amount of energy  Suppose we want to know what the Sun is made of. What should we do? o Compare the wavelengths of lines in the Sun’s spectrum to the wavelengths of lines produced by chemical elements in the laboratory. o Each chemical element (or ion or molecule) produces a unique set of spectral lines; the wavelengths of these lines can be measured in the laboratory. If the Sun's spectrum contains the set of lines for some particular element, we conclude that the Sun contains that element. We determine the Sun’s overall chemical composition by examining all the lines in its spectrum.  Any spectrum can be displayed either in photographic form as shown to the left or as a graph. Which of the following graphs could represent a portion of the Sun’s visible light spectrum? o o The smooth part of the curve represents the graph of the background rainbow of color; the dips in the curve represent the black lines where light is missing from the rainbow Three Basic Types of Spectra  Recall that one of the laws of thermal radiation states that a higher-temperature object emits photons with higher average energy (Wien’s law). This law is
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