1970-05-07 — Page 26

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170英文中學會考試題預習專欄

堅道英文書院主編

物理科

(二十七)

PHYSICS (27)

Optical Instruments

The purpose of optical instrumente

For an object to be clearly visible, a sufficiently jare ge and bright lange must be formed or focused on the retine at the back of the eye. Lonase and miorrors are arranged to zender visual aids. If an object cannot conveniently be observed because of its location or sina an image of appropriate size may be formed at a more

convenient location by some sort of optical instrumeu və Or an ifage of an object may be formed on a photographic film where it may be preserved by suitable chemical. action. Enay the sya is an optical instrument whose function fe part is to form on the retinal surface an

In general, optical image of the object being observed, Instruments are leness or mirrors or combinations of one of more of each. Sometimes priems are incorportaa because of their ability to change the direction of light rays

Several optical instruments will be described WiYA LÃO aide of diagrams. They arei

B.

Caner

Eye, human --

C. Siopia Magnifying Glass

Compound microscope-

Astronomical Telescopa

Terrestrial Telascopa

The Far Point

報日憍

be seen by

It is the farthest point that an object may b the undidad'sye,

For a normal eye the far point is at infinity, and the near point is taken to be a point 25 on from the sys. This distance is called the least distance of distinct dé vision,

Persistence of vision

The retinal remains stimulated by light energy for at least 1/20 second after its arrival. This renders the ability of the succession of images on a cinema soresp or the scanning of a TV tube to appear smooth, Binocular Vision

When a person viaje an object firat vith his right aya caly and then with the left eye only, two slightly dif ferent views of the object are obtained. When the ob jeat is viewed with bothe eyes simultaneously, a single resultant image is normally seen which has an appearance

This vision is the result of solidarity in relief.

of mental training, and if anything goes wrong with thế brain we see both images sinultaneously and experience diplopia,

The Common. Defects or khe human ay08:

MYOPIA (short-sightedness)

Sympton: can see only objects which are close

distant obiecta are blürred

In the myopic sya parailer 1701qent raya coms to focus in front of the retina. Kvodia is usually due to the fact that

the eye-ball is too long. ((ii) the curvature of the refractive surfaces bas

ing too great, or biii) over-fatigue of the

Correction:

ID

tliary musclea

This defeat is corrected by the use of concave spherical glasses placed in front of the eye to render the necess ary divergence to the parallefirays before they enter the eye,

日七月五年门十九一辆公生九十五洲民售 ***

PRESBYOPIA (aged

As a person becomes older the power or accommodation becomes steadily lens owing to the progressive red- uction in ciliary muscle power, and also poshibly. because the substance of the crystalline lens becomes less able to be moulded with the requirad curvature for near objects. Correction: Bi-focal lenses are, used,

Convex lens will reduce the divergence. of light before it enters the eye,

ABTTOMATISK

Astigmatism is usually due to the fact that the cur- vature of the cornez is greater in one meridian than the others. The result is that there are two foci, which take the form of focal lines at right-angles to one another. The eye tries to see both images simultaneously, with consequent eye-strain, result in headache".

Gorrection%

Problems

The defect is remedied by the use of a suitable combination of a spherical and cylindrical lens placed in front of the eye, so that the focal length length of the eye is rendered the same for both meridiana,

examples

17-1. A myopic eye can see only at 15 cm.wbis in

required to prepare a lens which enables the eye to read a sign at 100 cm. What is the focal length of the lens?

The image formed is a virtual image, the image distance is 15 cm. (negative for virtual) The object distance is 100 cm..

1/100 + 3/(-15) ≈ 1/£

1/fm (15-100)/1500

-85/1500

--17.6 cm

- means that a diverging lens of −17.6. focal length, or 170.176 ar 5.7 dianteza.,qux be used.

nhutter.

Lens

diaphrage

Parallel incident rays come to a focus and cross at Q, giving a blur circle on the retina at R

A person who can see clearly nothing closeET CHAR 60 cm wishes to examine the details of an object 20 cm away. What kind of spectacle lenses and of what power must he use?

