Question

An electromagnetic wave traveling through a vacuum has a wavelength of 1. 5 × 10–1 meter. The period of this electromagnetic wave is

Answer 1

Speed of EM wave in vacuum = 3 x 10⁸ meters per second.
Frequency = (speed) / (wavelength)
Period = 1/frequency

Period = (wavelength) / (speed) = (0.15) / (3 x 10⁸) = 0.5 nanosecond

( "nano..." = 10⁻⁹ )