If you are writing or studying, it makes very much difference where the light comes from. People who use books and pens every day have to be especially careful about the way the light shines on their work.
Every house gets its light either from daylight through the windows--which is the best to use--or from lamps or electricity; but whichever kind of light it is, the way it shines toward our book or work is a matter of great importance to the eyes.
Take a book, sit with your back toward the window, and try to read. Your shadow(影子) falls all over the page and makes it almost as bad for your eyes as if you were in a dark room. Now turn around and face the window. The page is in the shadow again, while the bright light is in your eyes.
Try sitting with your right side toward the window. This is very well for reading, but if you were writing, the shadow of your hand would fall across the page and bother(打搅) you a little.
There is just one other way:sit with your left side to the window. Now everything is perfect for reading and for writing, too.
Whatever kind of light is in the room, the rule about the right to sit is always the same.
When you sit with your face towards the window, .
考情点拨:事实细节题。应试指导:从第四段的内容可知，当你面朝窗户时，书页又处在了影子中。
把10gMgCl2和MgSO4的混合物溶于适量水中，滴加2mol／L的BaCl2溶液至沉淀完全，共消耗BaCl2溶液25mL。求混合物中MgCl2和MgSO4的质量各是多少克
固定在匀强磁场中的正方形导线框abcd，各边长为1．其中ab是一段电阻为R的均匀电阻丝，其余3边均为电阻可忽略的铜线。磁场的磁感应强度为B，方向垂直纸面向里。现有一与ab段的材料、粗细、长度都相同的电阻丝PQ架在导体框上，如图所示。PQ以恒定速度方向如何
一小物块以速度v0=10m／s沿光滑地面滑行，然后沿光滑曲面上升到顶部水平的高台上，并由高台上飞出，如图所示。问当高台的高度h多大时，小物块飞行的水平距离最大这个距离是多少(g=10m／s2)
某晶体盐的水溶液呈浅绿色，在此溶液中通入氯气，溶液变为棕黄色，把所得棕黄色溶液分成两份：一份加入KSCN溶液，溶液变深红色；另一份加入BaCl2溶液，产生不溶于稀HNO3的白色沉淀，则该晶体盐的化学式是_______。
欲除去乙烷中的乙烯，可将混合气体通过盛有_______的洗气瓶；欲除去乙炔中的硫化氢，可将混合气体通过盛有_______的洗气瓶。
浓硝酸常盛装在棕色瓶中，且贮放在冷暗处，原因是_______，
碳酸氢钠溶液和烧碱溶液反应的离子方程式为_______。
常温下，氨水中存在的离子有_______。
在酸或碱存在的条件下，甲酸甲酯跟水发生水解反应的化学方程式为_______。
有下列几种气体：H2、O2、HCl、NO、NH3、CH4，其中通常情况下能用来和水做“喷泉”实验的是_______。
