We know that v=λf where v is velocity, λ is wavelength and f is frequency.

Range of spectrum is 400 nm to 750 nm which equals 

400 nm=4×10⁻⁷ m
750 nm=7.5*10⁻⁷m

Here v=c=3*10⁸ m/s

So, Upper limit

So f=7.5*10¹⁴ Hz

Similarly The lower limit will be

Thus the range of spectrum in frequency is 4*10¹⁴ Hz-7.5*10¹⁴ Hz
or 400 Thz to 750 Thz
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It took me a lot of time to type this
Please mark it as brainliest answer
Also 1 Thz = 10^12 Hz