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ist10_ch11.qxd Unit #3 - Optics 11.3 Lenses 7/22/09 3:53 PM Page 449 Night vision goggles use lenses to focus light onto a device called an image intensifier. Inside the intensifier, the light energy releases a stream of particles. These particles then hit a phosphor-coated screen. The phosphors glow when the particles strike them. The person wearing the goggles sees a glowing green image (Figure 11.47). Activity: D21 Observing Lenses (pg. 449) Figure 11.47 The image intensifier of night goggles amplifies the particles before they hit the screen. The image appears as shades of green. D21 Quick Lab Observing Lenses Purpose Procedure To observe how concave and convex lenses affect light 1. Look though each lens at the printed text in this student book. Record your observations. 2. Look through both lenses at some printed text. Record your observations. Materials & Equipment • convex lens (bulges out) • screen, such as a piece of paper • concave lens (middle is thinner than the edges) • candle holder, such as sand and a metal tray 3. Try to use each of the lenses to project a candle flame or light onto a screen or piece of paper. Record your observations. Questions • light source, such as a candle 4. Which single lens would be most useful as a magnifying glass? CAUTION: If an open flame is used, it must be secured so that it cannot fall over. Keep all combustible materials away from open flames. Tie back long hair before using an open flame. 5. How should the convex and concave lenses be arranged to make a distant object appear closer? 6. What arrangement of lenses is most effective in projecting the image of a light source onto a piece of paper? Ray diagrams model the behaviour of light in mirrors and lenses. Lenses curved transparent material that is smooth and regularly shaped transparent glass or very hard plastic thin lens strong, hard, polished ‣ rays converge or diverge can magnify the image image produced by a thin lens can be real or virtual, inverted or upright, larger or smaller. thickness is slight compared to its focal length 449 image. sides of the lens, it is possible to make light rays diverge or converge as converging and diverging, depending on how they refract the light that enters them. they pass through the lens. The most important aspect of lenses is that the light rays that refract through them can be used to magnify images or to project images onto a screen. Relative to the object, the image produced by a thin lens can be real or virtual, inverted or upright, larger or smaller. sed Lens Terminology Figure 11.50 illustrates some o converging and diverging lense Two RayTypes Diagram diverging lenses (thinnest in the middle) converging lenses (thickest in the middle) asses n be eg, use refract erties m. hese d in ray 1 object ray 2 (a) Define: (pg. 450) ‣ axis of symmetry ‣ focal point (x 2) (F) ‣ focal length ( ) principal F axis F´ principle axis ‣ • The pri through surface axis of symmetry f • Both ki The foc focus or the sym the lens f (b) principal F´ axis F axis of symmetry f f Figure 11.50 (a) Converging lens and (b) diverging lens Lens Terminology F image 450 UNIT D • The ax line dra • The foc of symm along th same w lens, bo focal len Light and Geometric Optics Figure 11.50 illustrates some of the terms associated with both converging and diverging lenses: • The principal axis is an imaginary line drawn through the optical centre perpendicular to both surfaces. • TheLenses axis of symmetry is an imaginary vertical Concave principal F axis line drawn through the optical centre of a lens. • Both kinds of lenses have two principal focuses. aka: diverging lens The focal point where the light either comes to a light is refracted away focus or appears to diverge from a focus is given from the principle axis the symbol F, while that on the opposite side of ‣ rays will never meet the lens is represented by F!. ist10_ch11.qxd 7/22/09 3:53 PM Uses ‣ eyeglasses have one convex surface and one concave surface understanding how m a lens. The index ‣ telescopes and binoculars use on other side refraction of air. concave lenses to principal • The focal length, f, is the distance from the axis F´‣ axis improve the detail Image: smaller, of symmetry to the principal focus measured into the upright, lens,virtual the along the principal axis. Since light behaves the rface and toward (a)direction through a (b) same way travelling in either both types of thin lenses have two equal f at an angle, thelens, light focal lengths. Figure 11.53 (a) Concave lens diverging lens he normal. In other (b) and convex lens ptics t on entering the 1.53). f Diagrams Page 452 In yo thin lens. compared lens. You affecting the axis o Drawin Ray diagr You need steps in F Rules for Ray Diagrams ‣ ray parallel to PA Ray diagrams model the behaviour of light in mirrors and lenses. 