Decomposition of the wave-vector in terms of wavelength

[watty]

Im attempting a problem in relativistic optics that's asking me to write k_x, k_y and k_z in terms of lambda, c, and theta (theta being the angle between a source and an observer, at rest relative to each other). Lamda is the wavelength in this reference frame. I started off using trigonometry to say that { k_x = 2*pi*cos(theta) / lambda } and equivalent but with sin(theta) for k_y but then how do I express k_z ? i can't see homework to it without introducing an angle between the x and z axis!

help please!

 

[tiny-tim]

Hi watty!

(have a theta: θ and a lambda: λ and a pi: π )

uhh? you're in charge!

so just put k_z = 0. ​