2. Construction of Human Eye –iris, pupil, cornea, aqueous humour, lens, ciliary muscle, vitreous humour, retina Focusing –seeing near object ( near point ) –seeing distant object ( far point ) Eye defects –Short-sight –Long-sight

3. Construction of human eye eye lens Retina iris Cornea pupil Ciliary muscles Blind spot Vitreous humour

4. Light entering the eye is refracted successively at the cornea, the aqueous humour, the lens and the vitreous humour. The image is formed on the retina and the retina sends signal along the optical nerve to the brain, causing the sensation of sight. The image formed on the retina is inverted but is interpreted as erect.

5. Focusing near object Light from near object

6. Focusing far away object Light from far away object

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

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

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

12. Long sight Light from near object

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

14. Correction of long sight Light from near object Converging lens ( Convex 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 chaning the focal length of the eye 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.