Suppose you have two polarized filters at 90 degrees to each other. These block out all the light so you could say that this setup is a form of measurement, si

Suppose you have two polarized filters at 90 degrees to each other. These block out all the light so you could say that this setup is a form of measurement, since it provides knowledge that zero light is passing through. In classical mechanics, adding a 3rd filter between the two would have no effect since already no light is getting through. However with quantum putting a 3rd filter between your two original filters at 45 degrees will allow half of the photons to pass through all 3 filters. This is where things get strange and measurement is not very well defined. Are any of the 3 filters now performing a measurement? We know that half the photons will pass through but have no way of determining which will and which won't. This experiment works with polarized sunglasses btw, quantum in the palm of your hands.