Hi there!
We can use the following equation:
[tex]\large\boxed{v = \lambda f}[/tex]
v = velocity of wave (m/s)
λ = wavelength (μm)
f = frequency (Hz)
We can rearrange the equation to solve for frequency:
[tex]\large\boxed{f = \frac{v}{\lambda}}[/tex]
Plug in the givens. Make sure to convert μm to m:
1 μm = 0.000001 m
[tex]f = \frac{1800}{0.000045} = 40000000 Hz = \boxed{40 \cdot 10^6 Hz}[/tex]
