Physics Demystified, 2nd edition |

Stan Gibilisco |

Explanations for Quiz Answers in Chapter 13 |

1. When the earth's lower atmosphere (known as the
troposphere) refracts radio waves, we call the effect
tropospheric bending. It's a simple matter of definition! The
correct choice is C. |

2. Neutrinos have incredible penetrating power. They can pass through just about anything, including our whole planet, as easily as a ray of light passes through a pane of glass. The answer is D, "All of the above." |

3. The D layer has a negative influence on wireless broadcasting and communications at medium frequencies (MF) during the daytime, because it absorbs electromagnetic (EM) wave energy at those frequencies. At night, the D layer disappears, and so does its adverse effect. The correct answer is A. |

4. This problem involves the formula for photon energy in
terms of wavelength, given on page 391 in Chapter 12. If we let
We're told that
The correct choice is D. |

5. We can use the same formula to solve this problem as we used
in the solution to Question 4. In this case, however, the unknown value
is
We're told that 2.50 x 10 Using a little algebra, we can rearrange this equation to get
which solves to
The correct choice is A. |

6. Of the four sequences stated here, only choice C
expresses EM fields in order of increasing free-space wavelength. Gamma
(γ) rays have shorter wavelengths than x rays, which have
shorter wavelengths than ultraviolet (UV) rays, which have shorter
wavelengths than visible-light rays. You can see this sequence in
graphical form by turning to page 401 and looking at Fig. 13-2. |

7. We need the formula relating the speed of light
Let's consider
In this situation,
The answer is B. |

8. If a radioactive substance has a half-life
of x years, then its radiation intensity declines by 50%
after x years, and then the remaining radiation declines by
50% after another x years. Two successive 50% declines
represents a reduction to 25% of the original radiation intensity,
or an overall loss of 75% relative to the initial intensity. The
period of time in question (in this case 10 years) represents two
half-lives, so we know that the half-life must be 10/2 or 5.0 years.
The correct choice is B. |

9. For any source of EM radiation, the energy contained in a single photon varies inversely in proportion to the free-space wavelength, and directly in proportion to the frequency. Therefore, the radiation with the longest wavelength and the lowest frequency comprises photons with the least energy. Of the four types of EM energy mentioned here, extremely-low-frequency (ELF) radiation has by far the longest free-space wavelength and the lowest frequency. The answer is B. |

10. This problem works in the opposite sense from Question 9. The radiation with the shortest free-space wavelength and the highest frequency has photons with the most energy. In this case, that's x rays. The correct choice is A. |