The portion of the
electromagnetic spectrum in which the longest
wavelength is just below the
visible spectrum, extending from approximately 4 nm to approximately 400 nm.
Note: Some authorities place the lower limit of uv at values between 1 and 40 nm, 1 nm being the upper wavelength limit of x-rays. The 400-nm limit is the lowest visible wavelength,
i.e., the highest visible
frequency, violet.