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Файл:English for Optics Students. Английский для студентов, изучающих оптику. Учебное пособие
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Text 3
Radio interferometry
The Very Large Array, an interfero-
metric array formed from many smaller
telescopes, like many larger radio telescopes.
In 1946, a technique called astronomical interferometry was developed. Astronomical radio interferometers usually consist either of arrays of parabolic dishes or
two-dimensional arrays of omni-directional
antennas. All of the telescopes in the array are widely separated and are
usually connected together using coaxial cable,
waveguide, optical fiber, or other type of transmission line. Interferometry increases the total signal collected, but its primary purpose is to vastly
increase the resolution through a process called Aperture synthesis. This
technique works by superposing (interfering) the signal waves from the different telescopes on the principle that waves that coincide with the same
phase will add to each other while two waves that have opposite phases will
cancel each other out. This creates a combined telescope that is equivalent
in resolution (though not in sensitivity) to a single antenna whose diameter
is equal to the spacing of the antennas furthest apart in the array.
Acoustic interferometry
An acoustic interferometer is an instrument for measuring the physical
characteristics of sound wave in a gas or liquid. It may be used to measure
velocity, wavelength, absorption, or impedance. A vibrating crystal creates
the ultrasonic waves that are radiated into the medium. The waves strike a
reflector placed parallel to the crystal. The waves are then reflected back to
the source and measured.
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GLOSSARY
Unit Optics
angle
behavior
diffraction
electromagnetic radia-
tion
incidence
infrared
interaction
interference
conventional
converge
curved mirrors
fraction
frequency
hard X-rays
conservation of energy
conservation of momentum
normal
oblique angle
lens
particle-like
phenomena
photons
properties
quantum optics
radiation
ray
Unit Reflection
interface
magnify
media
metallic coating
propagation
refractive index
Unit Refraction
phase velocity
ratio
refraction
transmission medium
62
reflect
relevant
sine
ultraviolet
wave
wavelength
wave-like
X-ray
specular
thin-film optics
VHF
visible
wavefront
Unit Physical Optics
aperture
approximation
coherence
concave
convex
creeping wave
electromagnetic waves
light disturbance
lossy surfaces
orthogonal
perturbation
quantum mechanical effects
scatter
Unit Superposition and Interference
amplitude
constructive interference
correlate
crest
destructive interference
displacement
magnitude
odd multiple
plane wave
pointwise sum
superposition
trough
Unit Superposition and Interference
amplitude
constructive interference
correlate
crest
destructive interference
displacement
magnitude
odd multiple
plane wave
pointwise sum
superposition
trough
Additional tasks to Unit Superposition and Interference
Luminous
double-slit
gap
path lengths
63
path difference
fringe
KEYS
Keys to Unit Optics
Task 2.
1. How old is optics?
2. What is the main object of optics?
3. What devices are used in optics?
4. What areas of life is optics applied to?
5. What sciences is optics based on?
Task 4. 1-F, 2-K, 3-J, 4-D, 5-A, 6-C, 7-E, 8-B, 9-G, 10-I, 11-H
Task 6. 1-E, 2-I, 3-F, 4-H, 5-B, 6-G, 7-C, 8-A
Task 9. account for, reflect from, be followed by, a model of, lead to,
depend on, progress in, relevant to, deal with, divide into, be governed by.
Task 14
Verb Noun (doer) Noun Adjective
detective
descriptive
including
applicable
constructive
exhibitive
complete, completed
physical
travelling
detect
describe
include
apply
construct
exhibit
complete
–
travel
detector
descriptor
–
applicant
constructor
exhibitor
–
physicist
traveler
detection
description
inclusion
application
construction
exhibition
completion
physics
travel
64
Keys to Additional tasks to Unit Optics
Task 1. Explain the figures
a) b)
a) Diagram of specular reflection:
A light ray PO strikes a vertical mirror at point O, and the reflected ray
is OQ. By projecting an imaginary line through point O perpendicular to the
mirror, known as the normal, we can measure the angle of incidence,
and the angle of reflection,
. The law of reflection states that
r
in other words, the angle of incidence equals the angle of reflection.
