1. After leaving the lens...

On entering the lens the rays are bent toward the normal. On leaving the lens the rays are bent away from the normal. They appear on leaving the lens to come from a point behind the lens.

Exercises

1. Listen, read and translate the text.

2. Answer the following questions in a written form:

1. How may lenses be classified? 2. When do the rays converge to the principal focus? 3. When does the lens con­verge the rays rapidly? 4. What gives the lens a short focal length? 5. When has the lens a long focal length? 6. When are the emerging rays bent away from the principal axis?

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3. Make up and write all possible questions to the following sentences:

1. The unit used to denote (measure) quantity or amount of light is the lumen. 2. A lumen is the amount of light falling on a surface that has an area of 1 sq ft when every point of the surface is 1 ft from a point source of light of 1 candle. 3. The light may be absorbed by the material. 4. The mate­rial which sends no light on towards the eye and so looks black is said to be perfectly opaque. 5. Some light may pass through the substance. 6. The light undergoes rectilinear propagation.

4. Copy the following sentences putting "axis", "beam", "vir­tual", "focus", "lens", "principal", "rays", "converge", "diverging", "real" in the blanks:

1. A simple ... is usually a piece of glass bounded by spherical surfaces. 2. The principal ... of a lens is the line joining the centres of curvature of its surfaces. 3. If a paral­lel ... of light, parallel to the ... axis, is incident on a converging lens, the rays, after passing through the lens, all ... to a point on the axis which is called the principal ... . 4. In the case of a diverging lens, the ... will spread out after pass­ing through the lens, as if ... from a focus behind the lens. 5. The principal focus is thus ... for a converging lens and ... for a diverging lens.

5. Find English equivalents to the following word combina­tions:

естественный источник света, точечный источник света, однородная среда, распространение света, скорость света, световой луч, световые пучки, расходящийся пучок, сходящийся пучок, параллельный пучок, мнимый фокус, действительный фокус, угол падения, отражение света, угол отражения, поверхность плоского зеркала, вогнутая линза, выпуклая линза, преломление света, оптическая ось, фокус линзы, фокусное расстояние линзы, собирающая линза, рассеивающая линза, падающий фронт волны.

6. Answer the questions in a written form choosing the neces­sary gerundial construction given in the table:

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goes is known as a real focus. 3. The lens which converges the rays after leaving it is called a converging lens.

13. Listen to this short dialogue and learn it by heart:

— Lenses are magnifying glasses which are made of trans­parent material, aren't they?

— Certainly they are. This gives the lenses widely using in optical instruments. What kind of lenses do you know?

— There are many of them. Although they differ much in form they may be divided into two classes.

— I see. According to the way in which they act on a par­allel beam of light they are divided into converging and diverging lenses.

— Yes, they are. By the way, they have some focuses too.

— Of course. A point to which the rays converge is called a principal focus. And what else?

— An apparent focus is a point from which the light does not really come. What is a virtual focus?

- It is a point from which the light appears to come. Is there a real focus?

— Certainly, it is. A point through which the light actual­ly goes.

14. Suggested topics for oral narration:

1. Distinguish between a converging lens and a diverging lens. Describe, with the help of diagrams, their difference.

2. What is an apparent and real focuses? Show, by means of a ray diagram, that any lens has two focuses.