To convert a wavelength from nanometers (nm) to wavenumber in inverse centimeters (cm^-1), you can use the following formula:
Wavenumber (cm^-1) = 10^7 / Wavelength (nm)
Let's apply this formula to convert 400 nm to wavenumber:
Wavenumber (cm^-1) = 10^7 / 400 nm
Wavenumber (cm^-1) ≈ 25,000 cm^-1
Therefore, 400 nm is approximately equal to 25,000 cm^-1 when converted to wavenumber.