What is Lenses: Definition and 320 Discussions

A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (elements), usually arranged along a common axis. Lenses are made from materials such as glass or plastic, and are ground and polished or molded to a desired shape. A lens can focus light to form an image, unlike a prism, which refracts light without focusing. Devices that similarly focus or disperse waves and radiation other than visible light are also called lenses, such as microwave lenses, electron lenses, acoustic lenses, or explosive lenses.
Lenses are used in various imaging devices like telescopes, binoculars and cameras. They are also used as visual aids in glasses to correct defects of vision such as myopia and hypermetropia.

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  1. C

    Converging and diverging lenses

    Homework Statement a) A converging lens with a focal length of 15cm has an object 5cm tall placed 40cm to the left of it. To the right, 30 cm away, is a converging lens of focal length 10cm. use the thin lens and magnification formulae to locate the finale image. Find its magnification. b)...
  2. A

    PMMA Hydrophobic: Explaining Molecular Level

    I'm just wondering why PMMA (polymethylmethacrylate) is hydrophobic and does not attract water. I can't find anything that explains this on a molecular level :S
  3. R

    What is the Radius of Curvature of the Outer Surface of a Contact Lens?

    [SOLVED] Contact Lenses Problem Homework Statement A contact lens is made of plastic with an index of refraction of 1.58. The lens has a focal length of +26.0 cm, and its inner surface has a radius of curvature of +20.0 mm. What is the radius of curvature of the outer surface? Homework...
  4. S

    Light - Angle of Deviation and Lenses

    Hey guys, i have two question that me and a couple of my mates couldn't figure out for the life of us in class so i bring them here. The first one at first glance seemed fairly easy Homework Statement Calculate the angle of deviation at a glass air interface for an angle of incidence of...
  5. A

    Lenses Coursework Need Your Help

    "To use a graphical approach to determine whether there is a difference between the focal length of a lens when used with red and green light." What would I expect to see? What sort of results do you think I will get? What would be the variable? I need to use the equation 1/u +...
  6. M

    Lenses - Thin-Lens Equation and Magnification Equation

    [SOLVED] Lenses - Thin-Lens Equation and Magnification Equation I can't figure out what I am doing wrong, your help would be greatly appreciated :) Here is the Homework Problem A camera is supplied with two interchangeable lenses, whose focal lengths are 35.0 and 150.0 mm. A woman whose...
  7. M

    Exploring Lens Combinations: Measuring U and Understanding the Differences

    In a combination of lens experiment we measure U from the mean position b/w two lenses , Why ? If we measure U from the first lens or second lens will there be any difference b/w this case & the above mentioned one ?
  8. D

    Understanding Light, Lenses, and Mirrors: Final Exam Review

    Homework Statement I have a final tomorrow, and I there are a couple questions that I don't know. When the angle of incidence is the critical angle, the angle of refraction will be? If a screen is moved 3 times the distance from a light source, what will the intensity on the screen be...
  9. R

    Focal Lengths of Two Lenses of a Microscope

    A microscope is made in a tube 25.0 cm long. It magnifies 5.00 x 102 times, 50.0 times by the objective lens and 10.0 times by the eyepiece. What are the focal lengths of the two lenses?
  10. J

    Developing Variable Focal Length Lenses with Liquid Crystals

    I have been reading about the development of liquid crystal - variable focal length - lenses. Manipulating the electric field changes the orientation of the LC's (which are birefringent) and thus changes the index of refraction. The goal is to make small and thin zoom lenses for such things as...
  11. D

    Solving Light and Lenses Questions: Focal Length, Magnification, Distance

    Homework Statement 1)A philatelist uses a magnifying glass with a focal length of 10cm. In order to examine a rare postage stamp, he holds the lens 4 cm from his eyes. If the distance for the most comfortable viewing of the image is 30cm from his eyes, how far from the lens must the postage...
  12. I

    Combined Effect of Multiple Lenses: Find Focus

    So there are two lenses. The first lens is a converging lens and the second is a diverging lens with focal length f,-f respectively, separated by a distance D. The question asks to show the combined effect of the lenses is focusing however, I am not getting them to be always focusing. using...
  13. N

    How does Fermat's principle explain the shape of a lens in Feynman's derivation?

    I have a problem in understanding Feynman's derivation of a lens' shape using Fermat's principle. Feynman writes that we have to choose such a surface of the lens that all optical ways from the source S to the focal point F will have the same length (so all will be taken by the traveling light)...
  14. B

    How do I accurately measure the size of an object using CCTV lenses?

    [SOLVED] Understanding CCTV lenses Hey guys, I'm trying to get my head around the way a lens works... I've found the nodal points, and since the medium surrounding the lens is air constant refractive index (air) then they nodal points coincide with the principle planes, right? Also...
  15. M

    Stargazing Non-Magnifying Telescope Lenses: Components & Effects

    The components of a telescope are essentially two convex lenses? Out of curiosity (boredom) I am trying to figure out if the addition of another lens could make the image non-magnified. For instance, what type of lens/other would you add in order to make the image look as if you were...
  16. N

    Thin lens equation for diverging lenses.

