As starlight enters the earth's atmosphere, it undergoes refraction
Question
As starlight enters the earth's atmosphere, it undergoes refraction
only once, at a single sharp boundary
continuously, since the atmosphere has a gradually changing refractive index
not at all
only after sunset
Answer
continuously, since the atmosphere has a gradually changing refractive index
Detailed Explanation
The chapter states that starlight undergoes continuous refraction because the atmospheric refraction occurs in a medium of gradually changing refractive index.