Question

what is the energy of a wave of wavelength 400 nm

Answer 1

I probably should not have used v. It's actually Mu not v. Let me just rewrite everything.

E=hf where f=c/wavelength

Answer 2

I need a numerical value for this question

Answer 3

Indeed

Answer 4

Energy of a wave ??
Don't you rather mean energy of a photon?

Answer 5

E = h c / lambda = (Planck)(3x10^8 m/s)/(400x10-9m)

look up Planck's constant.