Draw a cylinder, with diameter D and length/height infinity, mark the top point Y and the bottom point Z Now Say a person falls into this cylinder/hole. (there are equal sources of light emitting into each end of the tube) From the falling person's perspective the point Z looks like an infinitesimal dot of light that the human eye cannot distinguish. At what height/length in the cylinder/hole does the person view the same thing from the point Y? What I want to know is how far does the person have to fall for the person can only see a dot of light at entrance Y. This is not some problem someone has given me, this is a genuine problem That I came up with. Also if there are any conditions that I am missing please take your own and If you have an answer for this please post what values you took for those parameters. Thank you.