Physics 30 Lesson 9 Optics Thin Lenses

Transcription

1 Reer t Pearsn pages 677 t 68. Physics 30 Lessn 9 Optics Thin Lenses The stuy lenses is very similar t that curve mirrrs. Hwever, while mirrrs invlve relectin, lenses invlve reractin. In act, r lenses there are tw reractins ne when the light ray enters the lens an ne when the light ray exits the lens. Reractin Reractin 2 Using the law reractin can yu explain reractin an reractin 2? Cnvex lens Nte that r a cnvex lens like the ne epicte abve the light ray is bent twar the principle axis twice, while r a cncave lens the light ray is bent away rm the principle axis. Using the law reractin can yu explain the reractins r a iverging lens? Cncave lens Cnvex lenses are cnverging lenses, an cncave lenses are iverging lenses. R.L. & A.K. 9 2//203

2 The equatins use r calculating the psitin an image rm a lens are ientical t the equatins r mirrrs. where cal length real (+), virtual ( ) istance rm lens t the bject i i istance rm the lens t the image hi i M real (+), virtual ( ) h h height bject h i height image real ( ), virtual (+) M magniicatin real, inverte ( ), virtual, upright (+) While the equatins r curve mirrrs an thin lenses are the same, it is imprtant t nte that real images rm in rnt a mirrr an real images rm behin a lens. This is ue t the act that mirrrs relect light back while lenses reract light thrugh t the ther sie the lens. I. Images Create by Lenses (Ray Diagrams) Ray travels rm the bject parallel t the principal axis. In passing thrugh the lens, the ray is reracte t the real cal pint n the ther sie the lens. Ray travels rm the bject parallel t the principal axis. In passing thrugh the lens, the ray is reracte away rm the virtual cal pint the lens. Ray 2 travels thrugh the virtual cal pint an is then reracte by the lens t emerge parallel t the principal axis. Ray 2 travels rm the bject twar the real cal pint. It then emerges parallel t the principal axis. Ray 3 travels thrugh the centre the thin lens straight thrugh. Ray 3 travels thrugh the centre the thin lens straight thrugh. R.L. & A.K //203

3 The iagrams belw illustrate the basic ray iagrams r bjects at ierent psitins relative t the lens. image rme rm a iverging lens R.L. & A.K //203

4 Example Utilising the rays r lenses, cmplete the ray iagram r the lens belw an escribe the image. slutin image is inverte, real an iminishe Image Frmatin Cnverging Lens R.L. & A.K //203

5 Image Frmatin Diverging Lens II. Calculatins Using the Lens Equatins Example 2 A 4.0 cm tall bject is place 50 cm away rm a cnvex lens that has a cal length 20 cm. Describe the image rme. h = 4.0 cm = 50 cm = +20 cm cnvex, cnverging lens 20 cm i i 50 cm i = cm hi i h ih hi The (+) i means that the image is real an inverte. The image is 2.67 cm tall, i.e. iminishe cm(4.0 cm) 50 cm = 2.67 cm R.L. & A.K //203

6 Example 3 A 5.0 cm tall bject is place 60 cm away rm a iverging lens that has a cal length 40 cm. Describe the image rme. h = 5.0 cm = 60 cm = 40 cm iverging lens i i 40 cm 60 cm i = 24 cm hi i h ih hi The ( ) i means that the image is virtual, erect an iminishe. 24 cm(5.0 cm) = + 2 cm 60 cm Example 4 An experiment is ne where an ptical evice, either a mirrr r a lens, is use. When the bject is place 20 cm rm the ptical evice, an erect image the bject is un n the ppsite sie the ptical evice. The image is ne-quarter the size the bject. What kin ptical evice is it an what is its cal length? The image is erect (i.e. virtual), smaller (i.e. i is less than ) an it is n the ppsite sie rm the bject (i.e. virtual an n ther sie happens r mirrrs nly). The nly way this can happen is with a cnvex (iverging) mirrr. i 4 20 cm 4 i is ( ) r an erect image = 5.0 cm i = cm 20 cm = 6.67 cm 0.5 R.L. & A.K //203

