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 e represent the energy that a single photon contains (in joules) and we let λ represent the wavelength (in meters), then

e = 1.9865 x 10-25 / λ

We're told that λ = 200 nm = 2.00 x 10-7 m. It follows that

e = (1.9865 x 10-25) / (2.00 x 10-7)
= 9.93 x 10-19 J

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 λ, not e, in the equation

e = 1.9865 x 10-25 / λ

We're told that e = 2.50 x 10-15 J, so we can calculate

2.50 x 10-15 = 1.9865 x 10-25 / λ

Using a little algebra, we can rearrange this equation to get

λ = (1.9865 x 10-25) / (2.50 x 10-15)

which solves to

λ = 7.95 x 10-11 m
= 0.0795 nm

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 c (in meters per second) to the frequency f (in hertz) and the wavelength λ (in meters) for an EM wave in free space. Once again, the formula is

c = f λ

Let's consider c = 3.00 x 108 m/s. We can rearrange the formula to obtain

f = 3.00 x 108 / λ

In this situation, λ = 40 m, so

f = 3.00 x 108 / 40
= 7.5 x 106 Hz
= 7.5 MHz

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.