Trắc nghiệm phần đọc đề kính hiển Vi [151_TEST 29_29-38]

Each advance in microscopic technique has provided scientists with new perspectives

on the function of living organisms and the nature of matter itself. The invention of the

visible-light microscope late in the sixteenth century introduced a previously unknown

Line    realm of single-celled plants and animals. In the twentieth century, electron microscopes

(5)      have provided direct views of viruses and minuscule surface structures. Now another

type of microscope, one that utilize x-rays rather than light or electrons, offers a

different way of examining tiny details, it should extend human perception still farther

into the natural world.

The dream of building an x-ray microscope dates to 1895, its development, however,

(10)    was virtually halted in the 1940’s because the development of the electron microscope

was progressing rapidly. During the 1940’s electron microscopes routinely achieved

resolution better than that possible with a visible-light microscope, while the

performance of x-ray microscopes resisted improvement. In recent years, however,

interest in x-ray microscopes has revived, largely because of advances such as the

(15)    development of new sources of x-ray illumination. As a result, the brightness available

today is millions of times that of x-ray tubes, which, for most of the century, were the

only available sources of soft x-rays.

The new x-ray microscopes considerably improve on the resolution provided by optical

microscopes. They can also be used to map the distribution of certain chemical elements.

(20)    Some can form pictures in extremely short times, others hold the promise of special

capabilities such as three dimensional imaging. Unlike conventional electron microscopy,

x-ray microscopy enables specimens to be kept in air and in water, which means that

biological samples can be studied under conditions similar to their natural state. The

illumination used, so-called soft x-rays in the wavelength range of twenty to forty

(25)    angstroms (an angstrom is one ten-billionth of a meter), is also sufficiently penetrating

to image intact biological cells in many cases. Because of the wavelength of the x-rays

used, soft x-ray microscopes will never match the highest resolution possible with

electron microscopes. Rather, their special properties will make possible investigations

that will complement those performed with light- and electron-based instruments.

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