Which instrument would you use to observe refraction effectively?

Using a microscope is the best choice for observing refraction effectively. A microscope is designed to magnify small objects, and as light passes through different materials, it bends or refracts at different angles. This bending of light can be clearly observed when looking at specimens under a microscope, especially transparent materials like glass slides with liquid samples.

In contrast, the other tools listed serve different purposes. A ruler is primarily for measuring lengths; a compass is used for navigation or drawing circles, and a scale measures weight. None of these instruments are designed to manipulate or observe the behavior of light in the way that a microscope does when examining the phenomenon of refraction.