26. A beam of 50-keV electrons is directed at a crystal and diffracted electrons are found at an angle of 50° relative to the original beam. What is the spacing of the atomic planes crystal? A relativistic calculation is needed for A. of the 16. The phase velocity of ripples on a liquid surface is V2πS/AP, where S is the surface tension and p the density of the liquid. Find the group velocity of the ripples.