• Is a natural optical instrument whose ability to form images depends on the crystalline lens.

1). Sclerotic.

  • Is a hard shell tough opaque coat of the eye.
  • Is the white part of the eye.
  • It forms cornea which is a transparent and special.
  • Protects the delicate part of the eye.

2) Choroid.

  • Is a black coat containing blood vessels that supply food and oxygen to the eye.
  • It thickens to form cilliary muscles.
  • It absorbs the stray light in the eye.

3) Retina.

  • Covers the inside parts of the eye and has cells (rods and cones) sensitive to light.
  • It is where the image or images are formed.
  • They pass to optic nerve which are then transmitted to the brain for interpretation.

4) Iris.

  • It is the colored part of the eye and has an opening known as the pupil through which light enters the eye.
  • It controls the amount of light entering the eye by adjusting the size of the pupil.

5) Aqueous humor.

  • Is a clear liquid between the cornea and the eye lens.
  • It helps the eyeball to maintain its shape.
  • It refracts light. It bends rays of light.

6) Crystalline lens.

  • It is a soft elastic convex lens.
  • It focuses the image on the retina by the use of cilliary muscles.
  • This muscle controls the shape of the lens by either contracting or relaxing hence change the focal length of the lens.
  • When these muscles contract, the eye lens bulges more. Its focal length decreases hence focus near objects.
  • When these muscles relax, the eye lens flattens. It increases its focal length hence focus distant objects.

7) Optical nerve.

  • Is nerve of sight and joins nerve of retina to those of the brains.
  • The image formed in the retina is inverted/upside down. The brain interprets and gives an upright image.

8) Forea.

  • Is the central part of retina where the best vision and best details take place.

9) Lachrymal gland.

  • It produces tears which lubricate the eyeball, wash the eye and kill any animal or bacteria or germs since it is antiseptic liquid.


  • Is the fine adjustment of the eye lens to accommodate images/objects at different distances on the retina.
  • This varies the shape of the eye lens.
  • The shape of the eye lens is controlled by cilliary muscles. These muscles make the lens to increase its focal length or reduce its focal length. It decreases with age.

                                                Near point/least distant of distinct vision.

  • Is the closest point which the normal eye can focus clearly on an object.
  • It is 25 cm from the eye.

                                                Far point.

  • Is the farthest point where the eye can focus clearly on an object.
  • It is referred to as infinity.

                                                Importance of two eyes.

  1. Provide a wide field of view.
  2. Enables one to see objects in three dimension.

Defect of the eye vision.

  • There are structural changes of the eye which make the focusing mechanism of the eye to be difficult.
  • These are;
  1. Short sight or myopia.
  2. Long sight or hypermatopia.
  1. Old sight or presbyopia.
  3. Color blindness.


  1. Short sight/myopia.
  • Is a defect where one can only see clearly near objects but cannot see distant objects or far objects.
  • The rays form a distant objects are focused in front of the retina. In this case one sees blurred images.


  1. The eyeball too long.
  2. The eye lens too thick thus produce short focal length.
  3. The defect can be corrected by using a concave lens. The concave lens forms a virtual image of distant object at near point. The image is now the virtual object of the eye lens and forms a real image on the retina.
  4. Long sight.
  • Is the defect where one can see clearly distant objects but cannot see near objects. Its near point is at infinity.
  • The images of near objects are formed behind the retina hence one sees blurred images.
  • Rays from distant objects are focused on the retina.


  1. The eyeball being too short.
  2. The eye lens too thin hence long focal length.
  3. The defect can be corrected by using a convex lens.
  • Is a defect where the eye forms distorted images for near and far objects.
  • It is caused by the curvature of the cornea varying in different direction. As a result, rays from different plane from an object focus in different positions by the eye.
  • The defect can be corrected by using a cylindrical lens.
  1. Old sight/presbyopia.
    • Is the loss of power of accommodation.
    • Affects both near and far vision of a person.
    • It is caused by a decrease in elasticity of the eye lens.
    • It can be corrected by using a bi- focal lens which has the lower part for viewing near objects and upper part for distant objects.
    • It is a defect where the extrinsic muscles of the eye that control the turning of the eyeball do not co- ordinate accordingly of the simulation.
    • It affects the paired rectus muscles which turn the eyeball up and down and lateral medial muscles which move the eyeball left to right. As a result, the eyeball faces different direction and hence focusing and accommodation are achieved with difficulties.
    • The defect cannot be corrected.
    • It is common in reptiles e.g. chameleon.
  2. Color blindness.
    • It is a defect where one cannot distinguish red and green color.
    • It is caused by a defective gene that controls green and red colors.
    • It is common to men.
    • The defect cannot be controlled or cured.

                                                THE CAMERA.

  • It is an optical instrument used to take photographs and uses convex lens.
  • It consists of;
  1. Light- tight box which is coated black on the inside.
  2. A convex lens (system of lenses).
  • A shutter.
  1. Photographic film.
  • The inside of the camera is blackened to absorb any stray light entering the camera.
  • The diaphragm is used to control the amount of light entering the camera.
  • The shutter allows light to reach the film from the object only for precise period when the photograph is tube taken.
  • The film is a light sensitive material where the image is formed. It decomposes under light to form a picture. This picture is referred to as negative. It is processed to develop a film which is positive. The film is processed in the studio to produce photographs.


  • Is done by adjusting the distance between the lens and the film. That is by varying the image distance or moving the camera away from or towards the object.

                                                Difference between the camera and the eye.

  1. Focal length of the eye lens is variable while that of the camera is fixed.
  2. The image distance of the camera is variable while that of the eye is fixed.
  3. The iris of the eye remain open while the shutter of the camera is opened when a photograph is to be taken.
  4. Focusing in a camera is done by varying either the image or object distance while in the eye is varying the focal length of the eye lens, that is, varying the thickness of the eye lens.
  5. The eye forms a temporary image while the camera forms permanent image.
  6. The diaphragm of the camera can be altered to vary the amount of light entering the camera while in the eye, the iris alters itself.


Similarities between the camera and the eye.

  1. The eye has a crystalline convex lens while the camera has a hard convex lens.
  2. The eye has a retina where images are formed while the camera has a film where the images are formed.
  3. The iris controls the amount of light entering the eye while the diaphragm controls the amount of light entering the camera.
  4. The eye forms a real and inverted image as the same as the camera.
  5. They both form diminished images or images smaller than the object.
  6. The choroid layer of the eye is blackened while the camera box is painted black on the inside.



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