2. Structure of Human Eye: –Eye Muscles, Optic nerve, sclerotic coat –Cornea, iris, pupil,, lens, retina, blind spot –aqueous humour, vitreous humour, ciliary muscle Focusing –seeing near object ( near point ) –seeing distant object ( far point ) Eye defects –Short sight –Long sight

3. Structure of human eye lens Retina iris Cornea pupil Focusing muscles Blind spot Vitreous humour

4. Light entering the eye is focused successively by the len onto the retina. The image formed on the retina is inverted but is interpreted as erect. The retina contains many light sensitive cells sends signal along the optic nerve to the brain, causing the sensation of sight.

5. Focusing near object Light from near object Clear image

6. Focusing far away object Light from far away object Clear image

7. Accommodation

8. Near Point and Far Point The average normal eye can focus objects easily from about 25 cm, i.e., the near point, to infinity, i.e. the far point. This range of distance of clear vision varies from one person to another and decreases with age.

9. Short sight Light from distant object blurred image The image is focused ____________ the retina in front of

10. The eyeball is too long. The lens lacks the ability to accommodate for a distant object.

11. Correction of short sight Light from distant object Concave lens ( Diverging lens )

12. Long sight Light from near object blurred image The image is focused ________ the retina behind

13. The eyeball is too short. The lens lacks the ability to accommodate for a near object.

14. Correction of long sight Light from near object Convex lens ( Converging lens )

16. Class Practice 1.Which of the following statements about the lens is/are correct ? (1)The image formed on the retina is real. (2)The pupil is smaller in bright light than in dim light. (3)When the object distance changes, the eye focuses an object by changing the focal length of the lens.

17. Class Practice 2.The near point of a long-sighted eye is 50 cm from the eye. A lens is used for correction. (a)What type of lens should be used ? (b)Find the focal length of the lens.