There are two polarize-rs that are inclined two each other at an angle of 90 degree, later one more polarizer is placed (in between the two polarser) bisecting the angle in which the other two polarisers are inclined. If an un-polarised light (of intensity I ) is passed through the first polariser then what is the intensity of the light coming out of the third polariser. (all the polarisers can completely polarise the light)

When the light will pass through the first polariser it will be completely polarized.The second polariser being at an angle of 45 will not allow any light to pass . Now the third polariser being perpendicular to the first polariser and at 45 to the second polariser it(the third polariser)does not allow any light to pass through because all the light is now blocked by the second polarizer and thus the third polariser creates a kind of barrier. Therefore the intensity of light coming out of the third polariser is zero. You should verify this answer but I think it is up to the mark.