The instrument that works based on the princi ple of reflaction and
refraction is called an optical in- strument.
Optical instrument helps peoples to see nor mally avery small object like viruses and bacteria; and to be able to see a very far
object like the moon, stars, and planets. The main parts of optical instruments
are mirrors and lenses.
In this chapter you will
learn about optical ins truments among others are eye, camera, magnifying
glass, microscope, telescope, and proyektor.
1. Eye.
Our eyes are included into
optical instruments because the eyes can do light refraction done by the eye
lens. While our eyes consist of several parts, those are cornea, iris, pupil,
eye lens, retina, and liquid
aqueous humor. Consider the following figure below !
a. The part of the
eye.
1). Cornea.
Cornea is the outer part
of eye ball
of thin layer which is clear
and can be penetrated by the light. Cor- nea functions in receiving
and`transmitting the light, also to protect the eye lens from feathers.
2). Iris.
Iris is a membrane in
front of the eye lens that forms a circular chink. Iris functions in giving a colour to the eye and arranging the intensity of
light entering the eye.
3). Pupil.
Pupil is the circular
chink formed by iris. The wide of pupil is controlled by iris based on the
intensi ty of light entering the eye. At dark or dim condition- (the light
intensity is small), the iris relaxes so that pu pil enlarged. As a result,
more light enters to the eyes. At bright condition ( the light intensity is
large ), the iris contracts so that pupil
becomes smaller(decreased) As a result, the light enterd to the eye decreases.
4). Eye lens.
The eye lens is made of
transparant material, fibered, and elastic,also convex in shape.The eye lens
functions in constructing the image of an object. The image formed by the eye
lens similar to the image for med by a convec lens, i.e. real, upside down, and smaller
in properties.
5.) Retina.
Retina is the back part
surface of the eye. It is functions as a screen to catch the image formed by
eye lens. Retina consists of yellow grains, which are very responsive to the
light.
6). Eye Nerve.
The eye nerve functions in
continuing the signals received by the retina to the brain. The brain translates the signals so that we can see the objact in
upright ( it is not upside down as caught by the retina ).
b. The
Accomodition Power of Eye.
An object can be seen
clearly bacause
its image formed by the eye
lens falss exactly at the retina.Why does the image of an object fall exactly
at the retina al though its distance is changed ? Because the eye lens is
elastic; it can be thin or thick, its focal length can change according to the
need, therefore the image of the object can be arranged to fall exactly at the
retina.
The ability of the eye lens, to
thicken or to thin in order to produce an image
exactly at the retina, is called the accomodation power of the eye. When the eye ac comodates maximally, the eye lens
thickens ( more convex ); as a result, the focal lengthof the eye lens in
creases. On
the other hand, when the eye accomodates minimally, the eye lens becomes thin; as a
result, the focal length of the eye lens decreases.
Pay-attention the figure below !
a. The eye lens
thickens if seeing a near object.
b. The eye lens becomes thin if seeing a far
objec
Because of the acommoda- tion power of the
eye, the eye has a near point (punc tum proximum) and a far point (punctum remotum). A near point
is the nearest point of an object to the eye that still can be seen clearly by the eye,
which the
eye accommodates maxi- mally.
A far point is the furthest point of the object to
the eye that still can be seen clearly by the eye, in which the eye does not accommodate. So the eye can see an
object clearly
if the object is placed
between the near point and the far point. A normal eye has a near point at a
distance of 25 cm and a far point at a distance of unlimited.
c. Eye Aberration.
If the accommodation power of the eye lens
decreases, then it has suffered an eye aberration.
1). Nearsighted Eye ( Myopia
)
People who suffers
nearsighted eye cannot see clearly the far objects. That aberration happens
becau se the image of the object falls in front of the retina.
Nearsighted can be helped by using glasses with a con cave lens ( a negative lens ).
Pay-attention the figure below !
a. For myopic suf- ferer, the image
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of the object -
falls in front of the retina.
b. A concave lens /a negative lens can help a myo pic sufferer.
2). Farsighted Eye ( Hypermetrophy
)
People
who suffers farsighted eye cannot see clearly the near objects. It happens
because the image falls behind the retina. Although the eye has accom- modated
maximally, the image of the object still falls behind the retina.
A farsighted can be helped by using glasses with a convex lens ( a positive lens ).
