PROPERTIES OF LIGHT

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saturnbutton1.JPG (21728 bytes)Atom saturnbutton1.JPG (21728 bytes)Black Body Spectra & Temperature
saturnbutton1.JPG (21728 bytes)Doppler Effect saturnbutton1.JPG (21728 bytes)Home Page

                           

saturnbutton1.JPG (21728 bytes)Atom Questions

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Q1.    How is the chemical composition of a star determined? Answer

Q2.    What is the average chemical composition of the universe?  Answer

Q3.    Explain in simple physical terms how absorption lines occur in spectra. Specifically, why are lines produced, and not broad bands or depressions?  Answer

Q4.    Why do individual atoms emit light at only certain specified wavelengths? What property of atoms requires this to occur?  Answer

Q5.    In general terms, how is the chemical composition of a star determined? What is the relative abundance of the two most abundant elements in most stars?  Answer

Q6.    Describe the standard model of an atom. What is the most unusual property of the atom, compared to our common experience?  Answer

Q7.    How are dark spectral lines formed? Explain the process in such a way that it is clear why a line is formed and not a broad range of colors.  Answer

Q8.    Explain how astronomers can determine what a star is made of. What is the most abundant element in most stars?  Answer

Q9.    What are the two most abundant elements in the universe? Why are the lines of these elements not always strong in the spectra of stars?  Answer

Q10.    Why is the rule that only certain electron orbits can exist in an atom so important to the study of astronomy?  Answer

Q11.    Explain how a bright line is produced in a spectrum?  Answer

Q12.    In what way are the rules for electron orbits "unusual"? How does this property of electron orbits affect the way light is absorbed by an atom?  Answer

Q13.    What special or unusual rule do electrons obey in an atom? How is it different from the rule that planets obey in their orbits around the sun?  Answer

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

saturnbutton1.JPG (21728 bytes)Black Body Spectra & Temperature Questions

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Q1.    In what two ways does the spectrum of a solid, glowing object change as it is heated to a higher temperature?  Answer

Q2.    What simple observation can be taken to determine the temperature of an astronomical object?  Answer

Q3.    How can the temperature of a glowing body be determined?  Answer

Q4.    Explain how the temperature of a remote object can be determined just by looking at it. How will its appearance change as it heats up?  Answer

Q5.    How is the concept of temperature defined? What is the average temperature in an interstellar cloud before star formation begins?  Answer

Q6.    What is meant by the term "temperature?" Why does a collapsing cloud get hot?  Answer

Q7.    Define the concept of temperature and of absolute zero.  Answer

Q8.    What is the definition of temperature? Describe the Kelvin scale of temperature.  Answer

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

saturnbutton1.JPG (21728 bytes)Doppler Effect Questions

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Q1.    How can you measure how fast an object is moving just from the light it emits?  Answer

Q2.    How is the speed of a moving star determined? Explain how the method works.  Answer

Q3.    What principle does a policeman use (even if he doesn’t know it) to measure your speed? How does it work?  Answer

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

saturnbutton1.JPG (21728 bytes)Atom Answers

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A1.    Atoms in the atmosphere of the star absorb light at specified wavelengths according to the permitted orbits of the atoms, forming a dark line spectrum. Each element absorbs light at a different set of wavelengths. By recognizing the pattern of lines of different elements, we can tell what the star is made of.

A2.    In the universe, approximately 9 out of every 10 atoms is a hydrogen atom and 1 out of every 10 atoms is a helium atom. All other elements account for less than one per cent of all the atoms.

A3.    Light is absorbed when an electron in an atom moves from a small orbit to a larger orbit within the atom. Because only certain, specified orbits are permitted for any atom, the electron must gain the exact amount of energy required for one of these permitted orbits. As a result, only certain wavelengths of light can be absorbed – those with the required amount of energy. The result is the occurrence of a thin, dark line in the spectrum of light passing through the atoms.

A4.    Electrons which orbit the nucleus of a particular element can only exist in certain, specified orbits unique to that element. As electrons move from one permitted orbit to another, they must release (or absorb) the exact amount of energy required for those orbits. Hence, they can emit (or absorb) only at certain specified wavelengths that correspond to energy differences between permitted orbits.

A5.    The allowed orbits of each element are unique to that element. Hence, each element has a unique spectrum of bright or dark lines in its spectrum. Astronomers can identify which elements are present in a star by recognizing the pattern of lines in their spectra, compared to the pattern of lines from each element. A careful analysis of stellar spectra shows that 9 out of 10 atoms in stars is a hydrogen atom, and that 1 out of 10 atoms is a helium atom.

A6.    An atom consists of a tiny but heavy nucleus surrounded by one or more electrons which orbit the nucleus. The number of electrons which orbits the nucleus equals the number of protons in the nucleus. The orbits of the electrons must lie at precisely defined distances from the nucleus; other orbital distances are simply not permitted.

