Answer:

Step-by-step explanation:
Hello,
The frequency and wavelength of electromagnetic waves propagating in free space might be related to each other by using the following equation:
c=λf
Solving for the frequency:
f=c/λ
Knowing that λ stands for the wavelength (642 nm for the mentioned incident light), c the light velocity (300.000 m/s) and f the frequency, we get the asked frequency as:
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Best regards.