Материал: Андрианова English Thesaurus of Student-Physicist 2013

This material is fabricated in thin sheets of long-chain hydrocarbons, which are stretched during manufacture so that the molecules align.

Polarizing material reduces the intensity of light passing through it. An unpolarized light beam is incident on the first polarizing sheet, called the polarizer. The light that passes through this sheet is polarized vertically. A second polarizing sheet, called the analyzer, intercepts this beam with its transmission axis at an angle to the axis of the polarizer. The expression, known as Malus's law, applies to any two polarizing materials having transmission axes at an angle to each other.

When an unpolarized light beam is reflected from a surface, the reflected light is completely polarized, partially polarized, or unpolarized, depending on the angle of incidence. For angles of incidence between 0° and 90°, however, the reflected light is polarized to s ome extent. For one particular angle, called the polarizing angle, of incidence the reflected beam is completely polarized. An expression relating the polarizing angle with the index of refraction of the reflecting surface is called Brewster's law.

When light is incident on a system of particles, such as a gas, the electrons in the medium can absorb and reradiate part of the light. The absorption and reradiation of light by the medium, called scattering, is what causes sunlight reaching an observer on Earth from straight overhead to be polarized. You can observe this effect by looking directly up through a pair of sunglasses made of polarizing glass.

Optical Activity

Many important practical applications of polarized light involve the use of certain materials that display the property of optical activity. A substance is said to be optically active if it rotates the plane of polarization of transmitted light. Optical activity occurs in a material because of an asymmetry in the shape of its constituent molecules. For example, some proteins are optically active because of their spiral shapes. Other materials, such as glass and plastic, become optically active when placed under stress.

An effect similar to rotation of the plane of polarization is used to create the familiar displays on pocket calculators, wristwatches, notebook computers, and so forth. The properties of a unique substance called a liquid crystal make these displays (called LCD's, for liquid crystal displays) possible. As its name implies, a liquid crystal is a substance with properties intermediate between those of a crystalline solid and those of a liquid; that

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is, the molecules of the substance are more orderly than those in a liquid, but less orderly than those in a pure crystalline solid.

1.How it’s necessary to consider light to explain the phenomena of interference, diffraction and polarization?

2.What do we need to create destructive interference?

3.What is used as coherent source?

4.How do zeroes and ones are presented on CD's and DVD's?

5.What do we call Fraunhofer diffraction?

6.How it's possible to obtain a linearly polarized beam?

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:

3.Fill in the gaps with the missing words from the list: diffraction, path difference, coherent sources, unpolarized, interfer-

ence effects

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1. Any random change in the light emitted by the source will occur in the two separate beams at the same time, and _____ can be observed.

2. . If the _____ is either zero or some integral multiple of the wavelength, the two waves are in phase and constructive interference results.

3.The positions of the dark and bright fringes are reversed relative to the pattern obtained from two real _____.

4.In general, _____ occurs when waves pass through small openings, around obstacles, or by sharp edges.

5.If the angle of incidence is either 0° or 90° (a no rmal or grazing angle), the reflected beam is _____.

UNIT 25

OPTICAL INSTRUMENTS

Read the following texts. Study the thesaurus on fig. 25, answer the questions after the texts

We use devices made from lenses, mirrors, or other optical components every time we put on a pair of eyeglasses or contact lenses, take a photograph, look at the sky through a telescope, and so on. In this Unit we examine how these and other optical instruments work. For the most part, our analyses will involve the laws of reflection and refraction and the procedures of geometric optics. To explain certain phenomena, however, we must use the wave nature of light.

The camera

The single-lens photographic camera is a simple optical instrument. It consists of an opaque box, a converging lens that produces a real image, and a film behind the lens to receive the image. Focusing is accomplished by varying the distance between lens and film - with an adjustable bellows in antique cameras and with some other mechanical arrangements in contemporary models. For proper focusing, which leads to sharp images, the lens-to-film distance depends on the object distance as well as on the focal length of the lens. The shutter, located behind the lens, is a mechanical device that is opened for selected time intervals. With this arrangement, moving objects can be photographed by using short exposure times, dark scenes (with low light levels) by using long exposure times. If this adjustment were not available, it would be impossible to take stop-

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action photographs. A rapidly moving vehicle, for example, could move far enough while the shutter was open to produce a blurred image. Another major cause of blurred images is movement of the camera while the shutter is open. To prevent such movement, you should mount the camera on a tripod or use short exposure times.

Most cameras also have an aperture of adjustable diameter to further control the intensity of the light reaching the film. When an aperture of small diameter is used, only light from the central portion of the lens reaches the film, so spherical aberration is reduced. The brightness of the image formed on the film depends on the light intensity, so we see that it ultimately depends on both the focal length and diameter of the lens. The ratio the focal length and diameter is called the focal ratio of a lens.

The eye

Like a camera, a normal eye focuses light and produces a sharp image. However, the mechanisms by which the eye controls the amount of light admitted and adjusts to produce correctly focused images are far more complex, intricate, and effective than those in even the most sophisticated camera. In all respects, the eye is a physiological wonder. The eye focuses on an object by varying the shape of the pliable crystalline lens through an amazing process called accommodation.

When the eye suffers a mismatch between the focusing power of the lens-cornea system and the length of the eye so that light rays reach the retina before they converge to form an image, the condition is known as farsightedness (or hyperopia). A farsighted person can usually see faraway objects clearly but not nearby objects. The condition can be corrected by placing a converging lens in front of the eye.

Nearsightedness (or myopia) is another mismatch condition in which a person is able to focus on nearby objects, but not faraway objects. Nearsightedness can be corrected with a diverging lens. The lens refracts the rays away from the principal axis before they enter the eye, allowing them to focus on the retina.

In the eye defect known as astigmatism, light from a point source produces a line image on the retina. This condition arises when either the cornea or the lens (or both) are not perfectly symmetric. Astigmatism can be corrected with lenses having different curvatures in two mutually perpendicular directions.

Optometrists and ophthalmologists usually prescribe lenses measured in diopters.

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