Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: When light is refracted its wavelength and speed change; however, its frequency remains the same as the frequency of the light source. Proof of this is that light retains its original color under water. For example, blue light is still blue under water. Answer: frequency
Light retains its original color under water because what remains the same when light is refracted?