Physics Help Forum lenses and images

 Light and Optics Light and Optics Physics Help Forum

 Mar 29th 2009, 09:56 PM #1 Junior Member   Join Date: Mar 2009 Posts: 5 lenses and images I've been trying to sort this problem out, but I've always had trouble with lenses... I know it's a relatively basic question, but I'm getting a weird answer. I'd really appreciate any help for this question. Thanks in advance! a)A lens is used to produce a real image of an object such that the image is twice as large as the object. The distance between the image and the object is 90 cm, the focal length of the lens is what? b) A lens is used to produce an image that is larger than the object. Is the lens converging, diverging, or can it be either converging or diverging? c) A lens is used to produce an image that is smaller than the object. Is the lens converging, diverging, or can it be either converging or diverging? Part b and c might be a trick questions (as in, "not enough information to decide"), I'm not sure...
 Mar 30th 2009, 12:02 AM #2 Physics Team   Join Date: Feb 2009 Posts: 1,425 a) 1/f = 1/ u +1/v where u = distance of lens from object, v = distance of lens from image, f = focal length. f = uv / u+v . Magnification = v/u = 2 in this case. Solve to get f = 20
 Mar 30th 2009, 12:05 AM #3 Physics Team   Join Date: Feb 2009 Posts: 1,425 True. Is the image real or virtual?
 Mar 30th 2009, 05:49 AM #4 Junior Member   Join Date: Mar 2009 Posts: 5 Thanks for helping with part a) Physicsquest!! Much appreciated. Part b and c don't mention anything about 'real' or 'virtual' images... b) just says that a lens is used to produce an image that is larger than the object, and c) just says that a lens is used to produce an image that is smaller than the object. And then you need to say what type of lens it is (converging, diverging), but like I said, it might be a trick question (as in, "it can be either converging or diverging" or "not enough information to decide") Thanks again for your help
 Mar 30th 2009, 10:33 AM #5 Physics Team   Join Date: Feb 2009 Location: India Posts: 365 A diverging lens always makes a diminished image where as a converging lens can make diminished or enlarged image depending upon the position of the object. Hence For part (b) the lens is converging. For part (c) the lens may be convergng or diverging.
 Mar 30th 2009, 07:24 PM #6 Junior Member   Join Date: Mar 2009 Posts: 5 Thanks so much for the help Parvez, very much appreciated! I see where I was going wrong for part a), and your explanation for part b and c was really helpful. Thanks again!!

 Tags converging, diverging, image, images, lense, lenses

 Thread Tools Display Modes Linear Mode

 Similar Physics Forum Discussions Thread Thread Starter Forum Replies Last Post Clyner82 Advanced Optics 1 Sep 26th 2016 03:21 AM Razvan Advanced Electricity and Magnetism 2 Mar 23rd 2014 09:36 PM bhav007 Light and Optics 0 Apr 22nd 2013 02:10 PM cyt91 Light and Optics 6 Jun 19th 2010 08:18 AM Legato Light and Optics 5 May 13th 2010 02:17 AM