The relationship between frequency and wavelength for an electromagnetic wave is
[tex]c=f \lambda[/tex]
where
f is the frequency
[tex]\lambda[/tex] is the wavelength
[tex]c=3 \cdot 10^8 m/s[/tex] is the speed of light.
For the light in our problem, the frequency is [tex]f=1.20 \cdot 10^{13} s^{-1}[/tex], so its wavelength is (re-arranging the previous formula)
[tex]\lambda= \frac{c}{f}= \frac{3 \cdot 10^8 m/s}{1.20 \cdot 10^{13} s^{-1}}= 2.5 \cdot 10^{-5}m[/tex]
