Laws of refraction:
Incident and refracted rays lie in the same plane.
When a ray of light passes at an angle into a denser medium, it is bent towards the normal, hence the angle of refraction is smaller than the angle of incidence.
Scattering is the random deflection of light rays by fine particles. When sunlight enters through a crack, scattering by dust particles in the air makes the shaft of light visible. Haze is a result of light scattering by fog and smoke particles.
Absorption of light as it passes through matter results in the decrease in intensity. Absorption, like scattering, may be general or selective. Selective absorption gives the world most of the colors we see. Glass filters which absorb part of the visible spectrum are used in research and photography.
Diffraction is the bending of waves around an obstacle. It is easy to see this effect for water waves. They bend around the corner of a sea wall, or spread as they move out of a channel. Diffraction of light waves, however, is harder to observe. In fact, diffraction of light waves is so slight that it escaped notice for a long time. The amount of bending is proportional to the size of light waves – about one fifty-thousandth of an inch (5,000 Å) – so the bending is always very small indeed.
When light from a distant street lamp is viewed through a window screen it forms a cross. The four sides of each tiny screen hole act as the sides of a slit and bend light in four directions, producing a cross made of four prongs of light. Another way to see the diffraction of light waves is to look at a distant light bulb through a very narrow vertical slit. Light from the bulb bends at both edges of the slit and appears to spread out sideways, forming an elongated diffraction pattern in a direction perpendicular to the slit.
Light can be imagined as waves whose fronts spread out in expanding concentric spheres around a source. Each point on a wave front can be thought of as the source of a new disturbance. Each point can act as a new light source with a new series of concentric wave fronts expanding outward from it. Points are infinitely numerous on the surface of a wave front as it crosses an opening.
As new wave fronts fan out from each point of a small opening, such as a pinhole or a narrow slit, they reinforce each other when they are in phase and cancel each other when they are completely out of phase. Thus lighter and darker areas form the banded diffraction patterns.
A pattern of waves will move outward, forming concentric circles, if small pebbles are dropped regularly from a fixed point into a quiet pond. If a board is placed in the path of these waves, they will be seen to bend around the edges of the board, causing an interesting pattern where the waves from the two edges of the board meet and cross each other. When an obstruction with a vertical slit is placed in the pond in the path of the waves, the waves spread out in circles beyond the slit.
D
iffraction patterns are formed when light from a point source passes through pinholes and slits. A pinhole gives a circular pattern and a slit gives an elongated pattern. A sharp image is not formed by light passing through because of diffraction. As the pinhole or slit gets smaller, the diffraction pattern gets larger but dimmer. In the diffraction patterns shown below the alternate light and dark spaces are due to interference between waves arriving from different parts of the pinhole or slit.
Fig.1.
Interference is an effect that occurs when two waves of equal frequency are superimposed. This often happens when light rays from a single source travel by different paths to the same point. If, at the point of meeting, the two waves are in phase (vibrating in unison, and the crest of one coinciding with the crest of the other), they will combine to form a new wave of the same frequency. The amplitude of the new wave is the sum of the amplitudes of the original waves. The process of forming this new wave is called constructive interference.
If the two waves meet out of phase (crest of one coinciding with a trough of the other), the result is a wave whose amplitude is the difference of the original amplitudes. If the original waves have equal amplitudes, they may completely destroy each other, leaving no wave at all. Constructive interference results in a bright spot; destructive interference producing a dark spot.
Partial constructive or destructive interference results whenever the waves have an intermediate phase relationship. Interference of waves does not create or destroy light energy, but merely redistributes it.
Two waves interfere only if their phase relationship does not change. They are then said to be coherent. Light waves from two different sources do not interfere because radiations from different atoms are constantly changing their phase relationships. They are non-coherent.
Interference occurs when light waves from a point source (a single slit) travel by two different paths (through the double slit). Their interference is shown by a pattern of alternate light and dark bands when a screen is placed across their path.
Fig.2.
Fig.3.
CHECK YOUR UNDERSTANDING
Exercise 1. True or false?
Frequency is the number of wave crests passing a point in one second.
When light strikes a mirror at an angle, it is reflected at some other angle.
The ratio of the speed of light in space (or in air) to its speed in the substance is always less than one.
Diffuse reflection takes place at surfaces that are rough compared with the wave length of light.
When the two sides of the glass are parallel, the light emerges in a new direction when it leaves the glass.
Light travels faster in empty space and slows down upon entering some matter.
Exercise 2. Choose the correct answer.
-
What is the nature of light?
Light is the form of electromagnetic wave.
Light is a magnetic field.
Light is an electric field.
Light is a stream of molecules.
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What happens with the light that is not transmitted or reflected?
