Question

   You pick up your older mobile phone to see who just sent you a text, but its dark ! Your sunglasses are blocking all pixels, all colors from your phone.

a. Aha, with a reminder from the textbook or the interwebs, you know what polarization the glare has, so you also know what polarization the light from your phone has. Explain.

b. You rotate your phone from its normal orientation by 10 degrees, then 30 degrees, then 45, then to 90 where you see its full intensity and successfully read your text. Calculate what fraction of full intensity (or equivalently what fraction of full power) makes it through your sunglasses in each case ?

c. Some energy did not make it to your eye. In the case of sunglasses, can you guess where does the energy go?

Answer 1

a. Answere . The Sun glasses are vertically aligned polarides, because light coming from horizontal reflecting surface are horizontally aligned. Now watching at the phone with sun glasses, and it is completely black it means the axis of both , the polarization of light and the polaride of the glasses are perpendicular. As glasses polarides are vertical axis then light from the mobile phone display is horizontally polarized.

b. Answer . Here the Malus' law is used. The intensity that is being transmitted from a polarizer is equal to the cosine square of the angle from the axis of polarized light to polaride times intrinsic intensity. That is

I = I₀cos² , where is angle from the axis of the polarized light to polarizer. So rotating 10 degrees the angle between axes are 80 degrees and intensity would be I = I₀ cos² (80) = 0.0301 I₀ . Similar for rotation of 30, 45 and 90 degrees will have angle between axes are 60, 45 and 0 degrees respectively. And intensity would be 0.25 I₀ , 0.5 I₀ and I₀  respectively. In 90 degree case we will have full intensity.

c. Answer. The energy which was not able to cross the sunglasses is used in heating up the molecules of the polaride.