First, I want to say sorry for my english, cause I do not live in an english-speaking country. 1. The problem statement, all variables and given/known data Here the problem definition: There is a person which has a body height of 1.7m and is standing in front of a cylindrical aquarium, which is 4 meters high. It is filled with water to 3.9m and on the top of it there is a grid. Now the person is standing directly in front of the cylindrical aquarium and as soon as the person would crouche she would be unable to see the grid. Now, one should calculate the diameter of the aquarium and it is said that the glass of the aquarium can be neglected. I included a picture which illustrates the situation. But there is just one person standing directly in front of the windows. 2. Relevant equations I guess: Snell's law//maybe Fresnel equations?! 3. The attempt at a solution But I am uncertain about the question, in which angle does the person look up in order to see the grid, because if the person looks up in a 90° angle (I assume that the person will have to look up in some way, even though it is not mentioned in the problem definition), I am not sure whether this person would see something at all, because of total reflection, of the light beam passing from the water in the air. Maybe you could tell me your ideas(I am not expecting a complete solution).