Explain with a sketch: a.Why the bottom of a glass filled with water appears closer?
Refraction is the word.
See image I created in FCAD.
Consider a ray of light from the bottom of the glass at point X travelling up to the surface of the water.
When it emerges from the water, it traverses a different medium-- air-- whose density is far lower than that of water.
Whenever light travels through media of different densities it bends at that interface of the mediums.
So in the above case light leaving water bends.
When viewd from point of observation A, light travels in a straight line, if you extend AY in a straint line -- AYX'; the apparent position where it appears to have originated is point X', which is closer to the top than the original point X.