题干

连接凸透镜或凹透镜上两个球面球心的直线叫做____.简称____.每个透镜的主轴上都有一个特殊的点:凡是通过该点的光,其传播方向____,这个点叫____,用字母____表示.
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主光轴,主轴,不改变,光心,O

同类题5

   Almost every day, we discuss the topic of health, especially for kids. But what is health? “Health” means eating well, getting enough exercise, and having a healthy weight. Let’s read the following rules. They can help you stay healthy.
   1. Eat a variety of food, especially fruit and vegetables. We all know that eating fruit and vegetables can help us stay healthy, but many of us only eat our favorite food. Remember that we can only get the nutrition (营养) we need by eating different kinds of food, especially fruit and vegetables.
   2. Drink water and milk most often. Everyone knows that water is important. Besides that, kids need plenty of calcium (钙) to grow strong bones, and milk is rich in it. Every day, you should drink at least three cups of milk, when you are 9 years old or older. You should also try to have less sugary drinks, like soda and coca. They include a lot of sugar. Sugar only includes calories (热量), not important nutrition.
   3. Listen to your body. When you are eating, notice how your body feels. When your stomach feels comfortably full, stop eating. Eating too much makes you feel uncomfortable. If you do it too often, it can make you unhealthy and fat.
   4. Limit screen time. What's screen time? It's the amount of time you spend watching TV, movies and playing computer games. The more time you spend on these sitting-down activities, the less time you spend playing sports, like basketball, and doing other activities like riding and swimming. Try to spend no more than 2 hours a day on screen time.