Many of the active Internet users admit that sitting at the computer takes a lot of time. Women are especially prone to this. They often sit on various forums, make virtual friends, create online chats with them to communicate. It is at a distance that it is possible to establish trusting relationships, which are so often not enough for people in the surrounding reality. In communication with anonymous interlocutors, it is easier for a person to open his soul. This is similar to how people used to find people to talk to on long-distance trains. But this is the age of technology, so it's easier to find people to talk to on the Internet. Yet over-reliance on such communication harms your personal life. Getting rid of computer addiction is like getting rid of other bad habits, but it has more in common with getting rid of overeating. You can spend a little time at the computer, but excess here will hurt.
Заменим в формуле тонкой линзы: 1/F = 1/d + 1/f, где F – фокусное расстояние линзы, d – расстояние от предмета до линзы, f – расстояние от изображения до линзы, полученного с этой линзы, 1/F на D, так как оптическая сила линзы D = 1/F. Заменим также f на Г ∙ d, так как увеличение линзы Г = f /d. Получим, 1/d + 1/(Г ∙ d) = D, тогда D = (Г + 1)/(Г ∙ d). Из условия задачи известно, что собирающая линза дает увеличенное в Г = 2,5 раза, действительное изображение предмета, причём расстояние от предмета до линзы d = 70 см = 0,7 м. Подставим значения физических величин в формулу и произведем расчёты, чтобы найти оптическую силу линзы:
D = (2,5 + 1)/(2,5 ∙ 0,7);
D = 2 дптр.