Final answer:
The reflected light from a puddle of water back into air does not undergo a phase shift, but a light reflecting from water to a more dense medium, like glass, will undergo a 180° phase change.
Step-by-step explanation:
When light in air reflects off the surface of a puddle of water, the reflected light undergoes a phase shift. This occurs because light is transitioning from a medium of lower index of refraction (air) to a medium of higher index of refraction (water). A 180° phase change (or a λ/2 shift) takes place when light reflects from a less dense medium to a more dense medium. Therefore, any light reflecting off the top surface of the water, as it reflects from water back to air, does not undergo a phase change. However, any light reflecting from the bottom of the water, where it interfaces with an underlying medium such as glass which typically has an index of refraction greater than that of water, will experience this phase shift.