Text. Reflection and refraction of light

Bodies like the sun and other sources of light with which we are familiar are said to be "self-luminous" since they emit light of their own accord.¹ On the other hand, the common objects around us are not self-luminous. Nevertheless, these bodies are vibrating as they reflect in all directions the light which falls on them from the sun. Mirrors and highly polished surfaces reflect light strongly in a particular direction, and we will now examine the laws governing this reflection.

Reflection of Light. When a beam of light, traveling in a homogeneous medium, comes to a second medium, some of the light is reflected. At a polished or silvered surface, nearly all the light is reflected. At the surface of clear glass, only a small part of it is reflected. The greater part of it enters the glass and passes through. Fig. 4 illustrates the terms we use in the study of reflected light. AB represents the surface of a plane mirror. PO, called the incident ray, is the direc­tion in which light falls on to the reflecting surface. P is the point of incidence and PN the reflected ray. The angles i and r which the incident and reflected rays make with

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P M the normal or perpendic­ular to the reflecting surface at the point of incidence, are called the angles of incidence and reflection respectively.

Laws of Reflection.

1. The incident ray, the reflected ray and the nor­mal at the point of inci­dence all lie in the same plane. 2. The angle of incidence is equal to the angle of reflection.

Refraction. Experiments have shown that light travels with the greatest speed in a vacuum and that it travels with different speeds in different media. When it passes oblique­ly from one medium to another in which it has a differ­ent velocity, there occurs a change in the direction of prop­agation of the light. This bending of the ray of light when passing from one medium to another is known as refraction.

Note

¹ of their own accord — самопроизвольно

Listen and read words and word combinations to be remem­bered:

reflection [ri'flekjn] отражение represent [_;repri'zent] изображать -

plane [plein] плоский refraction [ri'fraekjn] пре ломление

angle [aengl ] угол luminous ['lu:mines] светя щийся

incidence ['insidens] падение propagation [,propa'geijn] распростра-

beam [bi:m] пучок нение

bend [bend] изгибаться

are familiar хорошо осведомлены

self-luminous самосветящийся

clear glass прозрачное стекло

plane mirror плоское зеркало

incident ray падающий луч

reflected ray отраженный луч

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Models

1. Do bodies emit? Does he make? Did it represent?

2. Have they shown? Had he travelled? Was it reflected? Is he going? Exercises

1. Listen, read and translate the text.

2. Answer the following questions in a written form:

1. What bodies are said to be self-luminous? 2. What bodies reflect light in a particular direction? 3. What is called the incident ray? 4. What are called the angles of inci­dence and reflection? 5. What is known as refraction of light?

3. Copy these sentences putting "accord", "reflection", "angle," "beams", "bent", "light'', "self-luminous", "surface", "reflected", "straight", "plane", "refraction" in the blanks:

1. Light travels in a ... line. 2. We have made some exper­iments for the study of ... . 3. The moon is not ... body. 4. The sun emits light of its own... . 5. When light is scattered by reflection from an irregular surface it is called dif­fuse... . 6. Try to observe and compare the angle at which the ball strikes the surface with the ... at which it is ... . 7. You may make some experiments with light using a ... mirror. 8. The ... which strike the mirror are reflected at the same angle. 9. Place a stick in a tall jar of water, so that part of the stick is above the ... and observe where the stick enters the water and appears to be ... . 10. This is caused by the bending or ... of the light rays as they reach the air from the water.

4. Answer these questions in complete sentences in a written form:

a) (e.g. Does light strike matter? Yes, light strikes matter.)

1. May the light be absorbed by the material? 2. Do any things glow because of their high temperatures? 3. Can any objects emit light without the requirement of a high temperature? 4. Did Isaac Newton think that light energy was transmitted as a stream of particles or corpuscles? 5. Was it realized that light is a form of energy? 6. Will you study the transmission of light?

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b) (e.g. Where does light travel faster? Light travels faster in air.)

1. How does light travel? 2. How do mirrors and highly polished surfaces reflect light? 3. What kinds of light rays was told about? 4. When is nearly all the light reflected? 5. When is any of the light reflected? 6. How can you observe the laws governing reflection? 7. What had the last experiment shown?

5. Make up and write questions using the words given in the table:

What When Where	do did does	bodies like the sun a mirror the student rays light	emit reflect make lie travel	in different . medium? light? last experi­ment? in the same plane?
How	have is had shall	some of the light all the light experiments only a small part of light the angle of in­cidence the students we	shown reflected equal to learnt make	the future ex­periment? by the end of the lecture?

6. Translate this text and explain the use of tenses in it:

Electromagnetic radiation is very peculiar in that it needs no medium to carry it. It can pass through space empty of matter. This can be shown to be true by a simple experiment. Place a small cycle lamp under a bell jar which is attached to a vacuum pump, leaving the lamp switched on. When the pump has removed the air from the bell jar, you will still

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see the lamp shining just as brightly as before the pump was started. Thus the light can get across the space in the jar whether there is air present or not.

7. Make up and write 5 questions to the above text.