Pay-attention the figure below !
a. For hypermetrophy suf ferer, the image of the object falls behind the
retina.
b. A
convex lens ( positi- ve lens) can help the sufferer of hypermetro phy.
3). The Old Eye ( Presbyopia ).
This
eye aberration occurs because the increa- sing age makes the accommodation
power of the eye decrease. It is called presbyopia. The sufferer of an old eye
cannot see clearly the near and far object. In an old eye aberration, the near
point and the far point have changed. To be able to see the near and far object clearly, it must be helped with bifocal
glasses, i.e.glas ses with double function (to see near distance and far
distance ).
Example question :
1.
The furthest distance that a person can still see cle- arly is 75 cm. What is
the power of glasses he must use in order to
see a far object normally ?
Solution :
Given : So = ~
Si = -75 cm (symbol – because the image
that is formed by the convex lens is virtual).
Asked : P
Answer : 
= 0 - 
= 0 - 
f = - 75 cm = 
The Power of lens: P = 
=
=
So, the lens power of the glasses is 
2. A sufferer of nearsighted eye has a near point
of 40 cm. If he wants to read normally (at the distan- ce of 25 cm),then what is the
lens power of the glasses he
must use ?
Solution :
Given
: So = 25 cm
Si = - 40 cm, ( the
image is virtual )
Asked
: P
Answered : 
f = 
The power of lens : P = 
So, the lens power of the glass is 1.5
Diopters.
3. A porson can only see an object clearly in
front of his eyes at the furthers
distance of 100 cm.
What is the person’s power of the
glasses ? -1
Sn
= -Si
Sn
= 100cm, Si = -1m
4. The near point of the person is at a distance
of 120 cm in
front of his eyes, to be able what is the power of the glasses he must use ? -2,5
4). Astigmatism.
People who suffer astigmatism
cannot see clearly vertical and horizontal lines simultaneously This is caused
of curves of the eye cornea of astig- matism suffer is not perfect anymore.
This sufferer of astigmatism can be helped by using cylindrical lens glasses.
2. The Camera.
Same as the eye,
the
object observed by camera is
located
infront of lens that is in
front
of 2F2(So>2f)
and
image of the object
formed
is real image,
inversed, and
minimized.
In
the camera, the recor-
ded
object its image is caught
by
a film. This film functions as
screen
to catch image formed by the convex lens.
Film in the camera can record image
formed, because in the film there is chemical substance layer which is
sensitive to the light. Chemical substance layer in the film will change
according to the light it receives.
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Commonly camera consist of convex
lens, to focuse real image for always falls on screen of image catcher, diaphragm that has
function as iris in the eyes,
aperture has function as pupil in the eyes, and screen of image catcher.
3. Magnifying Glass.
Magnifying Glass is an
optical instrument of con- vex
lens which is used to see small things so can be seen larger, clear and not
inversed. For example to re- ad small article or seeing small figure/ objects.
To
observe an object by using the magnifying glass, the object observed must
lie between center point of
the convex lens (O) and focus point of the lens (F), so the image formed is
virtual, upright,en larged, and located behind the lens.When the image of the
object falls at the near point of the eye, the eye accommodates maximally. To
make the eyes do not accommodate, the object must be located at the focal point
of the lens. Accordingly, the ray enters to the eye is a parallel ray.
The
magnification of the image (M) formed by a magnifying glass is formulated as
follows :
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a. The figure when the eye accommodates maximally.
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● ● ● ●
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a.
The figure when the eye does not accommodate
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(accommodates minimally).
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● ● ● ●
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1. When the
eye doesn’t accommodate
(accommodates minimally), then :
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2. When the
eye accommodates maximally, then :
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where : Sn = the near point
f = the focal length of the magnifying glsas
M = the magnification of the
image
Example :
1.A book with small writings is located of 5 cm infront of a magnifying
glass which has focus distance of 7 cm. What is the i- mage magnification
formed by the magni- fying glass ?
Solution, Given : So = 5 cm and f = 7 cm
Asked : M = .........?
Answer :
3.5 x
So, the image
madnification formed is .......... times.
2. A magnifying glass with
power of lens 25 Diopters is used
to observe an object of size 2 mm. What is :
a.Magnification by the magnifying glass for the eyes with no accommodation ?
b.Magnification by the magnifying glass
for the eyes with maximum
accommodation ?
c. Height of image when the eyes has maximum
accommodation ?
