Explanation

Visible light spans wavelengths from about 380 nm (violet) to about 750 nm (red). Violet light lies at the short-wavelength end of the visible spectrum, approximately 380-450 nm, which is about 0.4 micrometres. The 0.7 micrometre value corresponds to red light, not violet. Hence the approximate wavelength of violet light in air is 0.4 um.