Home PHYSICS TOPIC 4: OPTICAL INSTRUMENT | PHYSICS FORM 3

# TOPIC 4: OPTICAL INSTRUMENT | PHYSICS FORM 3

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Simple Microscope
The Structure of the Simple Microscope
Describe the structure of the simple microscope
A magnifying glass, an ordinary double convex lens with a short focal length, is a simple microscope. The reading lens and hand lens are instruments of this type. When an object is placed nearer such a lens than its principal focus, i.e., within its focal length, an image is produced that is erect and larger than the original object. The image is also virtual; i.e., it cannot be projected on a screen as can a real
image.
The Mode of Action of a Simple Microscope
Describe the mode of action of a simple microscope
The
image formed by magnifying glass or simple microscope is virtual and
erect object place between principal focus (f) and convex lens.
• The normal district vision
• The position of the lens is usually adjusted so that V is about 25cm, which is the shortest distance of distinct vision.
Using the equation of lens (Lens formula).
I/U + I/V = I/F
Adopting the ‘real is positive’ sign convention we obtain:
V = (-Ve) since the image is virtual.
I/U – I/V = I/F
V= 25 –(Normal district vision)
I/U – I/25 =I/F
I/U = I/F + I/25
(I/U)=-1 (25 + F )
25F
U = 25F/F+25
The above formula shows the means of obtaining the distance of object, U.
Magnication (M) of simple microscope
Magnification is the ratio of the image distance to the object distance.
M = Image distance, V
Object distance, U
Hence
M = v/u …………………..(i)
From V = 25cm (distance of district vision)
From U = 25f/(f+25) ……………………… (ii)
Insert eqn (ii) into (i)
M = V/ (25f/(f+25)
M = 25/(25f/f+25)
M = 25/f + 1
Example 1
A
simple microscope with lens of focal length 5cm is used to read
division of a scale 0.5mm in size. How large will the division be seen
through the simple microscope?
Data given
• Focal length, f = 5cm
• Required to find magnification, M
Soln:
From
M = (25/f + 1)
= (25/5+1)
=(5+1)
= 6
The magnification of lens = 6
Let the size of the object be ho and that of the image be hi. Then:
M = h1/H ……………(i)
H1 = 6h
The Height , h = (0.5mm)
H1 = 6 (0.5mm)
HI = 3mm
Hence, each division will appear to have a size of 3.0mm viewed through the simple microscope.
A Simple Microscope
Construct a simple microscope
Parts of simple microscope
The Structure of the Human Eye
Describe the structure of the human eye
The
eyeball approximately spherical in shape.The wall of this sphere
consist of two layers, the outer layer or sclera and the inner layer or
choroid.The front portion of the SCLERA FORMS A TRANSPARENT CURVED
section called the camera.The choroid layer is balance in order to
prevent internal reflection and also to protect the light sensitive
parts of the eye.
The
aqueous and vitreous hum our are jelly – like substance that fills the
spaces within the eyeball.The aqueous humour is the salt solution of
refractive index n, 1.38.Vitrous hurmour is a watery , Jelly substance
of refractive index 1.34.Behind the cornea there is a colored diagram
called the iris.
The
iris has the central hole called the pupil. The iris contains muscles
which control the size of the pupil. The size of the pupil decreased in
the bright light and increased in the dim light.
Behind
the pupil and there is a crystalline lens held in position by
suspensory ligaments that are attached to the choroid layer.Near the
suspensory ligaments are the ciliary muscles.The function of the
suspensor ligaments there are the cilliary muscles.
The
function of cillary muscles is to control the thickness of the lens.
The lens become thick when the ciliary muscles contract and thin when
the ciliary muscles are relaxed.
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the back of the eye there is a retina (This is the part of the eye
which is sensitive to light).Image formed is inverted formed on the
Retina ( This is the part of the eye which is sensitive to light.)
Image
formed is inverted formed on the retina by successive refraction of
light at the corner, the aqueous hurmour the crystalline lens and the
Vitreous hurmour.Electrical signals are then transmitted to the Brain
through the topic nerve. Finally, the brain interprets these signals.
Accommodation Power of the Human Eye
Explain accommodation power of the human eye
Accommodation is the process whereby the eye alters its focal length in order to form images of objects at different distances.
(Thickening or Thinning of the lens causes a change in its focal length).
The
thickening or thinning of the crystalline lens is made possible by the
action of the ciliary muscles.To view neare object t, ciliuary muscles
contract, this makes the lens thicker.
In
the relaxed state of ciliary muscles, the crystalline lens become
thinner and enables the eye to see (view) distant objects. The farthest
point which can be seen clearly is called the far point of the eye and
the nearest point is called the near point of the eye.
The
corresponding distance from these points to the eye are referred to as
the maximum and least distance of district vision respectively.A normal
eye (i.e. without defects of vision) has a far point at infinity and
near point at a distance of 25cm from the eye.Structure of lens “ view
distant object”
The Defects of the Human Eye
Identify the defects of the human eye
Myopia or near-sightedness
• This defect causes person to see near object clearly while distant objects are not seen clearly.
• The strength of the cornea and the eye lens combination is too great even when muscles of the eye are completely relaxed.
• The focal length of the cornea and the eye – lens combination is always less than the distance to the retina.
• Images
of distant object are formed in front of the retina even when eye is
totally relaxed. However, an object that is closer can be brought into
focus.
• In this situation the focal length of the cornea and the
eye lens is so short that objects closer than the conventional (near
point of 25cm) can be brought into focus. That’s why this condition is
called Short sightedness (near sightedness).
• Since the problem
is that the strength of the eye – lens and the cornea combination is too
great, the solution is to provide eye glasses (or contract lenses) with
negative lens.
• The negative lens weakens the strength of the
cornea and eye – lens just enough so that the resulting focal length
when the eye muscles are relaxed matches the distance back to the retina
so that distant images are now in focused.
• The eye glass lenses are negative lenses that means they are thinner in the middle than at the edges.
• It
is easy to identify this kind of eye glass lenses since acting by
themselves they do not form a real image of an object at any distance.
Hyperopia or far-sightedness
• This defect causes a person to see distant objects only and short-distance objects are not seen clearly.
• In
the person with this condition, the strength of the cornea and the
eye-lens combination is too weak when the eye muscles are totally
relaxed. So the image of a distant object is formed behind the retina.
• The
solution in the opposite of myopia. Victims should wear positive eye
lenses which strengthen the corner and the eye lens just enough so that
the resulting focal length when the eye is relaxed matches the distance
to the back of the retina.
Astigmatism
• This
occurs when the focal length for the cornea and the eye’s lens for an
object oriented in some direction is not the same as for another located
in a perpendicular direction.
• The eye can not bring the
vertical and horizontal lines in a ‘+’ symbol in sharp focus at the same
time. (The axis of differing focal length need not be exactly
horizontal and vertical).
• The problem is that the cornea of the
eye lens is not symmetrical. The solution is to use eye glasses whose
lenses are not symmetrical in a complementary way.
• The cylindrical lens may be combined with an additional positive or negative lenses.
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</xml><![endif]–> Decreased accommodation

