Text 2. Geometric optics: sources, Transmission, and reflection of light

Key terms: optics, geometric optics, light, luminous sources, non-lumi- nous sources, rectilinear propagation, reflection, refraction, ray model of light, ray, beam, parallel (converging, diverging) beams, transparent (translucent, opaque) media, the incident ray, the reflected ray, the point of incidence, the normal, the angle of incidence, the angle of reflection

Optics is the study of the behaviour of light. Geometric optics is the branch of the study of optics that deals with reflection and refraction of light.

Sources of Light. Light is a form of radiant energy that the eye can detect. There are two classifications of light sources: luminous and non-luminous. Luminous sources produce light, so they are seen by their own light. Examples of luminous sources are the sun and a glowing light bulb. Non-luminous sources do not emit their own light, but reflect light from another source. As a result, they are seen by the light they reflect. The moon is an example of a non-luminous source of light.

Some Properties of Light. Rectilinear propagation is the term used to describe the fact that light appears to travel in straight lines through a uniform medium. The shadow of a stick cast by the sun is evidence of this. However, light can be made to change direction by bouncing it off the surface or by passing it through different media. Light travels in a different direction when it rebounds from a shiny surface, a process being called reflection. It also bends when it travels from one material to another at an angle other than 90°, a process being called refraction

The fact that light usually travels in straight lines has led to the Ray model of Light. Figure 5 shows light from a ray box, a device for projecting streaks of light towards optical instruments, such as mirrors and lenses. The path followed by the light is represented by a directed straight line called a ray. A bundle of rays is fig. 5. The Ray Model of Light called a beam. A beam in which the rays are parallel to each other is referred to as a parallel beam. A beam in which the rays move closer together and meet is called a converging beam, while one in which the rays spread out is called a diverging beam.

Types of Media. Light can travel through a vacuum and through some materials. Media are classified in terms of their ability to transmit light. Transparent media transmit light so well that objects are seen clearly through them. Window glass, water and coloured plastic are transparent media. Translucent media also transmit light. However, the light is scattered so objects cannot be seen clearly through them. Frosted glass and waxed paper are the examples of translucent media. Opague rnedia, such as aluminium, wood and asphalt do not permit light to pass. Some of the light is reflected. The rest is absorbed, causing the material to heat up.

Reflection. When light strikes a polished surface it is reflected. The incident ray is the ray of light approaching the reflecting surface, while the reflected ray is the ray of light leaving the reflecting surface. The point of incidence is the location where the incident ray strikes the reflecting surface. The normal N is a line drawn perpendicular to the surface at the point of incidence. The angle of incidence i is the angle between the incident ray and the normal while the angle of reflection r is the angle between the reflected ray and the normal. A diagram illustrating these features is called a ray diagram.

TASKS

I. Distinguish between a luminous and non-luminous source of light. Give an example of each, different from those in the text.

a) What is meant by the term «rectilinear propagation of light»?

II. What evidence do you have that light behaves in this way?

a) Distinguish between a ray and a beam.

III. Draw three diagrams to show the difference between parallel, converging and diverging beams.

a) What are the differences between transparent, translucent and opaque materials?

IV. Give an example of each, different from those in the text.

V. Draw a ray diagram and label the following terms: incident ray, reflected ray, normal, point of incidence, angle of incidence, angle of reflection.

VI. Look through these texts without using a dictionary, and say what is their main idea.

Reflecting Telescopes

The first reflecting telescope was made by Isaak Newton in 1668. Newton used a converging mirror with a diameter of 2.5 cm to collect and focus the light. He placed a small plane mirror at 45° angle to the path to deflect the light to the eye. An eyepiece (окуляр) was used to magnify and focus the final image.

Since the intensity of the light from a distant star is very weak, a broad beam of starlight must be focused to a point to obtain a visible image. The larger the diameter of the converging mirror and the better it focuses the light, the more distant the star that can be photographed.

Today, the largest parabolic reflecting telescope in the world, called the Keck telescope, is being built on top of a mountain in Hawaii. It will have a cross-sectional diameter of about 10 metres. Can you explain why modern reflecting telescopes use parabolic rather than spherical converging mirrors?

Solar Reflectors

Solar reflectors are designed to concentrate radiant energy as it comes directly from the sun. Archimedes is fabled to have used in 212 BC (Before Christ) 4000 shields as solar reflectors to set fire to a Roman fleet in a harbour. Each metallic reflector had a long focal length and was probably no longer than a sheet of newsprint. A reflector of only a square metre can collect enough solar energy to boil a litre of water in just 10 minutes. Imagine the heat collected by 4000 concave reflectors.

Larger solar reflectors make use of thousands of mirrors to focus light onto a small area. The French solar furnace in the Pyrenees uses an array (масив) of 63 movable mirrors, arranged in 8 terraces, to direct sunlight onto the fixed parabolic mirror surface of the building. The gigantic converging mirror then focuses the sunlight onto a boiler in an opening in the tower at the centre. The energy of the steam in the boiler is used to drive a steam turbine and produce electricity. Although the system is only 8% efficient, it produces about 64 kW of electricity on a sunny day.

