PHYSICS || HUMAN EYE

The Human Eye

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.

<!--[endif]-->At 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.

<!-- [if gte mso 9]><xml> Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 </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.

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PHYSICS || LENS CAMERA

The Lense Camera

The Structure of the Lens Camera

Describe the structure of the lens camera

Lens camera is an instrument which produces an image of object on the screen using light. The basic physical principle of all camera is the same in spite of the variation in the design of cameras.

The optical system of the camera are very similar to that of the lantern projector but with the direction of light reversed.The converging lens forms a real image of the object to be photographed.(This image is diminished (smaller than the object and inverted)

<!--[endif]-->The lens can be moved back and forward with the help of focusing any so that objects at different distances can be brought to the focus.A forced image is locate on the film or plate when the shuttled is open for a suitable amount of time as determined by the shutter speed.

<!--[endif]-->Light enters the camera Box and makes a picture of the object on the film “( The film is sensitive to light)

<!--[endif]-->The camera is equipped with a diagram or light entering the camera.It ensures that is incident centrally on the lens so that the distortion of the image formed is reduced

The Mode of Action of the Lens Camera

Describe the mode of action of the lens camera

The aperture stop, which is the limiting diameter of the aperture thought which light enters the camera (given as fraction of focal length F of lens) is also called F Number.

This F Number; is the fraction of focal length of the lens given as focal length divide by lens diameter.

F number = Focal length, F/Lens diameter, d

FN = F/d

Where d = is lens diameter.

• The Number Indicates the Number of times the focal length F of times the focal length F of the lens diameter ( or stop)
• The smaller the F - Number for a given focal length the larger the lens diameter
• The lens with a larger diameter has a greater light- gathering power or speed
• This for such a lens the shutter allows light in the camera for a short interval of time.

The Magnification of the Lens Camera

Determine the magnification of the lens camera

Magnification of a lens camerais obtained as the ratio of the Image distance and the object distance.

But from the lens formula:

Thus M = v/U

I/U + I/V = I/F

I/V = I/F - I/U

(I/V) ^–I = ( U - F / FU)^-I

V = FU/ ( U - F)

Example 5

A lens camera is to be used to take a picture of a man 2m tall if the lens of the camera Has a focal length of 10cm, calculate the minimum size of the film frame required, given that the man is 20.1m from the camera.

Solution:

Magnification is given by:

M = f/ (u-f)

Where

F= 10cm U = 201/m / 2010cm

M = ( 10/2010 – 10)

M = 1/20 ....................................i

Let the size of the frame be h when the height of man is 2m.

Then

M = 1/200

But h1/h = 1/200

h1 = (1/200) 2

h1 = (1/200)2

h1 (2/200)

h1 = (1/100) m

h1 = 1cm or 10mm

The film frame should be at least 10mm square.

Simple Lens Camera

Construct a simple lens camera

A simple lens camera

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PHYSICS || PROJECTION LANTERN

Projection Lantern

The Structure of the Projection Lantern

Describe the structure of the projection lantern

The projection lantern forms images of slides or camera film onto a distant screen. The film or slide to be projected is inverted and highly illuminated.

The Mode of Action of a Projection Lantern

Describe the mode of action of a projection lantern

Optical arrangement of projection lantern.

• The slice or film to be projected is inverted and highly illuminated.
• The concave mirror helps to concentrate the light which would otherwise be partly wasted.
• The lamp is placed at the principal focus of the concave mirror.
• The heat filter reduces the heat at falling on the slide or film so as to avoid it overheating.
• Since the image of the projection lantern is Highly magnified, it would not be very bright if there was not enough illumination.
• The condenser directs a maximum amount of light from the source of the slide and produce uniform illumination the screen. (The condenser is a double in order to reduce chromatic aberration).
• The projection lens forms the image of the slide on the screen.
• The light source is usually located at a distance of 2f from a condenser and invited so that the image on the screen is upright (erect).
• The focal length of the projection lens is ABOUT TWICE THE FOCAL length of the condenser since the screen is usually far from the lens.

The Magnification of a Projection Lantern

Determine the magnification of a projection lantern

Example 4

A lantern projector using a slide of (2cm x 2cm) projects a picture (1cm x 1cm) onto a screen 12m from the projection lens. How far from the lens must the slide be? Find the approximate focal length of the projection lens.

Solution

The magnification m is given by;

M = V/U ………………………………………i

Where

• H^I is the size of image
• H is the size of object
• U object distance
• V image distance

Thus

M = Hi …………………. ii

Then eqn (i) = eqn ii

• v/u =hi/h
• (1200/u)-1 = (100/2) -1
• (u/1200) = (2/100)
• U = (2/100) (1200)
• U = 24cm

The object distance, U = 24cm

Uses of a Projection Lantern

Mention uses of a projection lantern

Projection lantern are used in various areas. These include:

• Projection of films, slides and transparencies,
• projection of opaque objects, i.e. episcopic projection,
• used in searchlights and headlights,
• used in projection apparatus in industry for gauge and screw thread testing,
• used in physical experiments such as projection of a spectrum,
• used in polarisation experiments and interference experiments.

A Simple Projection Lantern

Construct a simple projection lantern

Projection Lantern

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