• This condition typically occurs in middle-aged people.
• The eye muscles gradually weaken with age, so that the range or accommodation is decreased.
• People with this condition cannot bring both near objects and far objects into focus.
• The
weakening of the eye muscles often causes the focal length of the eye
lens to increase as well so that many people of middle age tend to
become far sighted.
• Since the problem is adequate accommodation, no single lens can correct it and people with this problem usual needs bifocals.
• Bifocals are glasses with two different lens strengths, one for near and one for distant objects.
• The usual arrangement is that the bottom half of the lens is the near strength and the top half is the far strength.
The Correction of the Defects of Human Eye
Describe the correction of the defects of human eye
Myopia
is common name for impaired vision in which a person sees near objects
clearly while distant objects appear blurred. In such a defective eye,
the image of a distantobject is formed in front of the retina and not at
the retina itself. Consequently, a nearsighted person cannot focus
clearly on an object farther away thanthe far point for the defective
eye.
This
defect arises because the power of the eye is too great due to the
decrease in focal length of the crystalline lens. This may arise due to
either
1. excessive curvature of the cornea, or
2. elongation of the eyeball.
Correction:Thisdefectcan becorrectedby using aconcave (diverging) lens. A concave lens of appropriate power or focal length is able to bring the image of the object back on the retina itself.
Farsightedness,
also called hypermetropia, common name for a defect in vision in which a
person sees near objects with blurred vision, while distant objects
appear in sharp focus. In this case, the image is formed behind the
retina.
This defect arises because either
1. the focal length of the eyelens is too great, or
2. the
eyeball becomes too short, so that light rays from the nearby object,
say at point N, cannot be brought to focus on the retina to give a
distinct image.
Correction:This defect can be corrected by using aconvex(converging) lensof
appropriate focal length. When the object is at N’, the eye exerts its
maximum power of accommodation. Eyeglasses with converginglenses supply
the additional focussing power required for forming the image on the
retina.
The Human Eye and the Lens Camera
Compare the human eye and the lens camera
The camera
1. The eye and the camera has a have a convex lens which form a real and inverted image of an object.
2. The
eye and the camera are blackened inside to prevent internal reflection.
Rays of light which are not received on the retina or camera film are
absorbed by the choroid layer of the eye or the black surface inside the
camera.
3. The eye can regulate the amount of light that passes
through the crystalline lens by using pupil while in a camera the
diaphragm regulates light.
4. In the eye the image is formed in the retina while in the camera the image is formed on the photographic plate.
5. The
eye can change the focal length of its lens by the contraction and
relaxation of the ciliary muscles. In this way the eye can focus objects
at different distance. In a camera objects at different distance are
focused on by moving the lens forwards and backwards.