You need to be careful with this though. I don't fully understand the reasons, but you have to consider bending of the light. See the wiki: https://en.m.wikipe

You need to be careful with this though. I don't fully understand the reasons, but you have to consider bending of the light. See the wiki: https://en.m.wikipedia.org/wiki/Bedford_Level_experiment https://en.m.wikipedia.org/wiki/Bedford_Level_experiment I haven't done the experiment, but this brings it back to trusting scientists (light bends just above the water, but doesn't 4m above the water). I would need to do additional experiments to understand the refraction.