Final answer:
The clear plastic or glass cover on a microscope slide ensures a flat surface for sample viewing and prevents evaporation. Phase changes in reflected light occur at interfaces where the refractive index changes, such as at the bottom surface of the glass cover that contacts water. If the medium is carbon disulfide between crown glass, the phase change depends on the refractive indices involved.
Step-by-step explanation:
In the context of setting up a microscope slide, the clear plastic covering or glass cover that is placed over the water drop on the glass slide serves to create a uniform, flat surface for viewing the sample and to prevent evaporation of the water. The phase change in reflected light is likely to occur at interfaces where the refractive index changes specifically when light reflects from a medium of a lower refractive index to a medium of a higher refractive index, resulting in a half-wavelength phase shift.
For example, when light reflects off the top surface of the glass cover, which is in contact with air (a medium of lower refractive index), no phase change occurs. However, at the bottom surface of the glass cover, which is in contact with water (a medium of higher refractive index compared to air), a phase change will occur. The same applies to the glass slide interface below the water drop. If carbon disulfide were placed between two pieces of crown glass, the phase change upon reflection would depend on the comparative refractive indices of the carbon disulfide and the crown glass.