Chapter 10 introduction lenses
- 1. Physics Ex-52 Click LENSES Whereas mirrors produce images by reflection, lenses produce images by refraction. Depending upon their “thickness”, lenses are classified as “thick” or “thin”. In this course, we will only consider “thin” lenses. In effect, therefore, we disregard the thickness of the lens assuming it to be a line. PART-1 /2 Convex Concave
- 2. Physics Ex-52 Click There are two main types of lenses called convex (also known as converging ) and concave (also known as diverging). Convex lenses are thicker in the middle than at the edges while concave lenses are thicker at the edges than in the middle. As illustrated below, both convex and concave lenses each have three variations. PART-1
- 3. Physics Ex-52 Click When representing “thin” lenses in a diagram, it is sometimes more convenient to simply draw an arrow rather than the actual lens. Using this method, an arrow is used to represent the lens as illustrated below. PART-1 Convex Lens Concave Lens
- 4. Physics Ex-52 Click Convex lenses are thicker in the middle and thus converge light rays. Concave lenses are thinner in the middle and thus diverge light rays.
- 5. Physics Ex-52 Click Just as with concave mirrors, the characteristics of the image formed by a converging lens depend upon the location of the object. There are six "strategic" locations where an object may be placed. For each location, the image will be formed at a different place and with different characteristics. We will illustrate the six different locations and label them as CASE-1 to CASE-6. PART-1 Case-1: Object at infinity Case-2: Object just beyond 2 F’ Case-3: Object at 2F’ Case-4: Object between 2F’ and F’ Case-5: Object at F’ Case-6: Object within focal length (f)
- 6. Physics Ex-52 Click CASE-1: Object at “infinity” No image formed (All rays pass through F) No image NOTE Since the object is at “infinity”, all rays arrive parallel. Infinity simply means “far away”. Object
- 7. Physics Ex-52 Click CASE-2: Object just beyond 2F’ Image is real (formed by refracted rays) Inverted (upside down) Reduced (smaller than object) Located between F and 2F Object Image Note-1 A ray that comes parallel is refracted through F. Note-2 A ray that goes through the vertex goes right through. Note-3 A ray that goes through F’ is refracted parallel. NOTE In order to establish an image point, all we need are two intersecting rays. This ray is extra in locating the image.
- 8. Physics Ex-52 Click CASE-3: Object at 2F’ Image is real (formed by refracted rays) Inverted (upside down) Same size as object Located at 2F Object Image Again: In order to establish an image point, all we need are two intersecting rays. This ray is extra.
- 9. Physics Ex-52 Click CASE-4: Object between 2F’ and F’ Image is real (formed by refracted rays) Inverted (upside down) Magnified (larger than object) Located beyond 2F Object Image
- 10. Physics Ex-52 Click CASE-5: Object at F’ No image is formed (rays refract parallel) Object No image
- 11. Physics Ex-52 Click CASE-6: Object is within focal length Image is virtual (formed by extended rays) Upright Magnified Located on same side as object Object Image
- 12. Physics Ex-52
- 13. Physics Ex-52 Click One case only! Image is always virtual (formed by a direct ray and an extended ray) Always upright Always reduced Always located on same side as object Object Image
- 14. Physics Ex-52 Click Sign convention: 1- Object distance is always positive. 2- Image distance is positive if the image is on the side of the lens where light emerges (from lens). 3- Image distance is negative if the image is on the side of the lens where the light enters (the lens). 4- The focal length of a convex (converging) lens is positive. 5- The focal length of a concave (diverging) lens is negative. PART-1 Be sure to follow the following sign convention when solving problems concerning lenses.
- 16. Physics Ex-52 Question-1 A lens that is thicker in the middle than at the ends is known as: ClickClick A lens or a lens.Convex Converging
- 17. Physics Ex-52 Question-2 A lens that is thicker at the ends than in the middle is known as: ClickClick A lens or a lens.Concave Diverging
- 18. Physics Ex-52 Question-3 Trace the rays that emerge from the following glass mediums: ClickClick a) b) c) d)
- 19. Physics Ex-52 Question-4 For each case below, draw the appropriate lens that will produce the indicated rays. ClickClick a) b) Convex (converging) Concave (diverging)
- 20. Physics Ex-52 Question-5 How is the image formed by a mirror different from the image formed by a lens? ClickClick A mirror forms an image by reflection whereas a lens forms a an image by refraction.
- 21. Physics Ex-52 Question-6 For each convex lens illustrated below, draw the image. ClickClick a) Image Image is real, inverted, reduced and located between F and 2F.
- 22. Physics Ex-52 Question-6 For each convex lens illustrated below, draw the image. ClickClick b) Image Image is real, inverted, same size object and located at 2F.
- 23. Physics Ex-52 Question-6 For each convex lens illustrated below, draw the image. ClickClick c) Image Image is real, inverted, magnified and located beyond 2F.
- 24. Physics Ex-52 Question-6 For each convex lens illustrated below, draw the image. ClickClick d) Parallel All rays refract parallel, no image is formed.
- 25. Physics Ex-52 Question-6 For each convex lens illustrated below, draw the image. ClickClick e) Image Image is virtual, upright, magnified and located on same side as object.
- 26. Physics Ex-52 For each concave lens illustrated below, draw the image. ClickClick Image Image is virtual, upright, reduced and located on same side as object. REMEMBER Concave lenses always produce images that are virtual, upright and reduced. Question-7
- 27. Physics Ex-52 Question-7 For each concave lens illustrated below, draw the image. ClickClick Image Image is virtual, upright, reduced and located on same side as object.
- 28. Physics Ex-52 Type (real or virtual): _______________ Location: _______________ Magnification: _______________ Height: _______________ Question-8 An object that is 7 cm high is placed 20 cm in front of a convex (converging) lens whose focal length is 15 cm. Determine the characteristics of the image: Click Real Click 60 cm -3 (inverted and 3x larger than object) 21 cm Negative sign indicates inversion.
- 29. Physics Ex-52 Type (real or virtual): _______________ Location: _______________ Magnification: _______________ Height: _______________ Question-9 An object whose height is 4 cm is placed 50 cm from a concave (diverging) lens. If the focal length of the lens is 30 cm, determine the characteristics of the image: Click Virtual Click 18.75 cm 0.375 (upright and smaller than object) 1.5 cm Note that diverging lenses have a negative focal length.
- 30. Physics Ex-52 Type (real or virtual): _______________ Location: _______________ Magnification: _______________ Height: _______________ Question-10 An object that is 5 cm high is placed 70 cm in front of a convex (converging) lens whose focal length is 20 cm. Determine the characteristics of the image: Click Real Click 28 cm -0.04 (inverted and smaller than object) 2 cm Negative sign indicates inversion.
- 31. Physics Ex-52 ClickClick A lens produces the following optical effect. Which group of lens produces the above effect? Question-11 Plano convexPlano convexD)D) Double convexDouble convexC)C) Plano concavePlano concaveB)B) Double concaveDouble concaveA)A)
- 32. Physics Ex-52 ClickClick Beams of light rays are traveling through air parallel to the principal axis of four different lenses. The light rays enter the lenses and are refracted. In which diagram are the light rays correctly illustrated? Question-12
- 34. Resolving Power Red light vs. Green light HW 35-6(c): resolving power = λ/∆λ = N×m