    Hi, I was reading a section which explains about the "thin lens equation for diverging lenses" and it included an example; Formula: 1/f = 1/di + 1/do where f is the focal distance, di is the distance of the image from the center and, do is the distance of the object from the center. Example: f...
  17. O

    Questions on Lenses: Projecting a Slide on a Screen

    Homework Statement You wish to project a slide on a screen. One of the letters on the slide is a p. When you place the slide in the projector, how much the letter look when you insert the slide(as seen from behind the projector)? Explain. a) p b) d c) b d) q Homework Equations How converging...
  18. C

    Calculating focal length for aspherical lenses

    How is it done? I've done a little research on Wikipedia and found the lensmaker's equation, but one of the values needed to compute focal length is the radius of the lens curvature. Strictly speaking, an aspherical curvature wouldn't have a set radius, would it? The reason I'm asking is because...
  19. L

    Why Do Astronomical Telescopes Use Mirrors Over Lenses?

    Why do very large astronomical telescopes use a mirror rather than a lens for the objective optic?
  20. P

    Discover the Benefits of 'Day and Night' Contact Lenses

    Which contact lenses do you like the most? For me it's 'Day and Night' although I take it out in the night just to let the eyes breathe better.
  21. M

    Do lenses bend light in a special way?

    Corrective lenses and contact lenses work because of refraction. Do they bend light in a special way that will correct a person's vision for whatever reason it is?
  22. A

    Solving for x in a Combination of Two Convex Lenses

    Homework Statement A combination of two thin convex lenses of equal focal lengths,is kept separated along the optic axes by a distance of 20 cm between them.The combination behaves as a lens system of infinite focal length.If an object is kept at 10 cm from the first lens,its image...
  23. B

    Converging Lenses and image position

    Homework Statement Two converging lenses with focal lengths of 40 cm and 20 cm are 10 cm apart. A 2.0-cm-tall object is 60 cm in front of the 40-cm-focal-length converging lens. a. Use ray tracing to find the position and height of the image. do this accurately with a ruler. Determine the...
  24. M

    Focal Length of Thin Lenses in Different Mediums

    For this question, assume that the index of refraction of diamond is 2.39, that of glass is 1.65, that of water is 1.30, and that of air is extremely close to 1. A thin lens made of glass has a focal length in air of 16.1 cm. A lens of exactly the same shape is then made from diamond...
  25. M

    Optics, camera lenses and object height/size

    A camera with a focal length f of 6 cm is used to take the picture of a person who is 180 cm tall, standing at a distance 3 m in front of the camera. How big will the image of that person be on film? How far behind the lens (assumed to be very thin) must the film be placed? I tried using...
  26. 2

    What Lens Power Does a Student Need for Reading Close-Up Without Contact Lenses?

    Homework Statement Without her contact lenses, a student can focus an object located 0.80 m to infinity from her eyes. The power lens, measured in diopters, is the reciprocal of the focal length, measured in meters. Assuming that the image distance in the eye is 0.02 m, what lens power does...
  27. S

    How Do Two Lenses Affect the Focus of Light from Infinity?

    Question regarding two lenses in combination - the first is a diverging lens with f = -31.5 cm, and the second is a converging lens, 14 cm away, with f = 20.0 cm. The exact question is 'Where will an object at infinity be focused?' (coming in through the diverging lens), and I kind of understand...
  28. R

    How Does Lens Placement Affect Image Height and Orientation?

    A 1.5-cm tall object is placed 0.50 m to the left of a diverging lens with a focal length of 0.20 m. A converging lens with a focal length of 0.17 m is located 0.08 m to the right of the diverging lens. What is the height and orientation with respect to the original object of the final image...
  29. S

    Understanding Dark Fringe at Contact Lenses

    Homework Statement I'm unable to understand the production of dark fringe in the central point of contact of lens.The text says "the path difference between the rays reflected at the top & bottom of air gap at the point of contact is zero.Actually the ray reflected from the top of air film...
  30. M

    Combinination of two lenses, converging and diverging

    Homework Statement A converging lens and a diverging lens, separated by a distance of 30 cm, are used in combination. The converging lens has a focal length of 15 cm. The diverging lens is of unknown focal length. An object is placed 19 cm in front of the converging lens; the final image is...
  31. J

    How Do You Calculate the Separation Between Two Converging Lenses?

