http://en.wikipedia.org/wiki/Speed_of_light says the speed of light is 299,792,458 m/s

NOTE: Be sure to use the same distance units for speed and wavelength.

frequency1 = (299,792,458 m/s ) / (0.4 μm)

= (299,792,458 * 10^6 μm/s ) / (0.4 μm)

= 749,481,145 * 10^6 cycles / second

= 749,481,145 MHz (megahertz)

= approx 750 THz (terahertz).

frequency2 = (299,792,458 m/s ) / (0.7 μm)

= (299,792,458 * 10^6 μm/s ) / (0.7 μm)

= 428,274,940 * 10^6 cycles / second

= 428,274,940 MHz (megahertz)

= approx 428 THz (terahertz).

Therefore, the range of frequencies is roughly 428-750 THz (terahertz).