Question

How can a single Polaroid filter be used to show that the sky is partially polarized? (Interestingly enough, unlike humans, bees and many insects can discern polarized light, and they use this ability for navigation.)

Answer 1

When we look up into the sky and put the filter between our eye and the sky, and then rotate the filter (along the axis from our eye to what we are looking at). If the amount of light changes, then the sky must be polarized. Only light oriented along the polarization axis of the filter will pass. So if the sky is polarized, there will be one filter orientation at which that light passes and another orientation (90 degrees to the first) at which it does not pass. If the sky is not linearly polarized, then at any axis, exactly half of the light will pass and half will not, making no difference for any filter orientation. If the sky is perfectly polarized, then at one orientation no light will pass, and at another all the light will pass. If the sky is partially linearly polarized, then some light will always pass, but one orientation will allow more light. That is what we will see when a single polaroid filter was used.