    A coin is located 20.0 cm to the left of a converging lens (f=16.0 cm). A second identical lens is placed to the right of the first lens, such that the image formed by the combination has the same size and orientation as the original coin. Find the separation between the lenses. So, f=...
  32. H

    Solving Lens Question Homework: Converging, -0.582 m, Real Image

    Homework Statement "A certain lens focuses an object 2.85 m away as an image 48.3 cm on the other side of the lens. What type of lens is it and what is its focal length? Is the image real or virtual? Homework Equations 1/f = 1/do + 1/di The Attempt at a Solution I'm guessing it's...
  33. larkspur

    Future Eyeglass Lenses: Switchable Focus!

    http://www.gizmodo.com/gadgets/gadgets/liquidcrystal-switchable-focus-eyeglass-lenses-167757.php" would be so cool!
  34. J

    How can I determine the magnification of a lens?

    I do not know where to ask this question: For a lens (converging or Diverging), How do i know if magnification is positive or negative?
  35. K

    Gravitational lenses and objects in front of them

    Will the image of an object inside a gravitational lens, though on the side of the lens that is facing us, be enlarged/reduced in apparent angular width? If so, how is this quantified, and dimensionless GR values may it be related to (e.g. gravitational length contraction factor, etc.)? Will...
  36. T

    Mathematica Mathematical Demonstration - Lenses in contact

    Hi, I have been working on this for a little while and think I am approaching it incorrectly. If someone can help, I'd really appreciate it. This is for a basic introductory calculus based Physics class that I am in. 1. Mathematically demonstrate the following equation with no numerical...
  37. Amith2006

    Achromatism of Lenses: Answers to Your Questions

    Homework Statement I posted this question in Introductory physics section but no one is able to clear my doubt. That is why I have posted it here.Please guide me. # I have some doubts regarding achromatism of lenses: 1)If we want to form an achromatic combination of lenses in which both...
  38. Cyrus

    New Lenses for My Glasses - Blind as a Bat

    I got new lenses today for my glasses. It says: Sphere: OD -3.75 OS -3.75 I was walking around the mall for an hour while they made my lenses blind as a bat. :bugeye:
  39. N

    Calculate Separation of Two Biconvex Lenses

    I am trying past paper exam questions for my wave optics paper and have got stuck on this question. Two identical biconvex lenses of refractive index 1.5 and with radius of curvature of 20cm on each side are to be used to produce a real erect image of incoming parallel rays. If the image is...
  40. Z

    Axial Thickness Definition for Optics Lenses Problem

    What is axial thickness? Is it the thickness between the two principle planes of a lens or is it just the thickness of the lens? I need the correct defintion to finish a problem.
  41. cronxeh

    Challenges in Focusing X-Rays with Lenses

    What are the current technical difficulties that prevent us from focusing x-rays with lenses?
  42. M

    Finding Focal Length of 2 Lenses Combined

    Hello everyone, I'm confused on this issue. How can I tell if the focal length is going to be postive or negative when 2 lens are combined like the ones below: Here is the directions: You grind the lenses shown in Figure 34-49 from flat glass disks (n = 1.5) using a machine that can grind a...
  43. M

    Does Magnification Affect Both Vertical and Horizontal Dimensions in Lenses?

    I was wondering if magnification applies to the vertical and horizontal dimensions, or just the vertical. The following information is about the question I am dealing with. It won't answer my concern above, but give a sens of what is going on. The question is dealing with a thin lens...
  44. I

    Same focus for all circular lenses?

    In school physics i have learned that the focus for a circular convex(converging) lens is f=r/2 where r is the radius and f is focus. But wouldn't a pair of identical lens made of a different material eg glass and perspecs which have different refractive indexes have different point of focus?
  45. N

    Wearing Multiple Contact Lenses: Is it Safe?

    Is it okay to wear more than one in each/either eye?
  46. C

    Solving a Problem: Calculating Position of Image with 2 Diverging Lenses

    Are considered two identical diverging lenses, each one of focal distance -10cm and separated 25cm. which it is the position of the image of an object located in the infinite? The answer is: Virtual image and to 7,7 cm of the second lens. I need to know how to solve the problem, step by step.
  47. 1

    Help with Lenses Questions: Angle & Image Size

    Could somebody please help me on the following questions: 1. A camera with a 50mm focal length lens focuses on a man 1.8m tall standing 40m from the camera. (a) What angle does the man subtend at the camera lens? [I do not know what subtend means] (b) What is the size of the image...
  48. C

    Images and Lenses (explanation needed please)

    Okay, so I have to do a Lab Report, easy enough, I've done them before, but it isn't so clear, and when I tried doing it, I wasn't sure if I was even doing it right. (Personal Note: DO NOT TAKE PHYSICS CORRESPONDANCE! ><) Anyway, here's what I have to do. The Lab Report is on the...
  49. R

    Calculating Image Distance in a Two-Lens System

    An eraser of height 1.0cm is placed in front of a two-lens system. Lens 1(nearer the eraser) has focal length f1=-15cm, lens 2 has f2=12cm, and the lens separation is d=12cm. For the image produced by lens 2, what is the image distance i2? What I did is setup some equations. For lens 1...
  50. K

    What are gravitational lenses?

    What are gravitational lenses?
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