451focal extends from point ‣ ray travelling through centre, in not refracted (stays straight) Image is located at ray intersection ray 1 object ray 2 F image Figure 11.54 Concave lens ray diagram Conve A conver the centr cave Lenses How the Image Is Used Homework for a types Lens Some of eyeglasses Ray Diagram Convex Lenses Read 448 - 452 rayist10_ch12.qxd 1 telescopesand make use object rand predicting understanding how 7/22/09 3:56 PM Page 484 aka: converging lens Draw a diverging lens with a focal point of 4 cm, and of the diverging properties draw image at these situations: ays emerging from a lens. The index light is refracted of concave lenses. These toward the principle An Object which is 2 cm tall and 8 cm from the ray 2 axis han of refraction of air. axis of symmetry lensesthe areindex often used in An with Object 4 cm tall and 2 cm from the A of S ycombination passes from air into the lens, the F image converging lenses. from the lens surface and toward (a) (b) Cameras Our view of ourselves, ou es out of the lens at an angle, the light Figure 11.53 (a) Concave enormously lens by learning h ding away from the normal. In other optics have allowed us to (b) and convex lens Microscopes have allowe fractions, the first on entering the that was completely unk Activity: D24 - Focal Length telescopes, we have obse he lens (Figure 11.53). ‣ - - for a (pg. Lens 459) r predicting and understanding how Ray diagrams model the behaviour ays emerging from a lens. The index steps 2-9 han theRead index of refraction of air. y passes from air into the lens, the from the lens surface and toward es out of the lens at an angle, the light ding away from the normal. In other fractions, the first on entering the he lens (Figure 11.53). outshining nearby galaxi images of all these very l Object outside FP recent increased of light in mirrors and lenses. The451 ‣ image is real; power cameras has had i inverted incorporated into cellpho Object inside FP taken almost anywhere a ‣ image is virtual; around the world. This w upright about privacy, but it has communities of people in (a) (b)Cameras are also use vision systems to ensure Figure 12.24 This portable imaging device is equipped with a the Canadian Food Inspe Figure (a) Concave lens head-mount11.53 display for inspecting the to monitor food colour a sanitation of food-processing (b) and convex lens food. If the colour of the equipment. stored value, an alarm go inspected. Parts diaphragm focussing ring ‣ camera ‣ projectors ‣ magnifying glass 451 A came one end aperture detecto image (on film) For a di shutter lens object the foca must be do di the ima length o Figure 12.25 The parts of a simple camera that uses film the par aperture is the opening Suggested Activity • the iris controls the pupi D32 Inquiry Activity on page 493 in low light situations or The larger the aperture, t Ray diagrams model the behaviour of light in mirrors and lenses. Uses film spool film from the lens, it passes through the principal 2. The second ray travels from the tip of the ob optical centre of the lens and is not refracted 3. Draw the real image where the rays appear t Rules for Ray Diagrams Diagrams ‣ ray parallel to PA travels through F ‣ ray travelling through centre, in not refracted (stays straight) Suggested Activity • D25 Inquiry Activity on page 460 ‣ ray that goes through F, stays parallel object ray 1 ray 2 F image Figure 11.58 Convex lens ray diagram Image is located at ray intersection Learning Checkpoint 1. Describe the difference in shape between a convex lens 2. Which lens, convex or concave, can also be called a div Activity: D26 - Identifying the Properties of Images 3. Why do light rays bend twice when lenses are used? 4. DrawEquation a ray diagram for a convex lens when the object is Thin Lens (a) more than two focal lengths away from the lens (b) exactly two focal lengths from the lens Handout Focal Length ( ) pg 461 distance of object (do) ‣ 1 = 1 + 1 Thin Lens Equationd Qu # 10 distance of image (di) o di The distance of the object from the lens, do, the dista from the lens, di, and the focal length of a lens, f , ca using the thin lens equation. Given any two of the can use the thin lens equation to solve for the third: 1 1 1 = + f do di Remember that: 454 UNIT D ‣ A concave lens has a negative focal length and a negative distance to the image. ‣ A convex lens has a positive focal length and either a positive or negative distance to the image, depending where the object is placed. ‣ The image distance di is positive if the image is real and negative if the image is virtual. Light and Geometric Optics A convex lens has a focal length of 60.0 cm. A candle is placed 50 cm from the lens. What type of image is formed, and how far is the image from the lens? Example: ‣ A convex lens of a magnifying glass is held 2.00 cm above a page to magnify the print. If the image produced by the lens is 3.60 cm away and virtual, what is the focal length of the magnifying glass? Activity: D26 - Identifying the Properties of Images Handout pg 461 ‣ Qu # 10 A camera with a 200 mm lens makes a real image of a bird on film. The film is located 201 mm behind the lens. Determine the distance from the lens to the bird. Homework Read 452 - 458 ‣ pg. 461 - Qu # 10, 11, 12 ‣ pg. 454 - Qu #1-3 ‣ pg. 455-457 - Qu #1 from each group