Electromagnetic waves all travel at the speed of light when in vacuum, and essentially this same speed in normal air. Since wave speed is product of frequency and wavelength, c = f λ and λ = c/f = (3.00 x 108 m/s) / (7.5 x 1014 Hz) = 4.00 x 10-7 m = 400 x 10-9 m = 400 nm.
