TITLE: What is the best angle to see water
QUESTION [4 upvotes]: Sometimes when I clean my house, I miss some of the water since it seems invisible, but when I stand in different angle from the water I can see it clearly, whereas some angles offer partial transparency. 
My question is from which angle is it best to observe the water? (I suppose it has to do with the angle of the sun, but what if I my light comes mainly from light bulbs?)

REPLY [2 votes]: Summary: The best way to see the water is to try and look for the reflection of a light source (sun or light bulb) at a large angle of incidence.  
More Details: You see the water on your floor because it reflects light differently than the dry surface of the floor.  Namely, the water forms a smooth enough surface that the light reflected off of it can be imaged by your eye.  So, when you 'see' the water, you are seeing a specular reflection from some light source as compared to the diffuse reflection from the dry floor.  
In addition, the reflectivity of the water increases with increasing angle of incidence which is captured in the figure below.  If you stand directly above the water and the light bulb that you are trying to see is directly over your head (with a small offset), then the reflectivity of the water is very low at $\sim2\ \%$, and you may not notice the water.  If you try to use a light at the side of the room so that the angle of incidence is large, then the water's reflectivity goes up significantly, and it is much easier to see.  

Public domain image taken from http://en.wikipedia.org/wiki/File:Water_reflectivity.jpg
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