I. Translate and memorize:

a) to move - to remove -

1. There are still some people who suppose that it is the sun and the stars that move all the time while the earth is standing still. 2. Many people move to new houses - in our cities nowadays. 3. Please, remove your books and notebooks from the table. I mean, take everything off that table for this bulky (big) apparatus must be placed there.

b) to deal with

1. The articles we translate for our lessons deals with aspects of Optics. 2. This man is rather difficult to deal with, his nerves being in a poor state.

c) to avoid - unavoidable: to read - unreadable; to translate - untranslatable

d) advantage, advantageous, disadvantage, to advantage

This instrument can always be made use of to advantage for it is a versatile one.

II. Answer the questions:

1. What results from the luminous point being removed very far from the mirror? 2. Where is the principal focus located? 3. Do you know the difference between the words "principal" and "principle"? 4. Do all the rays meet at the principal focus? 5. What takes place (what happens) whenever we have to use a spherical mirror with a large aperture? 6. Why does it take place? 7. What does the fact that only the rays parallel to the optical axis meet at the principal focus result in? 8. What kind of mirror can be used instead of a spherical one in order to avoid what is called "spherical aberration"?

III. Compare and memorize:

able, unable, enable; to be an advantage, to take advantage of; single, simple, simplicity, simply; similar, similarity, similarly

1. The knowledge of the laws of nature enables us to use them to out advantage. 2. Petrov was unable to take the full advantage of the fine day because he could not spend it in the country.

IV. Say a few words about the Principal Focus.

Concave Spherical Mirrors

I. Answer the questions:

1. What is it necessary to know to construct the image of an object in a concave spherical mirror? 2. What can be done provided the center of curvature of the mirror and its principal focus are known? 3. How many cases are we to consider when dealing with a concave mirror? 4. Where is the image formed if the object is removed beyond the center of curvature? 5. What kind of image will be produced in this case? 6. What is the situation supposed to be in the second case? 7. Is the result similar to that obtained in the preceding case? 8. Does the result obtained in the second case differ from that obtained in the preceding case? 9. Where is the object supposed to lie in the third case? 10. Why is the image formed by a concave mirror called virtual as opposed to a real image? 11. Does the effect of a virtual image differ from that produced by a real image?

II. Translate and memorize:

a) 1. These fields extend far into the next region. 2. This canal is to be extended several miles to the north-east direction. 3. His success is due to a great extent to his friend who helped him a great deal. 4. No one knows the extent of one's endurance (выносливость) until it has been tried. 5. This street is but an extension of the one you live in.

b) 1. These two sisters are very much alike in everything except their color: one is a blond, while the other is almost a brunette. 2. These two problems are quite similar, except that one of them is slightly more complicated than the other. 3. Everyone has come to the lecture today except Petrov. 4. Many people have come to the meeting besides those who were personally invited to it.

III. Describe the three cases that may arise in the case of a concave spherical mirror.

IV. Translate without using a dictionary:

1. Separation of Light by a Prism.

When a beam of light passes through a prism it is bent through a certain angle from the direction it was originally travelling. This bending is known to arise from the fact that the velocity of light in air is not the same as in the material the prism is made of. If white light we know to be composed of a number of colors, passes through a prism, each color will be bent by the prism. Now, since lights of different colors do not travel with the same velocity in glasses, they will be bent different amounts on entering and leaving the prism. And although all the colors come to the prism in the same direction it is in different directions that they will leave it because of unequal bending they suffer. White light in passing through a prism is thus spread out into a band of colors known as a spectrum. The violet light having the shortest wavelength is bent most, while the red, whose wave length is the longest, is bent least, light with wavelengths between the red and the violet, naturally, occupying intermediate positions in the spectrum.

2. Achromatic Prism.

A. When light of more than one color falls on a prism the emerging beam is not only deviated but spread out into a variety of colors called a spectrum. If a second prism is placed in front of the first one with its vertex to the base of the first one the second prism will tend to gather the different colors together again and re-combine them into white light, the net dispersion produced by the two prisms being the difference between the dispersions produced by the individual prisms. The deviation caused by the second prism must be in the opposite direction to that resulting from the first one, and so the net deviation is the difference between the two. In this way it is possible to construct a prism to deviate light without spreading it out into a spectrum. Such a prism is called an achromatic prism.

B. Achromatic prisms are made by combining two prisms of two different kinds of glasses, i.e. one of crown glass and the other of flint glass. The angles of the two are so chosen that the dispersion produced by the crown glass is just equal and opposite to that produced by the flint glass.

This is possible for in prisms of equal angles flint glass produces greater dispersion than crown glass does. The two prisms being placed in a certain position with respect to each other, the net dispersion is zero.

On the other hand, the deviation to be yielded by the crown glass exceeds that produced by the flint glass so that there is obtained a net deviation without any dispersion. One sees all the rays emerge parallel to each other and combine to form white light. The rays, however, will be seen to have been bent from their original direction.