题目

Recently, one of my best friends, whom I’ve shared just about everything with since the first day of kindergarten, spent the weekend with me. Since I moved to a new town several years ago, we’ve both always looked forward to the few times a year when we can see each other. Over the weekend, we spent hours and hours, staying up late into the night, talking about the people she was hanging around with. She started telling me stories about her new boyfriend, about how he experimented with drugs and was into other self-destructive behavior. I was blown away! She told me how she had been lying一束宽度为a的平行光束沿透镜主轴入射到凸透镜上，如图所示．在透镜另一侧垂直主轴放一光屏．当光屏距透镜________ cm远时，屏上可得到宽度为a/2的光斑，屏在距透镜________处时，屏上可得到宽度为2a的光斑．(已知透镜焦距10 cm)