Place the plastic toy in the bathtub with enough water to cover the toy but not deeper than the height of the cup. Does the toy float or sink?
Look at the toy. Is it hard to see underwater?
Make waves in the water. How hard is it to see the toy now?
Stick the bottom of the cup into the water over the toy on the bottom of the tub, being careful not to get water inside the cup. How well can you see the toy now? Does the cup make a difference?
Put water in the cup and again place it over the toy in the bottom of the tub. Does the water affect how well you see the toy?
Light bends as it travels between substances of different densities. Since water is more dense than air, the light bends (or refracts) where the air and water meet, making it harder to see the objects below the surface.
The angle that the light hits the surface of the water affects how it bends or refracts. If the surface is moving, then the light is scattered and the objects under the water appear distorted.
The cup "calms" the water and provides a smooth surface for the light to travel through; thus creating a clear image of the object underwater. The air in the cup also prevents the light from refracting. Adding water to the cup increases the refracting and makes the toy harder to see.