A7.    If a light wave with exactly the correct amount of energy passes near an atom, an electron in the atom will completely absorb that light wave and use its energy to move into a larger orbit. Since the light wave must have a prescribed amount of energy, only one wavelength of light can be absorbed for each orbital change. Thus, one and only one wavelength will be emitted.

A8.    A spectrum of the star is obtained. The dark lines in the spectrum, which result when electrons move from one permitted orbit to another permitted orbit, show what the star is made of. Since each element has a unique pattern of permitted orbits, each element has a unique pattern of wavelengths at which it can absorb. This analysis of stars shows that hydrogen is by far the most abundant element in stars.

A9.    The most abundant elements in the universe are hydrogen and helium. The lines of these elements are not always prominent in the spectra of stars because the star may be too hot or too cool to show the lines. For example, if the star is too cool, the electrons of the hydrogen atoms will be in the lowest possible orbit, where they cannot absorb any visible light.

A10.    When an electron moves from one permitted orbit to another, it must gain or give up exactly the amount of energy required for the new orbit. The rule that only certain orbits can exist in an atom leads to the formation of spectral lines, which in turn allow us to identify what kind of atoms are present in a light source.

A11.    If an electron moves from a larger orbit to a smaller orbit, it must give up an amount of energy equal to the energy difference between the orbits. Since only certain electron orbits are permitted in an atom, an electron must give up a specified amount of energy, in the form of a light wave, to make the transition from the larger orbit to a smaller one. For a given transition, one and only one wavelength of light can be emitted to dissipate the required amount of energy.

A12.    The most unusual rule for electron orbits is that they can exist in only certain specified orbits in a given type of atom. Other, intermediate orbits simply do not exist. This means that electrons must gain or lose a specific amount of energy to move between permitted orbits. These specified energy differences mean that only specified wavelengths of light can be absorbed or emitted by atoms when electrons interact with light.

A13.    Electrons can orbit the nucleus in only certain exactly specified orbits. They simply cannot exist in orbits between the specified orbits. This rule seems very strange to us, because ordinary objects, like planets, do not obey such a rule. Planets can orbit the sun in any orbit; none are more favored or preferred than another.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

saturnbutton1.JPG (21728 bytes)Black Body Spectra & Temperature Answers

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A1.    As a glowing solid is heated, the average wavelength of the light it emits becomes shorter and the intensity of the light it emits increases.

A2.    Observe the average wavelength of the continuous spectrum emitted by an object. The hotter the object, the shorter (bluer) the average wavelength will be.

A3.    A dense glowing object emits light at all wavelengths. The distribution of wavelengths is governed by the so-called black body law. The curve of brightness versus color shows a peak at a certain wavelength. This peak wavelength depends only on the temperature of the object: the hotter the object, the shorter the peak brightness. Thus, the temperature of a glowing object can be determined by measuring this curve, or just the peak wavelength.

A4.    Every object emits light that is characteristic of its temperature. This so-called continuous spectrum usually approximates a black body curve. Since the brightest wavelength emitted by the body moves to shorter wavelengths as the object heats up, a measurement of the brightest color tells us what the temperature is. As the object heats up it also gets brighter at all wavelengths, so its important to find the brightest wavelength.

A5.    Temperature is a measure of the average speed of random motion of the atoms in a body. The temperature in an interstellar cloud before star formation begins is around 10 K above absolute zero.

A6.    Temperature is the average speed of the atoms in their random motion. Higher average speed means higher temperature. When a cloud of gas collapses, the atoms fall toward the center. Falling atoms gain speed. Collisions among the atoms convert this motion into random motion, which means the temperature increases.

A7.    Temperature is defined as the average speed of random motion of the atoms or molecules of a substance. Absolute zero occurs if there is no atomic motion.

A8.    Temperature is a measure of the average speed of random motion of the atoms in a substance. The Kelvin temperature scale has its zero point at absolute zero, where all random motion has stopped.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

saturnbutton1.JPG (21728 bytes)Doppler Effect Answers

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A1.    When an object is moving toward you, its light waves are compressed by that motion so their wavelength is shorter (bluer) than normal. Conversely, an object moving away from you emits light at longer (redder) wavelengths than normal. By measuring the amount of shift in the spectral lines from an object, we can determine how fast it was moving toward or away from us.

A2.    The speed of a moving object is measured through the Doppler effect. A wave from an object coming toward us is compressed (has a shorter wavelength) while a wave from an object moving away from us is expanded (has a longer wavelength). The pattern of spectral lines in a star tells us what the wavelength would have been if the star were not moving. The difference between these rest wavelengths and the actual wavelengths observed in the star tell us how fast it is moving.

A3.    The speed of an object is measured with the Doppler effect, in which the wavelength is compressed for objects coming toward an observer and expanded for objects moving away from the observer. The policeman sends out radio waves of a known wavelength. When they reflect off your car, their wavelength is altered by your motion. The amount of change of the wavelength tells him how fast you are going.