小站备考
托福
托福阅读
Official38阅读真题

托福official38阅读第1篇Microscopes题目解析

展开
Microscopes
Tip:单击查看句义;划选/双击查生词
Before microscopes were first used in the seventeenth century, no one knew that living organisms were composed of cells. The first microscopes were light microscopes, which work by passing visible light through a specimen. Glass lenses in the microscope bend the light to magnify the image of the specimen and project the image into the viewer's eye or onto photographic film. Light microscopes can magnify objects up to 1,000 times without causing blurriness.

Magnification, the increase in the apparent size of an object, is one important factor in microscopy. Also important is resolving power, a measure of the clarity of an image. Resolving power is the ability of an optical instrument to show two objects as separate. For example, what looks to the unaided eye like a single star in the sky may be resolved as two stars with the help of a telescope. Any optical device is limited by its resolving power. The light microscope cannot resolve detail finer than 0.2 micrometers, about the size of the smallest bacterium; consequently, no matter how many times its image of such a bacterium is magnified, the light microscope cannot show the details of the cell's internal structure.

From the year 1665, when English microscopist Robert Hooke discovered cells, until the middle of the twentieth century, biologists had only light microscopes for viewing cells. But they discovered a great deal, including the cells composing animal and plant tissues, microscopic organisms, and some of the structures within cells. By the mid-1800s, these discoveries led to the cell theory, which states that all living things are composed of cells and that all cells come from other cells.

Our knowledge of cell structure took a giant leap forward as biologists began using the electron microscope in the 1950s. Instead of light, the electron microscope uses a beam of electrons and has a much higher resolving power than the light microscope. In fact, the most powerful modern electron microscopes can distinguish objects as small as 0.2 nanometers, a thousandfold improvement over the light microscope. The period at the end of this sentence is about a million times bigger than an object 0.2 nanometers in diameter, which is the size of a large atom. Only under special conditions can electron microscopes detect individual atoms. However, cells, cellular organelles, and even molecules like DNA and protein are much larger than single atoms.

Biologists use the scanning electron microscope to study the detailed architecture of cell surfaces. It uses an electron beam to scan the surface of a cell or group of cells that have been coated with metal. The metal stops the beam from going through the cells. When the metal is hit by the beam, it emits electrons. The electrons are focused to form an image of the outside of the cells. The scanning electron microscope produces images that look three-dimensional.

The transmission electron microscope, on the other hands, is used to study the details of internal cell structure. Specimens are cut into extremely thin sections, and the transmission electron microscope aims an electron beam through a section, just as a light microscope aims a beam of light through a specimen. However, instead of lenses made of glass, the transmission electron microscope uses electromagnets as lenses, as do all electron microscopes. The electromagnets bend the electron beam to magnify and focus an image onto a viewing screen or photographic film.

Electron microscopes have truly revolutionized the study of cells and cell organelles. Nonetheless, they have not replaced the light microscope. One problem with electron microscopes is that they cannot be used to study living specimens because the specimen must be held in a vacuum chamber; that is, all the air and liquid must be removed. For a biologist studying a living process, such as the whirling movement of a bacterium, a light microscope equipped with a video camera might be better than either a scanning electron microscope or a transmission electron microscope. Thus, the light microscope remains a useful tool, especially for studying living cells. The size of a cell often determines the type of microscope a biologist uses to study it.

1.According to paragraph 1, what happens to the light when a specimen is being viewed with a light microscope?

你的答案:
正确答案:B
题目解析:
 后才能查看题目解析,还没有账号? 马上注册
【题目翻译】根据第1段,当用光学显微镜观察样品时,光会发生什么变化? A:光线继续保持不变,直接进入观众的眼睛或胶片。 B:一个玻璃透镜弯曲光线,形成一个放大的样品图像。 C:光线投射到胶片上,产生模糊的图像。 D:光的强度增加了一千倍。 【判定题型】:题目问的是文章中的具体细节信息,故根据题目问法可以判断本题为事实信息题。 【关键词定位】本题根据the light,specimen和a light microscope定位到第一段这几句:The first microscopes were light microscopes, which work by passing visible light through a specimen. Glass lenses in the microscope bend the light to magnify the image of the specimen and project the image into the viewer's eye or onto photographic film.意思是“最早的显微镜是光学显微镜,它的工作原理是将可见光通过样品。显微镜中的玻璃透镜弯曲光线以放大样品的图像,并将图像投射到观察者的眼睛或摄影胶片上”。 【逻辑分析】文章讲了光学显微镜的原理,就是折射光以放大标本的图像,并且把图像投射到观察者的眼睛里或者投射到胶卷上。 【选项分析】 A.选项A说光不需要折射直接进入人的眼睛,错误; B.选项B符合文章,正确。 C.选项C说投射到胶卷上的光形成模糊的影像,从后文我们知道不一定是模糊的,错误; D.选项D说光的强度增加了一千倍,无中生有,错误。

学习页面

Medi

terr

anean

加强 + 政府 + 名词后缀

加强的政府——管理

原文例句

加入生词

本文生词 0

色块区域是你收藏过的生词;

查询次数越多,颜色越深哦~

显示文中生词

登录后才能收藏生词哦,现在登录注册>

本文重点词 45

文中加粗单词为本文重点词;

根据词频与核心词范围精心挑选,托福考试必掌握词汇。

显示文中重点词
学习本文词汇

文中划选/双击的生词、加粗重点词已收纳至词盒

可随时点击词盒查看哦~

只有在词句精学模式下才能开启词盒功能哦~

我知道了

词盒
收藏
笔记
我的笔记
5000
保存
反馈