# Physics Exercises/Electromagnetic Waves

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1. Consider an oil film on water. If you see green light (532 nm) reflecting back when sunlight is normally incident, what is the minimum thickness of the film?
2. An astronaut on board the International Space Station is giving a presentation using his 5 milliwatt green laser pointer (at 532 nm). The pointer projects a bright dot of 1 millimeter diameter on a screen. During the presentation he accidentally shines his laser pointer out the window toward Earth. The Space Station is in the Earth's shadow at this moment and a person on Earth happens to be in the beam. If the Space Station is at an angle ${\displaystyle \alpha }$ above the horizon, how bright will it appear to this person? Express the brightness in terms of intensity (power per area) and assume that the collimation of laser pointer is diffraction-limited (in reality, collimation of laser pointer beam is not as good as that).