It is polarized.
It is refracted.
It is scattered.
It is absorbed.
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At what angles does the light leave an object in diffuse reflection?
The light leaves an object in diffuse reflection at the same angle as it strikes an object.
The light leaves an object in diffuse reflection so, that incident and reflected rays are on opposite sides of the normal.
The light leaves an object in diffuse reflection at many different angles.
When the light leaves an object in diffuse reflection angle of reflection equals to angle of the incidence.
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When a light ray crosses the boundary between two different materials what does it change?
It changes its speed.
It changes its direction.
It changes its wavelength and frequency.
It changes its wavelength, speed and direction.
-
How many laws of reflection do you know?
-
3 laws
2 laws
1 law
4 laws
INCREASE YOUR VOCABULARY
Exercise 1. Arrange the following words in pairs of antonyms.
Negative, light, top, shade, positive, small, diverge, base, more, thin, less, converge, thick, large.
Exercise 2. Translate the following words and word combinations into Russian.
a) recognize, diffuse, finite, converge, join, diverge, curvature, enlargement, truncated, incident;
b) image quality, image formation, light intensity, light diffraction, light propagation, pinhole size, image enlargement, surface curvature, object point, lens axis, light beam;
c) luminous point, straight line, truncated prism, bright image, practical value, coarse pinhole, certain limit, diffuse image, finite size, further enlargement, ground glass, dotted lines, infinite number, opaque screen, fine pinhole.
Exercise 3. Remember the meaning of the terms that you have found in the text.
Focus is a meeting point of rays of light; point, distance at which the sharpest outline is given (to the eye, through a telescope, through a lens, etc.).
Lens is a piece of glass or glass-like substance with one or both sides curved for use in spectacles, cameras, telescopes and other optical instruments.
Prism is a solid figure with similar equal and parallel ends and which are parallelograms body of this form made of glass which breaks up white light into the colours of the rainbow.
Diffraction is scattering of waves in back of a solid object after they strike it.
LANGUAGE ACTIVITY
Exercise 1. Summarize your knowledge of the Degrees of Comparison. Use comparative or superlative forms of adjectives or adverbs in brackets.
Sight is (important) of the senses.
Schmidt-Cassegrain telescope is (popular) among amateur astronomers.
As the conversation went on, he became (talkative).
Travelling is becoming (expensive).
You look (beautiful) than usual.
Your accent is the (bad) in the class.
If you’re very hungry, you can have the (big) steak.
My mother’s driving is getting (dangerous) as the years go by.
It’s getting (hard) to find time for everything you want to do.
Reflecting telescopes can be (powerful) than refractors.
Exercise 2. Insert the prepositions in, into, of, on, at, by where it is necessary.
When light strikes ...a mirror ... an angle, it is reflected ... the same angle.
Light travels faster ... empty space than ... matter.
Diffuse reflection is the kind of reflection ... which we usually see objects.
Light rays bend when they go from air ... water.
Many optical devices are similar ... principle though their design and purpose may differ.
Absorption ... light results in the decrease in intensity.
Exercise 3. State the function and translate the verb "to be" according to the list given below.
смысловой глагол
вспомогательный глагол, образующий страдательный залог
входит в состав общепринятых выражений и не переводится
глагол-связка
that is = то есть
вспомогательный глагол, образующий группу продолженных времён
to be to = must
The working element of the ruby laser is a cylinder of pink ruby containing 0.05 per cent chromium.
In the commonly used laser configuration a ruby rod is surrounded by the coils of a helical flashlamp operated usually for a few milliseconds with an input energy of 1000 to 2000 joules.
The narrowing of the linewidth is due to effect of the resonant cavity formed by the mirror.
Why is it difficult for scientists to compare performance of different kinds of lasers?
One of the most characteristic features of the He-Ne laser is that the output power does not increase monotonically with discharge current but reaches a maximum and thereafter decreases.
This occurs when an impurity atom has acceptor properties, that is, can attract electrons.
Powder metallurgy is finding new applications in various industries – in electronics, aviation, machine-building, etc.
If we are to achieve the aim we must confine our attention to one point only.
Exercise 4. Pay attention to the different usage of the verb "to have".
смысловой глагол
вспомогательный глагол, образующий перфектную группу времён
have to = must
Commercial applications for gas-dynamic laser have not yet been found.
Our brief mention made here has been to estimate the conceptual interest of creating a population inversion by a gas-dynamic expansion.
Excimer laser has two peculiar but important properties.
The ordinary F-centers have a very low fluorescence quantum efficiency.
Let us indicate the difficulties that have to be overcome to obtain Х-ray laser operation.
Exercise 5. Read the sentences, state the function of provided and translate these sentences.
