Answer to 5.4.2

The maximum frequency for the blue sample is about sqrt(2) smaller than for the red. The mass of the blue atoms is twice that of the red.

The area under the blue curve is roughly twice that under the red. The area under the density of modes must be 3 times the number of atoms per unit volume. To increase the number of modes by times 2, requires a decrease in lattice constant by a factor of 1/(cube root of 2) or about 1/1.25. The actual ratio was 1/1.18.