Exercises 4.4: Wave packets

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4.4.1 The figure shows the displacement versus time of one atom in a chain of 60 atoms, with periodic boundary conditions and a lattice constant of 2 Angstroms. The motion is of a packet moving to the LEFT. What is the group velocity of this packet if the time is measured in units of 10^(-14) s?

4.4.2 This is a zoomed portion of the same packet used in 4.4.1, but in which the displacement of three successive atoms is shown.

A: What is the phase velocity of this packet?

B: Which (by color) of the three atoms is on the left and which is on the right if the packet is moving to the LEFT?