Book 9 in which Millie goes inside a red laser beam is about the basic principles of lasers.

Millie watches how electrons and red photons cooperate to generate a laser beam. At first, she is intrigued by the process of optical pumping. Later, she realises its utility, when she understands the difference between light from a common bulb and light from a laser.

In this adventure, Millie revisits the concept of absorption, emission and energy levels introduced in Book 3, but here she sees their practical application.