Final answer:
The difference in paths taken by two originally in-phase light waves determines whether they interfere constructively or destructively. Reflection causes a phase change of 180°, affecting interference. Refraction changes the wavelength inside a medium, leading to a different phase.
Step-by-step explanation:
The difference in paths taken by two originally in-phase light waves is directly related to whether they interfere constructively or destructively. When the path length difference between the two waves is an integral multiple of the wavelength (λ), constructive interference occurs. On the other hand, when the path length difference is a half-integral multiple of the wavelength, destructive interference happens.
Reflection can affect the interference by causing a phase change of 180°. This means that when a wave is reflected, its phase is shifted by half a wavelength. Refraction, on the other hand, can also affect interference by changing the wavelength inside a medium. When a light wave enters a different medium, its wavelength changes, which can result in a different phase compared to another wave that has traveled a different distance in a different medium.