# Lenses

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## Lenses

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1. Lenses

2. Eyeglasses. Peep-hole. Watch. Camera. Security mirror. Magnifying glasses. 1 In daily life, you may have come across lenses that are thicker or thinner in the middle. Which of the following items do you think may contain lenses?     

3. 2 When you see through lenses, do the objects appear the same as they are? No !

4. Introduction • Lenses are very useful. • e.g. in cameras, projectors, telescopes, microscopes and eyes • The most common lenses: • spherical lenses (i.e. the surface is spherical)

5. 1 Convex and concave lenses Convex lens - thicker at the centre than at the edge.

6. 1 Convex and concave lenses Concave lens - thinner at the centre than at the edge.

7. 1 Convex and concave lenses a Converging or Diverging? convex lens concave lens (converging lens) (diverging lens)

8. 1 Convex and concave lenses a Converging or Diverging? convex lens concave lens (converging lens) (diverging lens)

9. 1 Convex and concave lenses a Converging lens bend the light inwards You may remember ‘converging’ as ‘concentrating’.

10. 1 Convex and concave lenses a Diverging lens bend the light outwards You may remember ‘diverging’ as ‘dividing’.

11. 1 Convex and concave lenses a Converging or diverging? Recall that light travels more slowlyin glass than in air. Light converges or diverges as some parts of the wavefronts lag behind. converges diverges

12. b Key features of lenses • Parallel rays are refracted inwards. principal focus • Refracted rays meet at a point called the principal focus F.

13. Centre of the lens is called the optical centre C. F optical centre

14. The distance of F from C is the focal length f of the lens. F C focal length

15. F' F F' F • A convex lens has 2 principal foci. • 1 on each side C principal axis • The line through the optical centre and 2 foci is called the principal axis.

16. Parallel rays are refracted outwards. principal focus • Refracted rays appear to spread from a point called the principal focus F.

17. Centre of the lens is called the optical centre C. F optical centre

18. The distance of F from C is the focal length f of the lens. F C focal length

19. F' F F' F • A concave lens has 2 principal foci. • 1 on each side C principal axis • The line through the optical centre and 2 foci is called the principal axis.

20. Q1 A light ray is incident on a… A light ray is incident on a convex lens. Which one represents the path of the light ray? X Y Z A Path X. B Path Y. C Path Z.

21. Q2 A light ray is incident on a… A light ray is incident on a concave lens. Which one represents the path of the light ray? X Y Z A Path X. B Path Y. C Path Z.

22. 2 Image formation by a lens O screen When the light from a point of an object enters our eyes, we can see the point. I

23. 2 Image formation by a lens I O a Real images Light rays converge to a point.

24. 2 Image formation by a lens I O screen Image can be captured by a screen. Hence called ‘real’.

25. 2 Image formation by a lens I O b Virtual images Light rays diverge from a point. No rays actually come from the image. Hence called ‘virtual’. convex lens

26. 2 Image formation by a lens I O b Virtual images Light rays diverge from a point. concave lens

27. I O screen Since only convex lensesconverge light rays, real images can only be formed by convex lenses.

28. Light rays through lenses Video Video Video Simulation Simulation

29. 3 Graphical construction of image formation a Construction rules In ray diagrams, we use symbols to represent lenses. convex lenses concave lenses

30. Construction rules for convex lenses Rule 1 A ray parallel to the principal axis... F C …is refracted through F.

31. Construction rules for convex lenses Rule 2 A ray passing through F'... F F' C …is refracted parallel to the principal axis.

32. Construction rules for convex lenses Rule 3 A ray passing through C... F F' C …travels straight on.

33. Construction rules for concave lenses Rule 1 A ray parallel to the principal axis... C F' F …is refracted so that it appears to come from F'.

34. Construction rules for concave lenses Rule 2 A ray directed towards F... C F' F …is refracted parallel to the principal axis.

35. Construction rules for concave lenses Rule 3 A ray directed towards C... C F' F …travels straight on.

36. 3 Graphical construction of image formation b Drawing ray diagram Click the following to see how to draw ray diagrams. convex lens concave lens

37. Q1 Which of the following... A B F F’ F F’ C F F’ Which of the following incident rays is mentioned in the construction rules?

38. Q2 Which of the following light… A F’ F B C Which of the following light rays is correct ?

39. Q3 How to find the focus of the... Step 1: Draw a light ray A parallel to the principal axis from the tip of the arrow. O I

40. Q3 How to find the focus of the... Step 2: By the construction, the ray A appears to come from the ______ and the extended part ________ the tip of the image. focus passes O I

41. Q3 How to find the focus of the... Step 3: The point where the extended refracted light ray A cross the ______________ is the focus. principal axis O I F

42. F I C F' O 3 Graphical construction of image formation c Magnification magnification m height of image (image size) = height of object (object size)

43. u v F I C F' O 3 Graphical construction of image formation Also, image distance v  m = magnification m = u object distance

44. 4 Position and nature of image f Convex lens refracts the rays to form an inverted image on a plane at the principal focus. convex lens focal plane Such a plane is called the focal plane.

45. construction ray C F' F I We can find the position and nature of image by drawing ray diagram ∴ The 2 parallel rays also converge toI.

46. 4 Position and nature of image I Images formed by a convex lens F 2F C 2F' F' at infinity object: image: at F , real, inverted and diminished

47. I O 2F F C 2F' F' object: beyond 2F ’ image: between F and 2F real, inverted and diminished

48. I O 2F F C 2F' F' object: at 2F ’ image: at 2F real, inverted and same size