Wavelength and Frequency are inversely related to each other as,
υ = c / λ ----- (1)
Where;
υ = Frequency = 2.40 × 10¹³ s⁻¹
c = 2.99 × 10⁸ ms⁻¹
λ = ?
Solving equation 1 for λ,
λ = c / υ
Putting Values,
λ = 2.99 × 10⁸ ms⁻¹ ÷ 2.40 × 10¹³ s⁻¹
λ = 0.00001245833 m
Or,
λ = 1.24 × 10⁻⁵ m