Task 3
1-E, 2-C, 3-F, 4-A, 5-G, 6-B, 7-D
Task 6.
1 deployed, 2 availability, 3 wirelessly, 4 considerations, 5 over, 6 optical, 7 error rate, 8 addresses
, or
ir
i
Task 9. 4, 3, 1, 2
Task 10.
Optics in aviation 1
The earliest forms of signaling 4
Marine application of optics 3
An advanced alternative to conventional means of communication 2
65
Keys to unit Reflection
Task 3. 1-D, 2-K, 3-B, 4-H, 5-J, 6-I, 7-F, 8-C, 9-E, 10-L, 11-G, 12-A
Task 7. 1-N, 2-F, 3-F, 4-N, 5-F, 6-T, 7-T, 8-T, 9-N, 10-T
Task 9. Double reflection: The sun is reflected in the water, which is re-
flected in the paddle.
Keys to Additional task to Unit Reflection
Task 2. A-9, B-6, C-2, D-8, E-3, F-1, G-7, H-5
Task 7. 1-B, 2-C, 3-A, 4-E, 5-D, 6-G, 7-F
Keys to Unit Refraction
Task 2. 1-F, 2-G, 3-E, 4-H, 5-B, 6-C, 7-E, 8-A.
Task 6.
Refraction is the change in direction of propagation of a wave due to a
change in its transmission medium.
The phenomenon is explained by the conservation of energy and conservation of momentum. Due to change of medium, the phase velocity of
the wave is changed but its frequency remains constant. This is most
commonly observed when a wave passes from one medium to another at
any angle other than 0° from the normal. Refraction of light is the most
commonly observed phenomenon, but any type of wave can refract when it
interacts with a medium, for example when sound waves pass from one
medium into another or when water waves move into water of a different
depth. Refraction is described by Snell's law, which states that for a given
pair of media and a wave with a single frequency, the ratio of the sines of
the angle of incidence θ
of phase velocities (v
ratio of the indices of refraction (n
and angle of refraction θ2 is equivalent to the ratio
1
/ v2) in the two media, or equivalently, to the opposite
1
/ n1):
2
sin
sin
vn
112
vn
221
.
In general, the incident wave is partially refracted and partially reflected; the details of this behavior are described by the Fresnel equations.
66
Refraction of light at the interface between two media of different re-
fractive indices, with
nn
21
. Since
the phase velocity is lower in the second medium
refraction θ
incidence
2
1
()vv
21
is less than the angle of
; that is, the ray in the
, the angle of
higher-index medium is closer to the
normal.
In optics, refraction is a phenomenon that often occurs when waves
travel from a medium with a given refractive index to a medium with another at an oblique angle. At the boundary between the media, the wave's
phase velocity is altered, usually causing a change in direction. Its wavelength increases or decreases but its frequency remains constant. For example, a light ray will refract as it enters and leaves glass, assuming there is a
change in refractive index. A ray traveling along the normal (perpendicular
to the boundary) will change speed, but not direction. Refraction still occurs
in this case. Understanding of this concept led to the invention of lenses and
the refracting telescope.
Task 9. Why does the straw look to be broken?
The straw appears to be broken because of the difference between the
angle at which light from it strikes the vertical edge of the glass versus the
horizontal surface of the water.
Keys to Additional tasks to Unit Refraction
Task 1. Describe the picture.
Why does the pencil seem to be longer
than it is?
Why do the light waves change their
direction?
An object (in this case a pencil) part
immersed in water looks bent due to refraction: the light waves from X change
direction and so seem to originate at Y. (More accurately, for any angle of
view, Y should be vertically above X, and the pencil should appear shorter,
not longer as shown.)
67
Refraction can be seen when looking into a bowl of water. Air has a refractive index of about 1.0003, and water has a refractive index of about
1.3330. If a person looks at a straight object, such as a pencil or straw,
which is placed at a slant, partially in the water, the object appears to bend
at the water's surface. This is due to the bending of light rays as they move
from the water to the air. Once the rays reach the eye, the eye traces them
back as straight lines (lines of sight). The lines of sight (shown as dashed
lines) intersect at a higher position than where the actual rays originated.