Solution

Obviously the person 18 far-sigated, since the normal reading distance for the average eye is approx. 25 cm, This suggests that apedtacles be used of such focal length tha an object diss tance of 20 cm. will correspond to an image distance of −60 cm.

3/2 + 1/v/

• 1/8

1/70 + 1/(-60

1/1

3760

2/60

+30 cm

(converging leas

The photographic camera is essentially a light-tight box with a converging lens suster mounted in a hole in front and a receiving screen for the image mounted at the rear.

The objective lona forse a real image of the object being photographed, This image is made to fall On the Bcreen, which is a filmcof light-sensitive mater 11, whereupon it is preserved by later treatment with chemicals in a precess called development into permane ent imageße

The parts and their functions are tanulated sus

Light-tight boz Black-painted to eliminate any

residue, reflecting raya

Converging lens

Iris diaphrag

Shutter

Pila

The Hunan, Eys

Kovable, to socommodate or to foous a real image on the film a hole of adjuntaħla sise, "for brightness control

a closa mechanism, controls the time, and thus, the energy for which the film is exposed

celluloid with chemical sensitive to light and may be developed to permanent imagen chemically

if the rays had come from the point S the image would be on the retina at R

The concave Lens L countersets the excessive.convergence. The rays. apparently diverge into the eve from S.

HYFERMETROPIA (long-sightedness)

Symptoms.

can see only objecta far away becausS ⠀⠀ of the lack of

convergence,

a person suffering rom simple hypermetropia 18 unable to see close-by objects clearly even with the exercising of his accommodation. adapted to the reception of convergent rays,

Hie eye is Thus parallel incident rays would cons to a focus behind the retina. Causes of this defeati

too short an eyebill

Curvature of the crystalline lens.

Crystalline Le

ПОБКАМ

Cornes

(11)

Crystal Tena

Yellow @po:

(111)

) stiffness of the muscles

Pupil

Iris

iliary Muscle

Uptic narvé

Correction: The curvature of the sys-lens may be

assisted by means of convex Lennan

Answer: The power of the converging lane is

1/0.3 - 3.33 diopters

An eye suffers from 4 dioptres of hypermetropia, and when fully accommodated the nearest distance of distinct vision is 30 in, in frantoøf the eye What glase must be used to make the eye equival- ent to a normal eye for distant ef-drotinet vision, and what will be the new position of tra near point when using the

Focal length of 1D lena = 40 in. Focal length of 4D lens - 10 in.

1/ÿ.

1/x

1/-30

1/2 - 1/10

1/10 1/30 7 inches

HLB Tange or V181on 16 rrom 74 inches to Infinity

The Simple Mamifying Glass

A converging lene forms an upright, virtual, and magni- fjad image of an object if the object lies closer to the lens than the focal point. The sys can focus this. so as to give a bigger image on the retinal than could be given by the unaided eye with the object placed at the mean point. The brain can therfore distinguish more detail.

con.

The human eye operates very much like the camera, verging lens system forms a real image on the retina, which is a film of nerve tissure spreading out over the rear interior of the eyeball, constituting an extension of the optic nerve. The light pattern on the retina produced by the real image activates the nerve ende in the optic cord, which transmite a message do the brain, Thue vision is explained.

The lens of the eye is no attached to muscles, the cil- iary muscles, that it can be squeezed or stretched to change its focal length and thereby give the eye they property of accommodation. The iris controls the mize of the pupil, and thus the amount of light admitted in- to the eyeball.

Accommodation

This is the ability or the eye to alter the focal lengti of the crystalline lens so that the objects at differen -distances can be seen clearly.-

The Near Point.

This the position of the nearest object that can be seen both clearly and without strain by the unaided eye

Rays from a distant point come to a focus behind the retina. The far point is virtual.

Lene L renders additional con vergence and brings the focus

to R

The Compound Microscop

The purpose of the microscope is to produce an enlarged virtual image of a very small but accessible object.

OBJECTIVE

EYE LENS

The diagram brings out the following pointer

(未完轉单第三

EYE