The athletic apparel company lucy worked with Boston’s Department of Conservation and Recreation to install the 10,000 motion-sensitive LED lights that you can now see on a quarter-mile stretch of the already-scenic Esplanade. The “light forest,’’ which is near the Hatch Shell and the Arthur Fiedler statue, is becoming quite a spectacle, especially because of the free exercise classes that are offered on the path twice a day. (Classes will be held at 6 a.m. and 6 p.m. this week, and at 9 a.m. and 4 p.m. this weekend). The lights will be up through Oct. 13. They went live late last week with a celebration that drew names such as Channel 7’s Kayna Whitworth, and lucy activewear execs Laurie Etheridge and Dawn Dzedzy.