Final answer:
A phase change in reflected light occurs when light reflects from a medium of higher refractive index to a lower one.
Thus, upon reflection, a phase change will happen at the top surface of the glass cover placed over a water drop on a microscope slide and at the glass slide below the water drop.
Step-by-step explanation:
When discussing the phase change of reflected light on a microscope slide, we consider the optical properties of the materials and the transition of light between them.
Generally, when light reflects off a boundary going from a medium of lower refractive index to a higher refractive index, there is a phase change of 180 degrees (or π radians). Conversely, when light reflects from a medium of higher refractive index to a lower one, there's no phase change.
In the case of a glass cover on a water drop on a slide, light reflects off the top and bottom surfaces of the glass cover and from the slide beneath the water drop.
Considering that the refractive index of glass is higher than that of air and water is typically less than that of glass, a phase change would occur at the top surface of the glass cover when light reflects off it into the air (a transition from higher to lower refractive index).
Additionally, a phase change will happen when light reflecting from the glass slide enters the water (again, a transition from higher to lower refractive index).
Therefore, the answer to the question is that a phase change will occur at both the top surface of the glass cover and at the glass slide below the water drop.