Additional Material
Read the text and get its central idea. Write sentences that best give the distinguish between converging and diverging lenses.
Magnifying Glasses
Magnifying glasses or lenses have been in use for centuries and were well known to the Greeks and medieval Arabs. Lenses of many different types play an important part in everyday life. Apart from benefit of spectacles which enable millions of people to read in comfort, our lives would be vastly changed if we had no cameras, projectors, microscopes or telescopes, all of which function by means of lenses.
Not all lenses can be used as magnifying glasses. There are some, used in opera glasses and in spectacles for short sighted persons, which
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give a diminished, erect virtual image. These are referred to as concave or diverging lenses, while magnifying glasses are called convex or converging lenses. The two types can be readily distinguished from one another; converging lenses are thickest in the middle while diverging lenses are thinnest in the middle.
Read and translate this text in a written form:
The Importance of the Study of Spectra
The study of the wavelengths of the radiation from a hot body comes under the general heading of Spectra. The number of spectra of elements and compounds which have been recorded runs easily into millions, and it is worth while stating at the outset the main reasons for the interest in the phenomenon.
It is now considered that an atom consists of a nucleus of positive electricity surrounded by electrons moving in various orbits, and that a particular electron in an orbit has a definite amount of energy. In certain circumstances the electron may jump from this orbit to another, where it has a smaller amount of energy. When this occurs radiation is emitted, and the energy in the radiation is equal to the difference in energy of the atom between its initial and final states.
UNIT 15
Text. Simple cell
About 1790, an Italian scientist called Galvani noticed the twitching of a dead frog's leg when the nerve was touched by two unlike metals in contact. He told his friend Volta about it, and after many experiments Volta concluded that an electric current had been generated in the muscle by the presence of the two metals and the chemical solutions in the nerve. About 1800 Volta made a great discovery and invention. He placed a number of copper and zinc discs together, with cloth or cardboard soaked in brine between them, and found that an electric current was generated when the terminals were joined. One pair of copper and zinc plates with brine between them forms a unit or cell; the whole arrangement was called "Volta's pile" and a number of such cells joined together to assist each other is called a "battery".
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N
owadays,
a simple cell can be made by dipping a copper plate and a zinc rod
inside dilute sulphuric acid in a beaker; the acid takes the
place of the brine (Fig. 6). The cell has an electromotive force
(e.m.f.) or potential difference (p.d.) of about 1 volt. When a small
torch bulb L
is
joined to the terminals of the copper and zinc the bulb lights up for
a time, but it soon becomes dim and goes out. The cell is said to be
"polarized". The main cause of the polarization is hydrogen
gas, which is produced at the copper plate when the cell is working.
This sets up what is called a "back" e.m.f. which opposes
the main e.m.f. Further, the gas has a high electrical resistance,
thus tending to diminish the current flowing.
Listen and read words and word combinations to be remembered:
conclude [kan'kluid] заключать beaker ['bi:ka] мензурка
terminal ['tэ:minə1 ] клемма dim [dim] тусклый
generate ['dgenareit ] генерировать join [djoin ] соединять(ся)
hydrogen ['haidridgan] водород cell [sel ] элемент
solution [so'lu:Jn] раствор dip [dip ] погружать(ся)
cardboard['ka:dbo:d] картон resistance [ri'zistsns] conpo-
diminish [di'minij] уменьшать(ся) тивление
unlike metals неодинаковые металлы
the whole arrangement все устройство
"Volta's pile" Вольтова дуга
dilute sulphuric acid раствор серной кислоты
electromotive force (e.m.f.) электродвижущая сила(э.д.с.)
potential difference (p.d.) разность потенциалов
to go out погаснуть
Give Russian equivalents to the following words without using a dictionary:
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nerve [no:v] assist [a'sist]
contact ['kontaekt] volt [volt]
metal [metl] polarization [,poularai'zeiJen]
chemical ['kemikal] polarize ['poularaiz]
disc [disk] produce [pra'dju:s]
zinc [zink] oppose [o'pouz]
pair [реa] tending ['tending]
Models