'The bottom of a pond appears raised.' due to refraction phenomenon.
Explanation:
Whenever light travel through different medium it bends its path. so when the light travels into pond i.e air(rarer) to water (denser) it bends its path creating an illusion.
The bending of this light ( away the normal) and again towards the normal after reflecting from bottom of pond creates an vision as if the bottom of pond is raised a little.
This phenomenon is called as the refraction phenomenon.
