QuestionUnpolarised light of intensity 80 W m-2 is incident on a polarising filter P. The light then passes through a second polarising filter A, whose transmission axis is initially parallel to that of P. Assume the filters are ideal.(a) State the intensity of the light between P and A. (1)
(b) Filter A is slowly rotated through 360°. Describe how the intensity of the light leaving A changes. (3)
(c) Filter P is removed and filter A is rotated again. Describe and explain what is observed. (2)Answer(a) The polariser absorbs half of the unpolarised light, so the intensity is 80 ÷ 2 = 40 W m-2. (1)(b) The intensity starts at a maximum of 40 W m-2. (1) It falls to zero when A has turned through 90°, because the filters are then crossed. (1) It rises to a maximum again at 180°, is zero again at 270° and returns to maximum at 360°, so there are two maxima and two minima in one full turn. (1)(c) The intensity stays constant, at 40 W m-2, as A is rotated. (1) The light reaching A is now unpolarised, with oscillations in all planes, so A always transmits the same fraction (half) whatever its angle. (1)Examiner tips- Give the angles at which the maxima and minima occur, not just "it gets brighter and dimmer".
- A minimum for crossed ideal filters means zero intensity.
- Part (c) is a common test: if the brightness changes when a single filter is rotated, the light reaching it must already have been polarised.
(b) Filter A is slowly rotated through 360°. Describe how the intensity of the light leaving A changes. (3)
(c) Filter P is removed and filter A is rotated again. Describe and explain what is observed. (2)Answer(a) The polariser absorbs half of the unpolarised light, so the intensity is 80 ÷ 2 = 40 W m-2. (1)(b) The intensity starts at a maximum of 40 W m-2. (1) It falls to zero when A has turned through 90°, because the filters are then crossed. (1) It rises to a maximum again at 180°, is zero again at 270° and returns to maximum at 360°, so there are two maxima and two minima in one full turn. (1)(c) The intensity stays constant, at 40 W m-2, as A is rotated. (1) The light reaching A is now unpolarised, with oscillations in all planes, so A always transmits the same fraction (half) whatever its angle. (1)Examiner tips- Give the angles at which the maxima and minima occur, not just "it gets brighter and dimmer".
- A minimum for crossed ideal filters means zero intensity.
- Part (c) is a common test: if the brightness changes when a single filter is rotated, the light reaching it must already have been polarised.