Q. The electric field associated with an e.m. wave in vacuum is given by E⃗ = î 40 cos(kz − 6 × 10⁸ t), where E, z and t are in volt/m, metre and seconds respectively. The value of wave vector k is
A) 3 m⁻¹
B) 2 m⁻¹
C) 0.5 m⁻¹
D) 6 m⁻¹
Explanation
[Solution from source] From E = E₀ cos(kz − ωt), ω = 6 × 10⁸ rad s⁻¹. In vacuum c = ω/k, so k = 6 × 10⁸/(3 × 10⁸) = 2 m⁻¹. 0.5 m⁻¹ comes from inverting the ratio. Answer (2).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.