Theory: Wave number (ν̅) is defined as the number of waves in unit length. Similarly, frequency, ν is the number of waves passed through a point in one second. Thus if c is the velocity the radiation, then          ν = ν̅c. Given that, ν̅ = 0.5 × 10⁸ m^-1, and we know that the velocity of the electromagnetic radiation is, c = 3 × 10⁸ ms^-1. So put the values in the above equation, ν = 0.5 × 10⁸ m^-1 × 3 × 10⁸ ms^-1 = 1.5 × 10¹⁶ s^-1.