Understanding Lasers: Chapter 12 Quiz

Source: Jeff Hecht, Understanding Lasers: An Entry-Level Guide, fourth edition (2019), Chapter 12 quiz, printed pages 470–473. The questions are paraphrased.

Quick answers

Question

Answer

1

b

2

c

3

a

4

b, \(1.008\ \mathrm{GB}\)

5

d

6

a, \(5.0\times10^5\) points/s

7

d in the printed key; see convention note

8

c

9

No listed answer; \(156{,}250\) channels

10

e, \(2.56\ \mathrm s\)

Worked reasoning

  1. Long-life vent monitor: b. A rarely serviced diode laser can last much longer than an incandescent bulb. Its directionality is also useful, but the reliability advantage is the book’s intended reason.

  2. Scanner rejection of room light: c. A narrow optical filter passes the scanner’s laser line while rejecting most broadband fluorescent light, greatly improving signal-to-background ratio.

  3. Why Blu-ray uses violet: a. Diffraction-limited spot size scales with wavelength, so a shorter wavelength reads smaller marks and closer tracks.

  4. Capacity from wavelength alone: b. Linear feature size scales as \(\lambda\), so areal density scales approximately as \(1/\lambda^2\):

    \[C_{DVD}=700\ \mathrm{MB}\left(\frac{780}{650}\right)^2 =1008\ \mathrm{MB}=1.008\ \mathrm{GB}.\]

  5. Other DVD improvements: d. Higher-numerical-aperture optics reduce the spot further, while improved coding and compression store useful content more efficiently.

  6. Maximum lidar point rate: a. The farthest target requires a \(600\ \mathrm{m}\) round trip:

    \[t_{rt}=\frac{2R}{c}=\frac{600}{3.00\times10^8} =2.00\ \mathrm{\mu s},\]
    \[f_{\max}=\frac1{t_{rt}}=5.00\times10^5\ \mathrm{s^{-1}}.\]

    The 1-ns pulse duration is negligible compared with this wait time.

  7. Distance scale of a 1-ns pulse: d in the key. Its free-space spatial length is

    \[\ell=c\tau=(3.00\times10^8)(10^{-9})=0.30\ \mathrm m.\]

    This matches choice d and the printed key. In a two-way time-of-flight range calculation, however, \(R=ct/2\), so the pulse-duration-limited range resolution is often quoted as \(c\tau/2=0.15\ \mathrm m\). The choices do not include that value.

  8. Single-drum colour printing: c. The photoconductor is written and developed successively with different toner colours, transferring the colour separations during multiple passes.

  9. Voice channels in 10 Gbit/s: no listed answer. Direct division gives

    \[N=\frac{10\times10^9\ \mathrm{bit/s}} {64\times10^3\ \mathrm{bit/s}} =156{,}250.\]

    Important

    Answer-key discrepancy

    The printed key selects d, \(178{,}000\), but that value does not follow from the two rates stated in the question. Protocol overhead would reduce, not increase, the number of payload channels.

  10. Earth–Moon round trip: e.

    \[t=\frac{2R}{c} =\frac{2(384{,}000\ \mathrm{km})}{299{,}792\ \mathrm{km/s}} =2.56\ \mathrm s.\]