Материал: Андрианова English Thesaurus of Student-Physicist 2013

The simple magnifier

The simple magnifier is one of the most basic of all optical instruments because it consists only of a single converging lens. As the name implies, this device is used to increase the apparent size of an object. Clearly, the size of the image formed at the retina depends on the angle subtended by the object at the eye. As the object moves closer to the eye, angle increases and a larger image is observed. The lens allows the object to be viewed closer to the eye than is possible without the lens.

The compound microscope

Greater magnification can be achieved by combining two lenses in a device called a compound microscope. The instrument consists of two lenses: an objective with a very short focal length (< 1 cm), and an ocular lens, or eyepiece, with a focal length of a few centimeters. The basic approach used to analyze the image formation properties of a microscope is that of two lenses in a row: the image formed by the first becomes the object for the second.

The microscope has extended our vision into the previously unknown realm of incredibly small objects, and the capabilities of this instrument have increased steadily with improved techniques in precision grinding of lenses. A natural question is whether there is any limit to how powerful a microscope could be. For example, could a microscope be made powerful enough to allow us to see an atom? The answer to this question is no, as long as visible light is used to illuminate the object. In order to be seen, the object under a microscope must be at least as large as a wavelength of light. An atom is many times smaller than the wavelength of visible light, so its mysteries must be probed via other techniques.

The telescope

There are two fundamentally different types of telescope, both designed to help us view distant objects such as the planets in our Solar System. These two types are (1) the refracting telescope, which uses a combination of lenses to form an image, and (2) the reflecting telescope, which uses a curved mirror and a lens to form an image. Once again, we will be able to analyze the telescope by considering it to be a system of two optical elements in a row. As before, the basic technique followed is that the image formed by the first element becomes the object for the second.

The largest optical telescopes in the world are the two 10-m-diameter Keck reflectors on Mauna Kea in Hawaii. The largest single-mirrored reflecting telescope in the United States is the 5-m-diameter instrument on

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Mount Palomar in California. In contrast, the largest refracting telescope in the world, at the Yerkes Observatory in Williams Bay, Wisconsin, has a diameter of only 1 m.

The Michelson interferometer

The Michelson interferometer is an optical instrument having great scientific importance. Invented by the American physicist A. A. Michelson (1852-1931), it is an ingenious device that splits a light beam into two parts and then recombines them to form an interference pattern. The interferometer is used to make accurate length measurements.

1.What are the components of the single-lens photographic cam-

era?

2.How can we prevent blurred images?

3.How the varying of the shape of the pliable crystalline of eye is

called?

4.Where do we use the simple magnifier?

5.Describe how a compound microscope works.

6.Why does the Michelson interferometer have great scientific importance?

Exercises

1. Rearrange the letters in the anagrams to form equivalents for the Russian words:

2. Match the words in A with the words in B to form word combinations:

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P A R T VI

Modern Physics

UNIT 26

RELATIVITY

Read the following texts. Study the thesaurus on fig. 26, answer the questions after the texts

Most of our everyday experiences and observations have to do with objects that move at speeds much less than the speed of light. Newtonian mechanics was formulated to describe the motion of such objects, and its formalism is quite successful in describing a wide range of phenomena that occur at low speeds. It fails, however, when applied to particles having speeds approaching that of light.

This Unit introduces Einstein's theory of special relativity and includes a section on general relativity. The concepts of special relativity often violate our common sense. Moving clocks run slow, and the length of a moving meter stick is contracted. Nonetheless, the theory has been rigorously tested, correctly predicting the results of experiments involving speeds near the speed of light. The theory is verified daily in particle accelerators around the world.

Experiments show that the speed of the electron - as well as the speed of any other particle that has mass - always remains less than the speed of light, regardless of the size of the accelerating voltage. The existence of a universal speed limit has far-reaching consequences. It means that the usual concepts of force, momentum, and energy no longer apply for rapidly moving objects. Less obvious consequences include the fact that observers moving at different speeds will measure different time intervals and displacements between the same two events. Newtonian mechanics was contradicted by experimental observations, so it was necessary to replace it with another theory.

In 1905, at the age of 26, Einstein published his special theory of relativity. Regarding the theory, Einstein wrote: The relativity theory arose from necessity, from serious and deep contradictions in the old theory from which there seemed no escape. The strength of the new theory

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lies in the consistency and simplicity with which it solves all these difficulties, using only a few very convincing assumptions.

The principle of Galilean relativity

In order to describe a physical event, it's necessary to choose a frame of reference. For example, when you perform an experiment in a laboratory, you select a coordinate system, or frame of reference, that is at rest with respect to the laboratory. However, suppose an observer in a passing car moving at a constant velocity with respect to the lab were to observe your experiment. Would the observations made by the moving observer differ dramatically from yours? That is, if you found Newton's first law to be valid in your frame of reference, would the moving observer agree with you? According to the principle of Galilean relativity, the laws of mechanics must be the same in all inertial frames of reference.

Speed of light

It's natural to ask whether the concept of Galileanrelativity in mechanics also applies to experiments in electricity, magnetism, optics, and other areas. Experiments indicate the answer is no. For example, if we assume that the laws of electricity and magnetism are the same in all inertial frames, a paradox concerning the speed of light immediately arises. In order to resolve this paradox, we must conclude that either (1) the addition law for velocities is incorrect or (2) the laws of electricity and magnetism are not the same in all inertial frames.

In the 19th century, physicists thought that electromagnetic waves also required a medium in order to propagate. They proposed that such a medium existed and gave it the name luminiferous ether. The ether was assumed to be present everywhere, even in empty space, and light waves were viewed as ether oscillations.

The second hypothesis is false - and we now believe that the laws of electricity and magnetism are the same in all inertial frames.

The Michelson-Morley experiment

The most famous experiment designed to detect small changes in the speed of light was first performed in 1881 by Albert A. Michelson (1852-1931) and later repeated under various conditions by Michelson and Edward W. Morley (1838-1923). We state at the outset that the outcome of the experiment contradicted the ether hypothesis. The experiment was designed to determine the velocity of Earth relative to the hypothetical ether. The experimental tool used was the Michelson interferometer. The Michelson-Morley experiment was repeated at different

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