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© Boardworks Ltd 2003 Lenses. © Boardworks Ltd 2003 3.2 (c) Thin converging lens Core Describe the action of a thin converging lens on a beam of light.

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Presentation on theme: "© Boardworks Ltd 2003 Lenses. © Boardworks Ltd 2003 3.2 (c) Thin converging lens Core Describe the action of a thin converging lens on a beam of light."— Presentation transcript:

1 © Boardworks Ltd 2003 Lenses

2 © Boardworks Ltd (c) Thin converging lens Core Describe the action of a thin converging lens on a beam of light Use the term principal focus and focal length Draw ray diagrams to illustrate the formation of a real image by a single lens

3 © Boardworks Ltd 2003 Supplement Draw ray diagrams to illustrate the formation of a virtual image by a single lens Use and describe the use of a single lens as a magnifying glass

4 © Boardworks Ltd 2003 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].

5 © Boardworks Ltd 2003  A lens can be thought of as a series of prisms.  The lens refracts all the rays to a point called the principal focus [F].  The distance between the centre of the lens and F is called the focal length [  ].  Imagine parallel rays of light from a distant object hitting the lens.  Draw normal lines [at 90° to the surface] for each ray.  Use the first refraction rule to work out the ray direction.  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. When light enters a less dense medium [e.g. air], it bends away from the normal. Using Refraction : lenses When light enters a more dense medium [e.g. glass], it bends towards the normal. F ƒ

6 © Boardworks Ltd 2003 What do you think happens when… Parallel light rays strike a convex lens? They pass through the focal point of the lens. Diverging light rays? Form a parallel beam if they pass though the focal point (F). F

7 © Boardworks Ltd 2003 Use a ruler to measure the distance between the lens and the screen - this is the focal length [ƒ]. Using Refraction : lenses - finding  ƒ Chose a distant object [to get parallel rays of light]. Hold a plain white screen in one hand. Hold the lens in the other hand and move it closer to the screen until a clear image appears.

8 © Boardworks Ltd 2003 Refraction : lenses 1. Find the focal length [ƒ] of your lens. 2. Fix the lens to the centre of a metre rule and mark the distances F and 2F either side of the lens. 2FFF 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.

9 © Boardworks Ltd 2003 Results Object position Image Position Real or virtual Magnified or diminished Inverted or erect >2F at 2F between 2F and F at F between F and lens

10 © Boardworks Ltd 2003 Refraction : lenses Object >2F away O 2FFF I The image [ l ] is formed between F and 2F away from the lens, is inverted and diminished.

11 © Boardworks Ltd 2003 Object at 2F O 2FFF I The image [ l ] is formed at 2F away from the lens, is inverted and the same size. Refraction : lenses

12 © Boardworks Ltd 2003 Object between 2Fand F away O 2FFF I The image [ l ] is formed further than 2F away from the lens, is inverted and magnified. Refraction : lenses

13 © Boardworks Ltd 2003 Object at F away O 2FFF The image [ l ] is formed at infinity - the rays never meet [we use this set-up for searchlights]. Refraction : lenses

14 © Boardworks Ltd 2003 Object between F and lens O I The VIRTUAL image [ l ] is formed on the same side of the lens as the object, is the right way up and magnified. 2FFF Refraction : A magnifying glass

15 © Boardworks Ltd 2003 Results Object position Image Position Real or virtual Magnified or diminished Inverted or erect >2F at 2F between 2F and F at F between F and lens between F and 2F at 2F > 2F at infinity same side as object virtual real magnified same size diminished erect inverted

16 © Boardworks Ltd FFF Magnification=Distance from lens to image Distance from object to lens Refraction : lenses

17 © Boardworks Ltd 2003 To do All P153 Answer all questions Extended only Draw an accurate ray diagram to show a magnifying glass where f=10cm and the object is 5cm from the lens

18 © Boardworks Ltd 2003 Learning check

19 © Boardworks Ltd 2003 Which of the following is the most dense? A.Air B.Water C.Glass D.Lead

20 © Boardworks Ltd 2003 When light changes direction as it moves from one medium to another we call this effect what? A.Reflection B.Refraction C.Diffraction D.Total internal reflection

21 © Boardworks Ltd 2003 What happens to the speed of light as it moves from air into glass? A.Decreases B.Increases C.No effect D.Decreases and increases

22 © Boardworks Ltd 2003 If a ray of light moves from air to glass parallel to the normal what happens? A.No change in direction B.It bends away from the normal C.It bends towards the normal D.It stops

23 © Boardworks Ltd 2003 If light travelling through a medium has a speed of m/s. What is the refractive index of the medium? A.2.6 B.0.5 C.2.0 D.1.5

24 © Boardworks Ltd 2003 Can you…… Draw ray diagrams depicting the refraction of light by lenses? Write a sentence using the terms principal focus and focal length Draw a ray diagrams to show how a lens forms an inverted image Draw ray diagrams to illustrate the formation of a virtual image by a magnifying glass

25 © Boardworks Ltd 2003


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