The relationship between frequency and wavelength for electromagnetic waves is
[tex]f= \frac{c}{\lambda} [/tex]
where
f is the frequency
c is the speed of light
[tex]\lambda [/tex] is the wavelength
For the light in our problem, the wavelength is
[tex]\lambda=639 nm= 6.39 \cdot 10^{-7} m[/tex]
and with the previous equation we can find the corresponding frequency:
[tex]f= \frac{c}{\lambda}= \frac{3 \cdot 10^8 m/s}{6.39 \cdot 10^{-7} m}=4.69 \cdot 10^{14}Hz [/tex]
And since [tex]1 THz = 10^{12} Hz[/tex], the frequency can be rewritten as
[tex]f=4.69 \cdot 10^2 THz[/tex]
