Download Homework #3 Chapter 2: Light and Motion Due

Homework #3
Chapter 2: Light and Motion
Due: 12 Sept 2013
Review and Discussion:
RD.2 Compare and contrast the gravitational and electric forces.
Answer:
The electric force is similar to the gravitational force in that it drops off by the inverse square of the
distance. It is different in that it can be either attractive or repulsive; unlike charges attract and like
charges repel. If the number of positive and negative charges are equal in an object, it appears to be
neutral. Gravity is always present, is never neutralized, and is only attractive.
RD.6 What is a blackbody? Describe the radiation it emits.
Answer:
A blackbody is an idealized object that absorbs all radiation falling on it. It also reemits all this radiation.
The radiation emitted occurs at all wavelengths but peaks at a wavelength that depends on the
temperature of the blackbody. The hotter the temperature, the shorter the wavelength of the peak
radiation.
RD.9 What is spectroscopy? Explain how astronomers might use spectroscopy to determine the
composition and temperature of a star.
Answer:
When light from a star is passed through a prism or similar device, a spectrum is produced. Spectroscopy is the
observation and study of these spectra. For example, by studying the patterns of dark absorption lines in the
spectrum of a star, a variety of information can be determined, including composition and temperature.
RD.11 What is the normal condition for atoms? What is an excited atom? What are orbitals?
Answer:
Normally, the number of electrons in an atom equals the number of protons in the nucleus, and the electrons are in
their lowest energy level, the "ground state." When an atom is excited, an electron absorbs energy from an outside
source and moves to a higher energy orbital. The precisely defined energy states or energy levels are referred to as
orbitals. They are the regions surrounding the nucleus that can be occupied by electrons.
RD.15 How do astronomers use the Doppler effect to determine the velocities of astronomical objects?
Answer:
The Doppler effect can make the radiation we receive from objects display a different wavelength than what we
expect. By measuring the amount of this "shift" in the wavelength, astronomers can determine whether an object is
moving toward or away from us. The greater the shift appears, the greater the relative speed. Therefore, by
measuring the Doppler shift, the relative velocity can be determined.
Conceptual Self Test:
CST.1
The wavelength of green light is about the size of an atom.
FALSE
CST.6
The energy of a photon is inversely proportional to the wavelength of the radiation.
TRUE
CST.10 A star much cooler than the Sun would appear
RED
CST.12 Visible spectrum of sunlight reflected from Saturn's cold moon Titan would be expected to be
Absorption
CST.14 According to Figure 2.11 in the textbook (Blackbody Curves), an object having a temperature
of 1000
emits mostly
Infrared light
Problems:
P2.01: A sound wave moving through water has a frequency = 256 Hz and a wavelength = 5.77 m.
(a) What is the speed of sound in water?
The relationship between frequency, wavelength, and wave velocity is  f = v.
Using the data given, the speed of sound in water is
v = (5.77 m)(256 Hz) = 1480 m/s. (Three significant figures)
P2.07: How many times more energy has a lambda X-ray than a f radio photon?
E = hf: energy is proportional to frequency. Find the frequency of a 1-nm gamma ray:
f = c/ = (3 x 108 m/s)/(1 x 10–9 m) = 3 x1017 Hz.
The 10-MHz radio photon has a f = 1x 107 Hz. The ratio of these two frequencies is
E_gamma / E_Radio = 3 x 1017 Hz / 1x107 Hz = 3 x 1010
P2.09: The H-line of a star is received on Earth at a wavelength of 656 nm. What is the star's
radial velocity relative to Earth?
The H line has a wavelength of 656.3 nm. Using the Doppler formula from Section 2.7:
(Apparent Wavelength / True Wavelength ) = 1 + v / c
656 nm
v
 1
656.3
c
Solving for v gives: 1.37  105 m/s, which is 137 km/s approaching