"ps" stands for "pico-second". It means 10⁻¹² second.
The period of this wave is 8.43 x 10⁻¹² second.
In a vacuum, the speed of any electromagnetic wave is 3 x 10⁸ m/s.
Wavelength = (speed) / (frequency)
But Frequency = 1 / period
So Wavelength = (speed) x (period)
Wavelength = (3 x 10⁸ m/s) x (8.43 x 10⁻¹² sec)
Wavelength = 2.5 x 10⁻³ meter
Wavelength = 2.5 millimeters