This causes the pencil to appear higher and the water to appear shallower
than it really is.
Keys to unit Physical Optics
Task 2. 1-F, 2-J, 3-I, 4-C, 5-D,G, 6-K, 7-A, 8-D,G, 9-B, 10-H, 11-E
Task 4. 1 is considered to propagate, 2 are not explained, 3 is approxi-
mately, 4 is also often applied, 5 can be used, 6 believed, 7 was predicted,
8 propagate, 9 are now generally treated, 10 is not valid, 11 tends not to include, 12 is used to estimate, 13 is then obtained
Task 5.
6 A Scattering
5 B Approximation in radio
Extra C The background of the subject
1 D Wavelengths
7 E The problem of inaccuracy
2 F The nature of light waves
4 G The subject of physical optics
3 H Some branches of physics
Keys to Additional tasks to Unit Physical Optics
Task 1. 1-C, 2-D, 3-F, 4-A, 5-B, 6-E
Task 5. 1-F, 2-D, 3-E, 4-B, 5-C, 6-A
Task 11. 1-F, 2-G, 3-A, 4-D, 5-K, 6-E, 7-J, 8-B, 9-H, 10-I, 11-C
Task 16. 1 analysis, 2 to deal with, 3 gravitation, 4 exactly, 5 celestial, 6
under, 7 interaction
68
Keys to Unit Interference
Task 1. 1-D, 2-I, 3-G, 4-A, 5-F, 6-H, 7-E, 8-C, 9-L, 10-B, 11-J, 12-K
Task 4. 1-B, 2-D, 3-A, 4-extra
Task 6. 1-E, 2- D, 3-B, 4-F, 5-H, 6-G, 7-A, 8-C
Task 8.
1. A simple form of interference pattern is obtained if two plane waves
of the same frequency intersect at an angle. Interference is essentially an
energy redistribution process. The energy which is lost at the destructive
interference is regained at the constructive interference. One wave is travelling horizontally, and the other is travelling downwards at an angle θ to the
first wave. Assuming that the two waves are in phase at the point B, then the
relative phase changes along the x-axis. The phase difference at the point A
is given by a
22sindx
2. The figure on the left shows interference fringes in overlapping plane
waves.
Task 10. 1–B, 2–A, 3–D, 4–C, 5–F, 6-E
Task 11. 1. A point source produces a spherical wave. The figure to the
right shows interference between two spherical waves.
2. The wavelength increases from top to bottom.
3. The distance between the sources increases from left to right.
4. If the light from two point sources overlaps, the interference pattern
maps out the way in which the phase difference between the two waves varies in space. This depends on the wavelength and on the separation of the
point sources.
Keys to additional tasks to unit Superposition
and Interference
Task 1. 1-D, 2-I, 3-G, 4-A, 5-F, 6-H, 7-E, 8-C, 9-L, 10-B, 11-J, 12-K
Task 4. 1-B, 2-D, 3-A
69
Task 6. 1-E, 2-D, 3-B, 4-F, 5-H, 6-G, 7-A, 8-C
Task 10. 1-B, 2-A, 3-D, 4-C, 5-F, 6-E
Keys to Additional tasks to Unit Superposition
and Interference
Task 1. 1-E, 2-D, 3-G-, 4-A, 5-B, 6-C, 7-F
Keys to Unit Diffraction and Optical Resolution
Task 1. 1-D, 2-H, 3-G, 4-E, 5-A, 6-C, 7-F, 8-B
Task 5. 1-B, 2-G, 3-E, 4-C, 5-F, 6-D
Task 6. 1-F, 2-F, 3-F, 4-F, 5-F, 6-T, 7-T, 8-F, 9-F, 10-F
Task 11. You can see solar glory at the steam from hot springs. A glory
is an optical phenomenon produced by light backscattered (a combination of
diffraction, reflection and refraction) towards its source by a cloud of uniformly sized water droplets.
Task 12. closely spaced tracks
produced by light
be extended
obstructing object
uniformly sized
encounters an obstacle
geometrical shadow
characteristic behaviors
be a consequence of the fact
near the edges
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