Carrie and Louise, two observers in a spaceship, view a light source placed close to Carrie. When the spaceship is travelling at a constant velocity, they both measure the frequency of the light source and obtain identical values.

The magnitude of the velocity of the spaceship increases.

State and explain any changes to the frequency of the light source, as measured by Louise, that occur during the acceleration.

speed of light is independent of speed of source;
so (because of acceleration) source appears to move away from Louise / OWTTE;
Doppler effect predicts a red-shift;
Louise measures a lower frequency;

or

accelerating spaceship is equivalent to being at rest in a gravitational field / OWTTE;
photons leaving the source therefore gain potential energy and lose kinetic energy;
since E=hf;
Louise will measure a lower frequency;