(a) Why is light from the blue sky polarized when viewed at 90° to the Sun? (b) A Polaroid sunglass lens transmits 40% of incident unpolarized sunlight. If the sunlight's intensity is 1000 W/m², what intensity reaches the eye, and what is its polarization state? Solution: (a) Sunlight's electric field vibrates in all transverse directions. Air molecules re-radiate as tiny antennas, which cannot emit along their vibration axis. For an observer at 90° to the Sun's direction, only vibrations perpendicular to the Sun–observer plane can radiate toward the observer — the other component is forbidden — so the scattered skylight is plane-polarized. (b) Ideal Polaroid would transmit 50% (500 W/m²); a real one transmits 40%, so 400 W/m² reaches the eye, plane-polarized along the lens's transmission axis. The absorbed 60% becomes tiny amounts of heat in the sheet.