PHYSICS 3

1. PHYSICS 3

2. Sunday, 10 August 2014 Convex Lenses Lesson objectives • Understand what effect convex lenses have on light rays. • Be able to draw ray diagrams for convex lenses.

3. Using Refraction : lenses  Imagine parallel rays of light from a distant object hitting the lens.  The distance between the centre of the lens and F is called the focal length [].  The lens refracts all the rays to a point called the principal focus [F].  Draw normal lines where the rays enter the air [at 90º to the surface].  Work out the direction of the refracted rays using the second refraction rule.  Use the first refraction rule to work out the ray direction.  Draw normal lines [at 90° to the surface] for each ray.  A lens can be thought of as a series of prisms. F When light enters a more dense medium [e.g. glass], it bends towards the normal. When light enters a less dense medium [e.g. air], it bends away from the normal. ƒ

4. Biconvex Lenses Biconvex lenses are converging lenses. When incoming parallel light rays are incident upon a biconvex lens... The rays are focussed to the principle focus F F – principle focus f – focal length, distance from F to lens

5. Drawing Ray Diagrams (1) • Draw one ray from the top of the object parallel to the centre axis. This is refracted through the principal focus. • Draw a second ray through the centre of the lens. This passes straight on – it is not refracted at all! • Where the rays meet the image is formed.

6. Drawing Ray Diagrams (2) Two light rays leave the object O and pass through the lens. Where they meet an image I is produced. This is a REAL, DIMINISHED and INVERTED image. Lens

7. Your Ray Diagram 1 A convex lens has a focal length of 4 cm and an object 3 cm high is placed 8 cm in front of the lens (i.e. at 2F). Find the position, size and nature of the image formed.

8. Convex Lens Ray Diagrams – Object at 2F Image is real, inverted and the same size as the object. So its 8 cm from the lens and 3 cm high.

9. Your Ray Diagram 2 A convex lens has a focal length of 4 cm and an object 2 cm high is placed 6 cm in front of it. Find the position, size and nature of the image formed.

10. Convex Lens Ray Diagrams – Object Between 2F and F The image is 12 cm from the lens, 4 cm high and is real, inverted and magnified.

11. Your Ray Diagram 3 A convex lens has a focal length of 6 cm and an object 1 cm high is placed 4 cm in front of it. Find the position, size and nature of the image.

12. Convex Lens Ray Diagrams – Object Between F and C The image is 12 cm in front of the lens, 3 cm high and is virtual, upright and magnified.

13. Optics Test 1. Which statement is true? A. Virtual images can be projected onto screens B. Erect images are upside-down C. Concave mirrors are diverging D. Biconcave lenses are diverging 

14. Optics Test 2. Which statement is true? A. Diminished images are smaller than the object B. Convex mirrors are converging C. Biconvex lenses are diverging D. Real images can not be projected onto screens 

15. Optics Test 3. What optical device is shown? A. Biconvex lens B. Biconcave lens C. Convex mirror D. Concave mirror 

16. 4. What do you think happens when… Parallel light rays strike a convex lens? They pass through the focal point of the lens. F Emerge as a parallel beam if they pass though the focal point (F). Diverging light rays? F

18. Summary between F and 2F real diminished inverted at 2F real same size inverted > 2F real magnified inverted at infinity same side as object virtual magnified upright

19. Using Refraction : lenses - finding  Hold the lens in the other hand and move it closer to the screen until a clear image appears. Hold a plain white screen in one hand. Chose a distant object [to get parallel rays of light]. Use a ruler to measure the distance between the lens and the screen - this is the focal length [ƒ]. ƒ

20. Refraction : lenses 1. Find the focal length [ƒ] of your lens. 2F F F 2F 2. Fix the lens to the centre of a metre rule and mark the distances F and 2F either side of the lens. 3. Place the candle >2F away from the lens and move the screen until an image appears and record observations. 4. Repeat for the candle at 2F, between 2F and F, at F and between F and the lens.

21. Results

22. Refraction : lenses 2F F F 2F Object >2F away O I The image [ l ] is formed between F and 2F away from the lens, is inverted and diminished.

23. Refraction : lenses 2F F F 2F Object at 2F O I The image [ l ] is formed at 2F away from the lens, is inverted and the same size.

24. Refraction : lenses 2F F F 2F Object between 2Fand F away O I The image [ l ] is formed further than 2F away from the lens, is inverted and magnified.

25. Refraction : lenses 2F F F 2F Object at F away O The image [ l ] is formed at infinity - the rays never meet [we use this set-up for searchlights].

26. Refraction : lenses 2F F F 2F I Object between F and lens O The VIRTUAL image [ l ] is formed on the same side of the lens as the object, is the right way up and magnified.

27. Refraction : lenses 2F F F 2F Magnification = Distance from lens to image Distance from object to lens

28. Using refraction : lenses summary  There are two main types of lens: Convex Concave  Convex lenses work by bending [refracting] rays of light to a principal focus.  The distance from the centre of the lens to the principal focus [F] is called the focal length [ƒ].  The image formed by a convex lens is inverted [back-to-front and upside-down].  The thicker the lens, the shorter the focal length[ƒ].

29. Convex Lens Ray Diagrams – Object Beyond 2F Image is real, inverted and diminished

30. Your Ray Diagrams Draw ray diagrams with your object positioned: • At 2F – describe the image, • Between F and 2F – describe the image, • Between the lens and F – describe the image.

31. Optics Test 3. What optical device is shown? A. Biconvex lens B. Biconcave lens C. Plane mirror D. Concave mirror 