ASTR 101 Exam II - Chapter 5
Which of the following statements about thermal radiation is always true?
A hot object emits more radiation per unit surface area than a cool object. (This is part of the first law of thermal radiation (the Stefan-Boltzmann law).)
Suppose you look at a detailed spectrum of visible light from some object. How can you decide whether it is an emission line spectrum or an absorption line spectrum?
An emission line spectrum consists of bright lines on a dark background, while an absorption line spectrum consists of dark lines on a rainbow background.
Each of the following describes an "Atom 1" and an "Atom 2." In which case are the two atoms different isotopes of the same element?
Atom 1: nucleus with 7 protons and 8 neutrons, surrounded by 7 electrons; Atom 2: nucleus with 7 protons and 7 neutrons, surrounded by 7 electrons (They are isotopes both atoms have the same atomic number but different atomic mass numbers.)
Which of the following statements is true of green grass?
It absorbs red light and reflects green light. (We see the reflected green light.)
What is the purpose of interferometry?
It allows two or more small telescopes to achieve the angular resolution of a much larger telescope. (This has been done for decades with radio telescopes, and is now being done with other types of light as well.)
Which of the following is not an advantage of the Hubble Space Telescope over ground-based telescopes?
It is closer to the stars. (Distance to the stars has absolutely nothing to do with it, as should be apparent if you consider the scale of the solar system and the distances to stars to scale (as discussed in Chapter 1).)
What is the purpose of adaptive optics?
It reduces blurring caused by atmospheric turbulence for telescopes on the ground. (It does this by changing the shape of a telescope mirror to compensate for the way that the atmosphere causes the light from a star to move around.)
Betelgeuse is the bright red star representing the left shoulder of the constellation Orion. All the following statements about Betelgeuse are true. Which one can you infer from its red color?
Its surface is cooler than the surface of the Sun. (Red light has lower energy than yellow or white light, so the red color of Betelgeuse tells us that its peak thermal radiation comes at lower energy than the peak thermal radiation of the yellow/white Sun. A lower energy of peak radiation means a lower temperature.)
What do astronomers mean by light pollution?
Light pollution is light from human sources that makes it difficult to see the stars at night.
Suppose that Star X and Star Y both have redshifts, but Star X has a larger redshift than Star Y. What can you conclude?
Star X is moving away from us faster than Star Y. (The redshifts mean that both stars are moving away from us, and a larger redshift means a faster speed.)
Which of the following statements best describes the two principle advantages of telescopes over eyes?
Telescopes can collect far more light with far better angular resolution. (That is, telescopes have a larger light-collecting area than the eye and a better (smaller) angular resolution.)
How does the light-collecting area of an 8-meter telescope compare to that of a 2-meter telescope?
The 8-meter telescope has 16 times the light-collecting area of the 2-meter telescope. (The 8-meter telescope is 4 times larger in diameter, so its light collecting area is 4^2 = 16 times greater.)
What does angular resolution measure?
The angular size of the smallest features that the telescope can see. (Therefore a smaller angular resolution is better.)
Which of the following conditions lead you to see an absorption line spectrum from a cloud of gas in interstellar space?
The cloud is cool and lies between you and a hot star. (Atoms or molecules in the cloud therefore absorb specific wavelengths of light from the hot star.)
The diagram represents energy levels in a hydrogen atom. The labeled transitions (A through E) represent an electron moving between energy levels. Suppose that an electron in a hydrogen atom absorbs 10.2 eV of energy, so that it moves from level 1 to level 2. What typically happens next?
The electron returns to level 1 by emitting an ultraviolet photon with 10.2 eV of energy. (Electrons typically return to the ground state unless something else interferes with this return.)
Which of the following best describes why we say that light is an electromagnetic wave?
The passage of a light wave can cause electrically charged particles to move up and down. (That is, electrically charged particles can respond to (or generate) electromagnetic radiation.)
The angular separation of two stars is 0.1 arcseconds and you photograph them with a telescope that has an angular resolution of 1 arcsecond. What will you see?
The photo will seem to show only one star rather than two. (Because the angular separation of the stars is smaller than the telescope's angular resolution, the light of the two stars will be blurred together to look like a single star.)
