Answer
See the answer below.
Work Step by Step
From figure 6-10, $\lambda=1875\ nm$
$c=\lambda\times\nu$
$\nu=2.998\times10^8\ m/s\div 1875\times10^{-9}\ m=1.60\times10^{14}\ 1/s$
This wavelength is in the infrared spectrum.
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