Colin's conundrum
Regular commentator Colin Cuthbert from Toulouse International school sent me a photo of an unusual phenomena. You take a polariser hold it in front of your computer screen and rotate it until the planes of polarisation are at 90° so no light passes. Then introduce a second polariser between the screen and polariser, rotate it and at 45° you see light. It seems strange that the introduced polariser has rotated the plane of polarisation. What has happened is the polariser introduced at 45° to the plane of polarisation allows through the component of light in the direction of its plane of polarisation according tro Malus' law this will have intensity Icos2θ where I is the original screen intensity. The original polariser is now at 45° to the introduced on so will also let through the component in line with its plane. The final intensity should be Icos245co245= I/2. When comparing the brightnesss of the screen with both polarisers and then withj the original one taken away, there doesn't seem to be much difference, I guess my eye can't detect the difference between 0.7I and 0.5I. Always easier to see with a diagram. That reminds me of a teacher who used to teach down the corridor from me, who thought that explanations become clearer if repeated louder.
And another thing; I've just discovered why the light from my phone doesn't appear to be polarised. To enable people to use their phone at all angles whilst wearing polaroid sunglasses, new phone displays are covered by a film that changes the plane polarised light into circularly polarised which will pass through the sunglasses at all angles.