Laboratory measurements show hydrogen produces a spectral line at a wavelength of 486.1 nanometers (nm). A particular star's spectrum shows the same hydrogen line at a wavelength of 486.0 nm. What can we conclude?
The star is moving toward us. (The wavelength is shifted from 486.1 to 486.0 nm, which means a shift to a shorter wavelength. A shorter wavelength means a shift to the blue end of the spectrum (a blueshift) so that the object is moving toward us.)
Studying a spectrum from a star can tell us a lot. All of the following statements are true except one. Which statement is not true?
The total amount of light in the spectrum tells us the star's radius. (We cannot measure radius from a spectrum without additional information.)
Which of the following is always true about images captured with X-ray telescopes?
They are always shown with colors that are not the true colors of the objects that were photographed. ("True colors" make sense only for visible light, not X rays.)
Suppose you watch a leaf bobbing up and down as ripples pass it by in a pond. You notice that it does two full up and down bobs each second. Which statement is true of the ripples on the pond?
They have a frequency of 2 hertz. (Remember that hertz are units meaning "cycles per second.")
The Chandra X-ray Observatory must operate in space because:
X rays do not penetrate Earth's atmosphere. (To detect X rays, the observatory must be above Earth's atmosphere.)
Which of the following statements about X rays and radio waves is not true?
X rays travel through space faster than radio waves. (This is the right answer, because the statement is not true. All light travels at the same speed, so radio waves and X rays travel at the same speed.)
The diagram represents energy levels in a hydrogen atom. The labeled transitions (A through E) represent an electron moving between energy levels. Which labeled transition represents an electron that absorbs a photon with 10.2 eV of energy?
You can tell because the electron jumps up from 0 eV to 10.2 eV. (B)
According to the laws of thermal radiation, hotter objects emit photons with _________.
a shorter average wavelength (Higher temperature objects emit higher energy photons, which means shorter wavelength.)
If we say that a material is opaque to ultraviolet light, we mean that it _________.
absorbs ultraviolet light
Consider an atom of carbon in which the nucleus contains 6 protons and 7 neutrons. What is its atomic number and atomic mass number?
atomic number = 6; atomic mass = 13 (Atomic number is the number of protons, and atomic mass number is the number of protons plus neutrons.)
From shortest to longest wavelength, which of the following correctly orders the different categories of electromagnetic radiation?
gamma rays, X rays, ultraviolet, visible light, infrared, radio
The spectra of most galaxies show redshifts. This means that their spectral lines _________.
have wavelengths that are longer than normal (Redder means longer wavelength.)
Which forms of light are lower in energy and frequency than the light that our eyes can see?
infrared and radio
An atom that has fewer electrons than protons is called a/an _________.
ion (An atom that has a number of protons that does not equal the number of electrons is an ion.)
Suppose you know the frequency of a photon and the speed of light. What else can you determine about the photon?
its wavelength and energy (Once you know frequency, you can calculate both wavelength and energy. (Frequency is related to wavelength by the formula: wavelength × frequency = speed of light; frequency is related to energy by the formula: energy = Planck's constant × frequency.))
When considering light as made up of individual "pieces," each characterized by a particular amount of energy, the pieces are called _________.
photons (A single photon carries a particular amount of energy, corresponding to a particular frequency and wavelength.)
Thermal radiation is defined as _________.
radiation with a spectrum whose shape depends only on the temperature of the emitting object (It's called thermal radiation because it depends on temperature.)
Suppose you built a scale-model atom in which the nucleus is the size of a tennis ball. About how far would the cloud of electrons extend?
several kilometers (This means that an atom is mostly empty space, since most of the mass of an atom resides in the nucleus.)
Suppose you want to know the chemical composition of a distant star. Which piece of information is most useful to you?
the wavelengths of spectral lines in the star's spectrum (Different chemical elements and ions produce different sets of spectral lines.)
Which of the following best describes the fundamental difference between two different chemical elements (such as oxygen and carbon)?
they have different numbers of protons in their nucleus (Atomic number is the number of protons, and each chemical element has a different atomic number.)
The stars in our sky twinkle in brightness and color because of _________.
turbulence in the Earth's atmosphere (Stars twinkle because turbulence keeps changing the precise amount by which light bends as it passes through the atmosphere.)