 

UNIT 13

Grammar: 1. The Gerund

2. Different functions of one

Texts: 1. Washington, the Capital of the USA

2. The Speed of light

Section 1

GRAMMAR EXERCISES

I. Translate the following sentences into English paying attention to the forms and functions of the Gerund:

1. Speaking foreign languages is of importance for all civilized men. 2. Science requires experimenting. 3. What apparatus is used for measuring electric current? 4. Before being sent up the balloon was filled with a special gas. 5. The idea of choosing the place for the Capitol Building in the USA belongs to its first president George Washington. 6.1 know of his having been sent to work there. 7. What is the reason for her having left the meeting so suddenly? 8. We heard of the experiment having been started last week. 9. He improved his report by changing its beginning and end. 10. They objected to his remaining at home. 11. Instead of restoring the old theatre they decided to build a new one in the centre of the town. 12. New possibilities for applying atomic energy open up. 13. It is possible to set up power stations based on utilizing the heat of the Sun.

II. Translate the sentences paying attention to the words after which Gerund is used (if any), and to the form of the Gerund. Which of these sentences contain the Gerundial complexes? Pay attention to the ways of translating them into Ukrainian.

1. His being sent there was quite unexpected to me. 2.1 like your being always in time for classes. 3. I didn’t know of your having been so deeply impressed by my words. 4. Excuse my interrupting you. 5. We have never heard of his having belonged to this scientific society. 6. He doesn’t like being interrupted. 7. Upon passing his examinations successfully he was accepted to the University. 8. You must not come to the party without being invited. 9. He left the room without saying goodbye. 10. We know nothing of his having published the article. 11. We were sure of the bridge having been destroyed during the storm. 12. The work could not be done without the necessary experiments having been carried out. 13. The professor insisted on our comparing the results of the experiments. 14.1 am surprised at your having done such a silly thing. 15. Do you mind my opening the window? It’s so stuffy in the room.

III. Which preposition would you use before the Gerunds or the complexes with the Gerund in these sentences: at, in, on, to, for, from, upon, before.

1. wonder... your having missed so many lessons. 2. Thank you... helping me. 3. He insisted... my going there with him. 4. She succeeded... writing the text without any mistake. 5.1 am sure... Peter’s returning you the money in time. 6. Instead... buying this expensive dress you should have bought something for your child. 7. In spite... the weather being very bad, all were... going on excursion. 8.1 am afraid... your being ill. 9. Our illness may prevent us... completing our work in time. 10. Your progress in English depends... your regular working at it. 11.... writing the letter he took it to the post office. 12. There are many ways of translating the Gerund into Ukrainian.

IV. Translate these sentences into English using the Gerund or the complexes with the Gerund.

1. Замість того щоб написати листа, він подзвонив мені. 2. Перед тим як вийти з дому, він вимкнув світло. 3. Я нічого не знав про те, що він уже повернувся додому. 4. Мене дивує те, що ти робиш (зробив) так багато помилок при перекладі цього тексту. 5. Мені вдалося купити квитки на цю виставу. 6. Те, що її посилають у відрядження, нікого не здивувало. 7. Я чув, що ваш товариш прийняв вашу пропозицію. 8.Маленькі діти люблять, коли їм читають казки. 9. Я пам’ятаю, що бачила десь цього чоловіка. 10. Він має погану звичку перебивати інших людей, коли вони розмовляють.

V. Translate these sentences into English using the substitution table given below.

A. 1. Необхідно отримати кращі результати. 2. Потрібно більше працювати. 3. Можна було взяти будь-яку книжку. 4. Слід поліпшити вимову. 5. Можна було взяти будь-який інструмент. 6. Необхідно використати ці дані.

ONE must use this data.

can could take any book one liked, may obtain better results, could improve one’s pronunciation, will have to work hard, take any instrument.

В. 1. Неможливо вступити до університету без іспитів.

Не можна було проводити це дослідження без останніх даних. 3. Неможливо відповісти на це запитання відразу. 4. Не можна доїхати до станції на цьому автобусі. 5. Не можна запізнюватися на лекції. 6. Не можна користуватися цими приладами.

ONE can’t use these devices.

mustn’t answer the question at once.

couldn’t get to the station by this bus.

be late for the lectures.

enter a university without exams.

carry on this research without that latest

data.

VI. Translate these sentences, paying attention to the functions of «one»:

1. One should never cross the street in this place. 2. I don’t like this book, give me another one, please. 3. This text is much more interesting than the one we read last week. 4. Pete is one of the best students in our group. 5. This dress is much more interesting than the one I bought last week. 6. One never knows what may happen with these boys. 7. One mustn’t return home so late. 8. One needn’t do this work.

Section 2