Final answer:
A phase change upon reflection will occur at both the top and bottom surfaces of crown glass when the fluid between them is carbon disulfide, which has a higher refractive index than crown glass, consistent with Fresnel's equations.
Step-by-step explanation:
When considering the interaction between light and different media, a phase change upon reflection can occur depending on the refractive indices of the materials involved. If the refractive index of carbon disulfide (the fluid between the two pieces of crown glass) is higher than the crown glass, then there will be a phase change of 180 degrees upon reflection at both interfaces where light enters from a less optically dense medium (crown glass) to a more optically dense medium (carbon disulfide). However, if the refractive index of carbon disulfide is lower than that of crown glass, such a phase change will only occur at the top surface where the less optically dense carbon disulfide is in contact with the air, a medium of even lower optical density.
Therefore, without specific values for the refractive indices at hand, the answer would typically be either 'a. Phase change at both glass surfaces' if the refractive index of carbon disulfide is higher than that of the crown glass or 'd. No phase change at either glass surface' if the carbon disulfide has a lower refractive index. However, in practice, carbon disulfide has a refractive index that is higher than crown glass, so the correct answer is most likely 'a. Phase change at both glass surfaces'. This is an application of Fresnel's equations, which describe the way light behaves at the boundary between two different media.