For years I have been mercilessly lampooned by friends and acquaintances alike for my unorthodox lifestyle choice of having no TV. In an age of increasingly large plasma flat-screens and surround sound, digital home entertainment systems which accost you the minute you walk into someone’s house, people regularly look at me like I’m either severely handicapped or chronically hard done by when I mention that I have no television. I can see the mixture of genuine pity, raw pathos and sheer disbelief in their faces as they stare at me open-mouthed. To be sure, television is a great invention, if handled in moderation. The composite etymological derivation (from the Greek and the Latin words – literally meaning “to see from afar”) tells of a tremendous technological feat which certainly deserves to be applauded. What’s more, if one is discerning, it can be the source of some quality entertainment, instruction and enjoyment. Some of the nature documentaries and arts programmes on BBC 2 are truly fantastic and are well worth the license fee alone. But the sad reality is that young people are rarely discerning and, by dint of poor time management skills, often end up wasting an inordinate amount of precious, never-returning time watching trash, their brains wallowing in a trough of mental lethargy.
．这列波的传播速度是7.5m／s
m，不考虑光的干涉和衍射，试问：
，请你求出圆形亮区的半径。