Given : P = 25
Diopters f = 
and ho = 2
mm
and ho = 2
mm
Asked : a. M when the eyes don’t accommodation ?
b. M when the eyes accommodate maximum
c. hi = ........ ?
Answer: a.
M =
b. M = 7.25
c. M = 14.5 mm
4. Microscope.
Microscope is an optical instrument which con- sist of two pieces of convex lens that
is used to ob- serve very small objects so seen larger and clearer.
Parts of the micros-
cope can be seen at
the figure biside.
The concave mirror
functions in directing
the light to the object
observed.
The convex lens close
to the observed object
is called the objective
lens, while the
con-
vex lens close to eye is
called the ocular lens.
The objective lens forms
the image of the observed object,which properties
are real, upside down, and enlarged. To get such image, the object must be
positioned between the focal point and the center point of the objective lens
curvature.
The
real image formed by the objective lens becomes the object for the ocular
lens.The ocular lens functions as magnifyinr glass, if the eye accommoda- tes
maximally, then the object is located between the center point of the ocular
lens (Ooc) and the focal point of the ocular lens. If the eye doesnot accommodate, the object is located at the
focal point of the ocular lens
Pay-attention the figure below !
a. The image
construction in microscope for maximum
accommodated eye.
b. The image
construction in microscope for the
eye –
that does not accommodate.
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The microscope
magnification is the multiplication of
the objective lens magnification
and the ocular lens mag-
nification,
and it is formulated as follows :
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Because the ocular lens of a
microscope has a
function as a magnifying glass,
the magnification of the
microscope is
formulated as follows :
1. When the
eye accommodates maximally :
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To make the eye accommodates maximally, the distance of the objective
lens and the ocular lens is formu-
lated as
follows :
2. When the
eye does not accommodate :
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In order to make the eye does not
accommodate,the distance of the objective lens and the ocular lens is :
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Where :
Mmic = the magnification of the microscope
Mobj. = the magnification of the objective lens
Moc. = the magnification of the ocular lens
Sn = the near point of eye
fobj. = the focal length of the objective lens
foc. =
the focal length of the ocular lens
Si obj. = the distance of the image by objective lens
S0obj. = the distance of the object in front of the ob
jective lens.
d =
the distance of the objective lens and the
ocular lens.
5. Telescope.
Telescope is one of the optical instruments that is used
to observe objects located very far so appears nearer and clearer.
There
are two kinds of the telescope, i.e. a re- fraction telescope and a reflection
telescope. The dif- ference of their operational manners is that the light received
by the refraction telescope only undergoes re fraction, while the light
received by the reflection te – lescope undergoes reflection and refraction.
a. The Refraction
Telescope.
There
are 4 kinds of refraction telescope, i.e. :
1. the astronomical telescope or the star
telescope.
2. the terrestrial te lescope.
3. the theatre teles cope or Galileo telescope.
4. the prism telescope or binocular.
1.
The Astronomical Telescope.
The
star telescope or the astronomical telesco- pe is an optical instrument used to
observe the sky ob
jects like
stars, moon, and planets, so that we can see them nearly and clearly.
The astronomical telescope
consists of 2 convex
lenses. They function as an objective lens and ocular lens ,
in which the focal length of the objective lens is larger than the focal length of the ocular lens ( fob
> fok ).
The operational manner of the astronomical
telescope.
The
images of the very far sky objects fell at the focal pint of the objective lens
are real and upside down. To make the eye not easily gets terid, the obser
vation is conducted in a condition where the eye does not accommodate. Accordingly, the image of the object formed by the objective lens
becomes the object to the ocular lens
, and is found at the focal point of the ocu- lar lens. As a result, the focal point of the objective lens is pinched together to the
focal point of the ocular lens as shown at
the figure above.
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The magnification
of the astronomical telescope is for mulated as follows :
Thedistance ( d ) of the ocular lens and the objective lens :
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2.
The Terrestrial Telescope.
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The terrestrial Telescope is an optical instrument to observe any objects
in the earth which positions are
so far from the observer, so that it looks closer and clearer.
The terrestrial telesco pe consists of 3 con- vex lenses;
i.e. as an objective lens, an o- cular lens,and a reser ved lens. The reser-
ved lens only functi- ons in changing the position of the object image from
upside – down into upright.
The terrestrial telescope.