при условии, если (что);
давать, обеспечивать.
If this condition is not satisfied, however, laser action can still occur under pulsed operation provided condition mentioned above is fulfilled.
Copper vapor lasers provide the most efficient (~ 1%) green laser source so far available.
Polymethine dyes provide laser oscillation in the red or near infrared region.
Chemical lasers provide an interesting example of direct conversion of chemical energy into electromagnetic energy.
They are potentially able to provide either large output power or large output energy.
A resonator provides for a stronger coupling between the radiation and the excited atoms.
The elliptic cylinder is made of highly reflective material and is provided with reflective end plates.
The discharge is provided by radio-frequency generator which is usually operated in the 25-to-30 nm region.
Provided the temperature is changed, the force attracting electrons to atoms is also changed.
Unit 3
WORD-STUDY
Exercise 1. Check the transcription in the dictionary and read the words listed below.
Nouns
horizon, grating, microscope, screen, binoculars, importance, magnification,
advantage, enlarger, spectrum.
Adjectives
intellectual, predictable, virtual, transparent, fundamental, terrestrial, objective,
photographic.
Exercise 2. Make different parts of speech from the following words according to the models.
Noun + ive = adjective
effect, act, excess, success.
Adjective + ly = adverb
sufficient, special, notable, rotational, vibrational, intermediate, spontaneous, optical.
Noun + less = adjective
use, aim, help, shape, color.
Noun + ic = adjective
ion, electron, atom, science.
Exercise 3. Read and translate the words paying attention to the meaning of the prefix “ semi”.
Semiconductor, semi-conductive, semi-conductivity, semiautomatic, semicircle, semifinal, semi-period, semitransparent, semi-permanent, semi-reflecting.
UNDERSTANDING A PRINTED TEXT
List of Terms:
angle of incidence – угол падения
concave mirror – вогнутое зеркало
converging – схождение
convex eyepiece lens – выпуклая линза окуляра
far-off objects – отдаленные объекты
focal length – фокусные расстояния
grating – решетка
magnifier – увеличитель
refractive index – показатель (индекс) преломления
refractor – рефрактор (телескоп)
screen – экран
curved – искривленный, изогнутый
SCAN-READING
Optical Instruments
An optical instrument uses mirrors, lenses, prisms or gratings, singly or in combination, to reflect, refract or otherwise modify light rays. Optical instruments, especially microscopes and telescopes, have probably broadened man’s intellectual horizons more than any other devices he has made.
Perhaps the best way to understand the operation of optical instruments is by geometrical optics- a method that deals with light as rays instead of waves or particles. These rays follow the laws of reflection and refraction as well as the laws of geometry.
Images formed by mirrors and lenses may be either real or virtual and of a predictable size and location. A real image, as formed by a camera or projector, is an actual converging of light rays and can be caught on a screen; virtual images cannot. The rays from object points do not pass through corresponding points of a virtual image. Images seen in binoculars are virtual.
Optical prisms are transparent solids of glass or other material whose opposite faces are plane but not necessarily parallel. They are used to bend light rays by refraction or internal reflection. The amount of bending depends on the refractive index of the prism, the angle between its faces, and the angle of incidence of the light. Since the refractive index depends also on the wavelength, prisms are often used to disperse a light beam into its spectrum.
Lenses form an image by refracting the light rays from an object. Curved glass lenses were first used as simple magnifiers in the 13th century, but it was not till nearly 1600 that the microscope was devised, followed by the telescope a decade or so later. Mirrors, which form an image by reflecting light rays, had already been known for several centuries and were easier to understand. A lens, however, has an advantage over a mirror in that it permits the observer to be on the opposite side from the incoming light.
Microscopes, projectors and enlargers are similar in principle, but they differ in purpose and design. In each, a positive lens forms a real image of a brightly illuminated object. With projectors, the image is caught on a screen; with microscopes, it is viewed through an eyepiece; and with photographic enlargers, the image is projected on light sensitive paper, where it - is recorded in semi-permanent form.
But description of light as traveling along rays is only approximately true; it gave us the simplest way of explaining making an image.
Light and color are so much a part of our lives that we often overlook their fundamental importance to many businesses such as astronomy, optics photography, television and many others.
Telescopes enlarge the image of far-off objects. Two types of telescopes in common use are refracting telescopes and reflecting telescopes. Refracting telescopes are often used as terrestrial (land-use) viewers. They consist of an objective lens, a long tube, and an eyepiece lens. Light rays from an object are refracted through a convex objective lens and form a real image in the tube of the telescope. However, the real image is less than one focal length of the convex eyepiece lens. As a result, the eye of the viewer sees the image of the object as a virtual image, inverted and enlarged. The magnification of a refracting telescope is found by dividing the focal length of the objective lens by the focal length of the eyepiece lens.
