Detailed Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Q: Passage: Green light has a higher frequency than microwaves. It is possible to calculate it, but since the speed of an electromagnetic wave is constant we know that waves with higher wavelengths must have a lower frequency based on the wave equation. Answer: frequency
A:
What of a electromagnetic wave can be calculated using the wave equation due to their speed always being constant?