The
operational manner of the terrestrial telescope. eye
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The magnification of the terrestrial
telescope is formulated as follows :
The distance of the ocular lens to the objective
lens
( the length of the telescope ) is :
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3.
Galileo Telescope.
The function of Galileo
telescope is similar to –
terrestrial telescope.
Galileo (a physicist) dis
covered a method to shor-
ten the size of the terres –
trial telescope by elimina-
ting the reserved lens and
replacing the ocular lens-
from a convex lens into a
concave lens.
Thus, the object does not
look upside down but up-
right as its real position.
The
figure of the telescope was used by galileo.
4.The PrismTelescop ( Binocular ).
To
make the image of the object does not look upside down, the prism telescope is
used a couple of the right angled prism. On those couple of the prisms, there happens
four times of total reflection. The prism telescope consists of a convex lens as the
objective lens, a couple of
right angled prisms as the image reserved, and a convex lens as the ocular lens
as shown the figu re below : a. Parts of the prism telescope and b total re
flection in the prism.
The
advantages of prism telescope compare the to
terrestrial telescope or Galileo telescope are that its size is more practice
to be carried away everywhere, and the image of the object looks clearer
because the light beam from the object undergoes total reflection by the prism.
5.
Periscope.
A periscope is a placed at the sub marine
to see the the objects on the sea level. This te- lescope almost si- milar with the prism telescope, i.e. has
two convex lenses and a
couple of prism, but the light only undergoes total reflection twice as seen
the figure above.
b. Reflecting
Telescope.
A reflecting teles cope is used to o- serve the sky ob- jects. It consists of a concave mirror, a flat mirror, and a
convex lens.
The concave mirror functi ons in collecting the beam of light come from the sky objects. Before those light
beam collected at the focal point of the concave mirror it is first reflected by
a flat mirror, which is positioned between the center point (O) and the focal
point(Fob) of the concave mirror.
Afterward, the light is reflected into the ocular lens, which is in the
shape of a convex lens as shown at the figure above.
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The concave mirror is used as a
substitution for the objective lens, because the concave miror is easier to
make, cheaper, the light gets no colour dis persion in it, and in the same size
a mirror is lighter than a lens.
Equation example :
1. The power
of the objective lens of an astronomical telescope is + 2 Diopter, while the power of
its ocu- lar lens is +20 Diopter. What is the magnification and the length of
the telescope, if the eye of the observer does not accommodate ?
M = 10
D = 55cm
2. A terrestrial telescope with a focal length of the objec- tive lens
of 30cm, a focal length of the reserved lens of 2cm, and a focal length of
ocular lens of 2.5cm is used to see a far object. Calculate the magnification
and the lenght of the terrestrial telescope if the eye does not accommodate !
M = 12
D = 40.5 cm
6. Projector.
Projector is an optical
instrument functioned in projecting an object such as picture to the screen
Hence, the image of the object (picture) looks bigger and clearer on the
screen.
a. Episcope.
object/picture The operational manner of episcope
Episcope is a projector
functioned in projec – ting untransparent pictures like photograph, poster,
brocure, and other printing pictures which are not transparent.
The light is radiated by
a light
source collected using a concave mirror and directed into the picture. That light is reflacted
by the picture to the flat mir- ror. Then, the flat mirror directed the light
to the convex lens, so that it is formed a real, bigger, and upside down image.
In order to make the image is not upside down, the position of the picture is
reser ved, so that the image of the picture caught at the screen is in upright
state.
b. Diascope.
Diascope is a
projector functioned in projecting transparent pictures like slide and film.
1) Slide Projector.
The
operational manner of a projector slide
The working principle of a slide projector is as
follows :
The light radiated by
the lamp is collected by a concave mirror. Then,it is transmitted into a con
denser lens. The condenser consists of two or three convex lenses. The function
of the condenser lens is to form a real, bigger, and upside down image. After
ward, that real image is caught by screen. To make the image of the slide is
not upside down, the slide must be put in reserved position. Slide is the
result picture of a photographic activity using a positive film called diapositive. Slide is
especially to be projected at a screen using a projector.
2. Film Projector.
In a film projec tor, the substitution of
the slide mounting is faster.
Hence, the pictures pro- jected on the screen look as if they are living.
3. Overhead Projector.
In an overhead projector, 2 flat mirrors are
placed in such a way that makes an overhead -
projector is more practice Overhead
projector.
than a slide projector.
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