7 III. Practice Prblems a. (real, inverte, smaller) b. (real, inverte, same size) c. (real, inverte, larger). (n image rms) e. (virtual, erect, larger) R.L. & A.K //203

8 . (virtual, erect, smaller) IV. Han-in Assignment. Cmplete the iagrams belw by rawing in the light rays as they emerge rm the lenses. Label each the lenses belw as either cnvex r cncave. Label each the lenses belw as either cnverging r iverging. 2. Using the rules r light rays sketch ray iagrams r the llwing cases. State whether the image is: Real r virtual, erect r inverte, an larger r smaller than the bject. a. b. R.L. & A.K //203

9 c.. e.. 3. An bject 8.0 cm high is place 80 cm in rnt a cnverging lens cal length 25 cm. Determine the image psitin an its height. (36 cm, 3.6 cm) 4. A lamp 0 cm high is place 60 cm in rnt a iverging lens cal length 20 cm. Calculate the image psitin an the height the image. ( 5 cm, 2.5 cm) 5. A typical single lens relex (SLR) camera has a cnverging lens with a cal length 50.0 mm. What is the psitin an size the image a 25 cm canle lcate.0 m rm the lens? (5.3 cm,.3 cm) R.L. & A.K //203

10 6. A cnverging lens with a cal length 20 cm is use t create an image the sun n a paper screen. Hw ar rm the lens must the paper be place t pruce a clear image? (20 cm) 7. The cal length a slie prjectr's cnverging lens is 0.0 cm. (a) I a 35 mm slie is psitine 0.2 cm rm the lens, hw ar away must the screen be place t create a clear image? (5.0 m) (b) I the height a g n the slie ilm is 2.5 mm, hw tall will the g's image n the screen be? ( 62.5 cm) (c) I the screen is then remve t a pint 5 m rm the lens, by hw much will the separatin between ilm an lens have t change rm part (a)? (0.3 cm) *8. A cnvex lens has a cal length + 20 cm an a magniicatin 4. Hw ar apart are the bject an the image? (45 cm) 9. A prjectr is require t make a real image, 0.50 m tall, a 5.0 cm bject place n a slie. Within the prjectr, the bject is t be place 0.0 cm rm the lens. What must be the cal length the lens? (9. cm) 0. An bject 5.0 cm high is place at the 20 cm mark n a metre stick ptical bench. A cnverging lens with a cal length 20 cm is munte at the 50 cm mark. What are the psitin an size the image relative t the metre stick? (0 cm, 0 cm). A camera lens has a cal length 6.0 cm an is lcate 7.0 cm rm the ilm. Hw ar rm the lens is the bject psitine i a clear image has been pruce n the ilm? (42 cm) R.L. & A.K //203

11 Activity Cnvex lens Purpse: T etermine the cal length a uble cnvex lens. Apparatus: Set up yur wn apparatus base n the iagram belw. Be sure t return all materials t their apprpriate places ater yu have cmplete the lab. light surce lens meter sticks stan Thery: The thin lens equatin is: i Therere, i we measure the bject istance an the image istance we can etermine the cal length the lens. Prceure:. Place the bject at sme pint away rm the lens. (Start at arun 60 cm.) Recr the bject istance (istance rm lens t bject). 2. Using a white piece paper as a screen, mve the screen until a sharp image the bject (the bulb ilament) appears n the screen. Recr the image istance (istance rm lens t the screen). 3. Repeat steps ne an tw until a ttal three ierent psitins have been lcate. Observatins: Create an apprpriate ata table t rganise yur results. Analysis:. Calculate the cal length r each psitin an then in an average result. Shw all wrk an calculatins. 2. Draw scale ray iagrams r each psitin shwing the rmatin each image. R.L. & A.K. 9 2//203