3 The human eye :-The human eye is the sense organ which helps us to see the colourful world around us.The human eye is like a camera. Its lens system forms an image on a light sensitive screen called retina. The eye ball is almost spherical in shape with a diameter of about 2.3cm. Light enters the eye through a transparent membrane called cornea. Behind the cornea is a muscular diaphragm called iris which has an opening called pupil. The pupil controls the amount of light entering the eye. The eye lens helps to focus the image of objects on the retina. The ciliary muscles helps to change the curvature of the lens and to change its focal length.
4 Parts of the Eye Lens Eyelid Sclera Eyelashes Orbital Muscles Iris CorneaOptic NervePupilEye Socket
10 RetinaLayer of tissue on the back of the eye that has cone and rod cells which gather informationRods: allow us to see black, white, & shades of greyCones: allow us to see red, blue, and green colorsThey work together so we can see many shades of colors
11 Optic NerveNerve that connects the eye to the brain
13 Orbital Muscles Allow eyes to move Humans are the only animal who can “roll” their eyes
14 How do we see? 1. Light enters the eye through the cornea. 2. Light continues through the pupil which is controlled by the iris.3. Light passes through the lens which refracts (bends) the light causing the picture to be upside down.4. The upside down picture travels to the retina which flips the image right side up.5. The optic nerve sends signals to the brain, which interprets the images.
15 Working of the eye :-The eye lens forms a real inverted image of the object on theretina. The light sensitive cells in the retina then produce electricalsignals which are carried by the optic nerves to the brain. The brainprocesses the information and sends the message to the eye and thenwe see the object.
16 Power of accomodation of the eye :- The ability of the eye lens to see both near and distant objects byadjusting its focal length is called the power of accommodation of theeye.The eye lens is composed of a fibrous jelly like material. Its curvaturecan be changed to some extent by the ciliary muscles. The change inthe curvature of the eye lens can change its focal length. When themuscles are relaxed, the lens becomes thin and its focal lengthincreases and when the muscles contract, the lens becomes thick andits focal length decreases.Near point :-The minimum distance at which the eye can see objects clearly iscalled the near point or least distance of distinct vision. For a normaleye it is 25cm.Far point :-The farthest distance upto which the eye can see objects clearly iscalled the far point of the eye. For a normal eye it is between 25cm andinfinity.
17 2) Defects of vision and their correction :- i) Myopia or near sightedness :-Myopia is a defect of vision in which a person can see nearbyobjects clearly but cannot see distant objects clearly because theimage is formed in front of the retina.This may be due to:-i) Increase in curvature of the eye lensii) Increase in the length of the eye ballIt can be corrected by using suitable concave lens.Myopic eyeCorrection using concave lens
18 ii) Hypermetropia or far sightedness :- Hypermetropic eyeCorrection using convex lensHypermetropia is a defect of vision in which a person can seedistant objects clearly but cannot see nearby objects clearly becausethe image is formed behind the retina.This may be due to:-i) Decrease in curvature of eye lensii) Decrease in the length of the eye ballIt can be corrected by using a suitable convex lens.
19 iii) Presbyopia :- Presbyopia is a defect of vision in old people in which they are not able to see nearby objects clearly dueto the increase in the distance of near point.This is due to the weakening of the ciliary musclesand decrease in the flexibility of the eye lens. It can becorrected by using suitable convex lens.Sometimes they are not able to see both nearby anddistant objects clearly. It can be corrected by usingbifocal lenses consisting of both concave and convexlenses. The upper part is concave for correction ofdistant vision and the lower part is convex for correctionof near vision.