A reflecting telescope works in much the same way, but it uses mirrors instead of objective lenses to collect the light rays from an object. The incident light rays enter the telescope's tube and strike a concave mirror at the base of the tube. As the rays are reflected off the base mirror, they strike a mirror in the tube. The newly reflected light rays then converge at a focus in front of the eyepiece and the viewer sees an enlarged image.
Reflecting telescopes can be more powerful than refractors because large mirrors can collect more light than lenses can.
CHECK YOUR UNDERSTANDING
Exercise 1. Answer the questions.
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What do optical instruments use to modify light rays?
What kinds of images can be formed by mirrors and lenses?
What has influence upon the amount of bending?
What is an advantage of a lens over a mirror?
What is similar in microscopes, projectors and enlargers?
What telescopes are used most often in everyday life?
Why can reflecting telescopes be more powerful than refracting ones?
Exercise 2. Topics for discussion.
What do you know about optical prisms?
What can you tell about the use of telescopes?
INCREASE YOUR VOCABULARY
Exercise 1. Read the collocations and translate them.
Intellectual horizons, transparent solids of glass, a predictable size and location, refractive index, curved glass lenses, in semi-permanent form, far-off objects, a convex objective lens, incident light rays.
Exercise 2. Match the synonyms.
Verbs
1
|
2
|
broaden
|
watch
|
bend
|
alter
|
devise
|
widen
|
modify
|
achieve
|
observe
|
curve
|
reach
|
reflect
|
pass
|
invent
|
|
go
|
|
enlarge
|
|
enter
|
|
leave
|
|
travel
|
Nouns
1
|
2
|
aid
|
observer
|
device
|
beam
|
glasses
|
help
|
enlargement
|
fall
|
ray
|
eyepiece
|
viewer
|
spectacles
|
incidence
|
increase
|
|
mechanism
|
|
pupil
|
|
goggles
|
|
side
|
|
speed
|
Exercise 3. State the part of speech of the following words and determine their meaning without using a dictionary.
approximate, to approximate, approximation, approximative, approximately;
most deficient, deficiency;
to emit, emitter, emission, emissive, emissivity;
the use, to use, user, using, usage, useless, useful, usefulness.
Exercise 4.
a) Make up singular-plural pairs.
Phenomenon, spectra, index, spectrum, radius, indices, phenomena, radii.
LANGUAGE ACTIVITY
Exercise 1. Summarize your knowledge of the functions of the verb “to be”. Translate the following.
The microscope has been improved and refined continuously throughout its history, that is practically since the 17th century.
The main characteristics to be refined have always been the resolution and the elimination of aberrations.
The projection lens is to be especially corrected for curvature of field and distortion.
The purpose of the condenser is to concentrate the light coming from the mirror to a point approximately 1.25mm above the surface of its top lens.
The magnifying power of microscopes is being increased from year to year.
Exercise 2. State different meanings of “that (those)”.
тот, этот
что
который
заменитель существительного (that of)
то есть [that is = i.e. (id est - лат.)]
именно, только лишь (it is …that – усилительный оборот)
то, что
It is also worth pointing out that ruby lasers were extensively used in the past for military rangefinders.
Gas lasers are usually excited by electrical means, i.e., pumping is achieved by passing a sufficiently large current through the gas.
One of the most characteristic features of the He-Ne laser is that the output power does not increase monotonically with discharged current but reaches a maximum and thereafter decreases.
Since these pure rotational lasers are relatively less important than the other categories, we shall not discuss them further in the sections that follow.
That metals are good conductors of electricity is known to everybody.
It was the nature of p-n junction that happened to be one of the most difficult things for scientists.
At that time the engineers were testing a new semiconductor for the application in industry.
Semiconductor is a material having an electrical conductivity intermediate between that of metals and insulators.
Output energy and peak power in Q-switched operation are comparable to those obtainable with a Nd:YAG rod of comparable dimensions.
This was the laser scheme that was proposed in the original paper of Schawlow and Townes.
Exercise 3. Summarize your knowledge of Present Simple or Present Continuous. Put the verbs in brackets into the correct tense.
A lens (to be) a piece of glass, plastic or other transparent material curved on one or both sides.
Lenses (to refract) the light rays from an object forming an image.
I (to use, never) my mobile phone if I (to drive).
The walls of the house (to absorb) heat day after day.
The news bulletin (to begin) at 6 p.m.
In spring the days (to grow) longer and the nights (to become) warmer.
Jack often (to go) to the theatre. On Saturday he (to go) to see a new play.
Electromagnetic waves (to travel) in all directions through the Universe.
Some people still think the sun (to go) round the earth.
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