# [Unit 12.3] lens

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• 1. Lig ht Lens

2. Types of converging lens:

• A converging
• (or convex) lens
• is thicker in the
• middlethan
• at the edge.

(a)biconvex(b) plano-convex (c)concavo-convex 3. Technical Terms: The optical centre, C, is the centre of the converging lens. principal axis Optical centre, C 4. principal axis Technical Terms: Theprincipal axisis a horizontal line passing through the optical centre Optical centre,C 5. Technical Terms: The principal focus, F, of the converging lens is the point on the principal axis to which all rays originally parallel and close to the axis converge, after passing through the lens. F F: focus Focal length C F 6. Technical Terms: The focal length, f, of the lens is the distance between the optical centre and the principal focus. F Focal length f C F 7. Standard rays (For construction of ray diagram) F C F step 1 step2 step 3 8. FF object image Object distance u Image distance v Technical Terms: The object distance,u , is the distance between the optical centre and the object. The image distance,v , is the distance between the image and the optical centre. 9. Construction rules

• Incident ray through the optical centre, C
• Incident ray parallel to the principal axis
• Incident ray directed towards focus, F

F F C object Image 10. Formation of image by a converging lens:

• Converging lens of focal length = f
• Object distance = u
• Image distance = v ( 6 Cases)

Given: (continue on next slide) 11. 2FFF image Object at infinite position Image nature : real, diminished and inverted imageImage distance:v = f Uses:telescope v=f u=f Case 5a: 12. 2FFF object image u 2f v u > 2f Nature of image :real, inverted and diminished Image distance:f2f Uses : slide projector, film projector, OHP Case 5d: 15. FF object u f u=f No image is formed.Image distance :(Image is formed at infinity) v = Uses : Theatres, spotlights Case 5e: 16. FF object image u f v 0When an object is placed within the focal length of a convex lens, the image formed is virtual, upright and enlarged.

• This principal is used in a magnifying glass.

20. Virtual Image: The image formed by this way is a virtual image, please explain? 21. To find the focal length of a convex lens:

• Approximate method
• Place a screen at the back of a convex lens. Adjust the position of the lens until a clear image of distance object is obtained on the screen. The distance between the lens and the screen gives the focal length of the convex lens.

22. Approximate methodto find the focal length of the convex lens Focal length 23.

• Accurate method:
• In this experiment, an illuminated object is obtained by using a piece of cardboard with a cross wire at its centre to cover the face of a torchlight.

24. Steps to measure the focal length:

• Place a plane mirror at the back of a convex lens.
• Adjust an illuminated object in front of the convex lens until a clear image is obtained at the same position as the object.
• Measure the distance between the convex lens and the object(image). It gives the focal length of the convex lens.

25. FF object image Worked example 1: Given: focal length of convex length, f, object distance, u and its size. Find by graphically the size and the nature of its image produced.The image obtained: Enlarged, inverted and real. Image distance, v > f 26. FF image object Worked example 2: Given: focal length of convex length, f, image distance, v and its size. Find by graphically the size and the position of the object.Graphically: 27. object image F focal length f Given: the size and position of distance of an object and its image . Find by graphically the focal length, f, and the position of the lens.Worked example 3: Graphically: 28. GCE O-Level Past Examination Paper Science (Physics) 29. Nov 1998 10. The human eye has a converging lens system thatproduces an image at the back of the eye. If the eyeviews a distant object, which type of image isproduced? A real, erect, diminishedB real, inverted, diminishedC virtual, erect, diminishedD virtual, inverted, diminished B 30. November 1989 10. In the diagram, XY is a convex lens and F are the principal foci. An object is placed at O. At which point is thebase of the image formed? D X Y 31. GCE ONov 1997 11.The diagram shows the action of a magnifying glass. Which point is the principal focus of the lens ? D 32. Nov 1996 13(a) Draw a labelled diagram to show what is meantby the focal length of a thin converging lens.[2] Focal length is distance from centre of lens to focus. (continue in next slide) 33. 13(b) A thin converging lens can be used to produceeither a real or a virtual image.Explain thedifference between these types of image.[2] (Cont. )Q. 13Nov 1996 Real image is theimage that can project on the screen. Virtual image is theimage that produce by reflection of light and cannot projected on the screen.Draw two ray diagrams, one to show theformation of a real image, the other to showthe formation of a virtual image, using a thinconverging lens.[6] (continue in next slide) 34. (Cont. )Q. 13Nov 1996 13(b) When f > u, image: Virtual,upright and Enlarged. When u > f, image: Real,inverted and reduced. 35. 6(a) Draw rays on the diagram below to show what ismeant by the focal length of a converging lens. GCENov 1991 lens Rays parallel to axis axis Focal length (continue on next slide) 36. 6(b) The diagram below shows the image I of anobject O produced by a converging lens.Complete a ray diagram to show two rayspassing from the object to the image. Mark thepositions of the lens L and the relevant focalpoint F on the diagram.[4] (Cont. )Q. 6 Nov 1991 O I lens F L F 37. Nov 1990 2.The diagram below represents a thin converging lens L (shown as a vertical line) which is being used to produce an image I of a point object O. On the diagram, complete the path of theray shown and draw a second ray to locate the position of the principal focus (the focal point) of the lens. Label this point F.[4] F L

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