The weather is clear and the river is crystal clear. We can see the fish in the river swimming in the white clouds. The cloud we see is actually a _ _ _ _ _ _ cloud formed by _ _ _ _.

(1) Light will be reflected when it meets the water surface, which is equivalent to a flat mirror, and the appearance of white clouds on the water surface through the water surface is a virtual image;

(2) When the light reflected by the fish enters the air from the water, it is refracted on the water surface, and the refraction angle is greater than the incident angle. When the refracted light enters the human eye, the human eye will look against the direction of the refracted light and see the virtual image of the fish above the actual position of the fish.

So the answer is: the reflection of light; Virtual; Refraction; Go up.