1. The Professional Development Service for Teachers is funded by the Department of Education and Science under the National Development Plan By Mark Jordan © Refraction Part 2 Lenses

2. The Professional Development Service for Teachers is funded by the Department of Education and Science under the National Development Plan OUTLINE OF THE DAY To Be Completed by Class Teacher

3. List the uses of lenses Look closely at the lenses you have been given and answer questions below: o How are the lenses shaped? o How are the lenses alike/different? o Experiment with light & separate into convex/concave Place a convex lens between window and a white piece of paper. Place the lens close to the paper and then slowly move the lens towards the window. Draw a sketch of image you see. Using prior knowledge (mirrors) compare the image from the window with the image of the man’s eye above.

4. 4 C1C1 C2C2 C1 C1 & C2 C2 are the centres of the spheres of which the surfaces of the lens form a part The line through C1 C1 & C2 C2 form the principal axis Converging Lens Principal axis

5. 5 C1C1 C2C2 Converging lens F F A ray of light on entering the lens is refracted towards the normal and on leaving away from the normal. Since surfaces are inclined towards each other the ray is refracted towards the principal axis. Rays parallel to the principal axis converge towards a point called the principal focus F. Since light can travel equally well in both directions, there are two foci. Normal

6. 6 F = Focal point To locate the position of an image in a convex lens we use two of the following rays of light FF 3 through the focus emerging parallel to principal axis. 1 parallel to the principal axis emerging through focus 2 striking the centre of the lens passes straight through (if lens is thin)

7. 7 F = Focal point object Images in Convex lens vu f Image Real, inverted & diminished Image formed in convex lens when the object is placed outside twice the focal length FF

8. 8 Image formed in convex lens when the object is placed at twice focal length F = Focal point object Image Real, inverted & same size as object Images in Convex lens vu f FF

9. 9 Image formed in convex lens when the object is placed between f and 2f F = Focal point object Image Real, inverted & magnified vu f FF

10. 10 Image formed in convex lens when the object is placed at the focus F = Focal point object Image at Infinity Images in Convex lens u f FF

11. 11 Image formed in convex lens when the object is placed inside the Focus F = Focal point object Images in Convex lens u f Image Virtual, magnified & upright FF

12. 12

13. 13 raybox lens screen u v Experiment to find the focal length of a convex lens

14. 14 F = Focal point 1. A ray which strikes the lens travelling parallel to principal axis is refracted as if it came from focus 2. A ray striking the centre of the lens passes straight through ( if lens is thin ) 3. A ray heading for the focus on striking the lens is refracted parallel to principal axis To locate the position of an image in a concave lens we use two of the following rays of light Convave lens FF

15. 15 Image formed in concave lens when the object is placed in front of lens F = Focal point object Images in Concave lens Image Virtual, upright & diminished v u f FF

16. 16 Cornea Iris Pupil Lens Optic nerve Retina Suspensory ligament

17. 17 Accommodation is the ability of the lens in the eye to change it’s focal length so that light from close up and distant objects always fall on the retina

18. 18 light from distant object falls short of retina light from distant object falls on retina with help of a diverging (concave) lens Short-sight defect Corrected

19. 19 light from near object falls ‘behind’ retina Long-sight defect Corrected with help of a converging (convex) lens falls on retina light from near object

20. The Professional Development Service for Teachers is funded by the Department of Education and Science under the National Development Plan