What is Lens: Definition and 830 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. Seydlitz

    Why Does an Object at a Lens's Focal Point Create an Image at the Same Location?

    Homework Statement A small object is located on some distance from a converging lens. At some distant behind a converging lens lies a flat mirror. The resulting image is exactly at the same location as the small object. Why can we deduce that the object is located in the focal point of the...
  2. B

    Focal Length of a Plano Convex Lens

    Hi, The focal length of the Plano-Convex lens as calculated by the lens maker equation: Does it work when light crosses the lens from the convex side, planar side, or both? Thanks.
  3. F

    Second focal point of a concave lens

    Why is the focal point to the left of a concave lens called the second focal point? Isn't it just a matter of the name? So can't the focal point on the left of the lens be called F1 instead of F2?
  4. N

    Moving an object and screen to find the focal length of a lens

    Homework Statement A convex lens is used to project the image of an object on a screen with a magnification m1 . Leaving the lens fixed, the object is moved along the optical axis by a distance Δs and the screen is moved until the image is located again. If the new value of the...
  5. A

    Lens and mirror question, can't understand solution

    Homework Statement A lens of focal length 0.8m is 1.0 m to the left of a mirror of focal length -0.500m. An object is placed 1.00 m to the left of the lens. Light goes through the lens, bounces from the mirror, and goes through the lens again, this time from the right. Homework Equations...
  6. L

    X-Ray Burning Lens: Why Can't it Focus?

    I know that High-energy (hard) X-ray light cannot be focused as easily as visible light by using a burning glass. But why is this? If there was x ray light spread over a normal sized lens with the same intensity as sun light why couldn't it focus? would the material needed for say 50cm^2...
  7. M

    Why does a positive lens focus two images at different lengths?

    Missed this part of the lab and I need to complete my lab write up. We have a light source at one end of a track, and at the other end a screen. there is a positive lens in between them. When we set the screen at a distance away from the light source, let's say one meter, and slide the lens...
  8. Q

    Does focusing light with a lens create a light gradient?

    *disclaimer I am not a physicist Had a weird thought the other day - when you focus light with a lens, for example a magnifying glass, you basically increase the 'concentration of photons' at a certain point, right? But then energy is conserved... so wouldn't focusing some of the light on...
  9. mishima

    Preserving Cow Eyeball Lenses: What's the Best Method?

    Hi, I was thinking about getting a lens from a cow eyeball dissection kit. I was wondering if I need to do anything special to prevent it from decomposing. What exactly is it made of?
  10. C

    Thin lens equation and image formation

    Homework Statement An object 4.29 cm high is projected onto a screen using a converging lens with a focal length of 32 cm. The image on the screen is 51.7 cm. Where are the lens and the object located relative to the screen? ho=4.29 cm hi=51.7 cm ƒ=32 cm Homework Equations 1...
  11. Luck0

    How the interference of light changes if there's a lens?

    Suppose we have the Young's experiment with a convergent lens between the slits and the screen. How would the interference fringes change? Sorry about the silly question, but I'm having a hard time with geometrical optics. Thanks!
  12. G

    What is the solution for object distance in a diverging lens?

    Homework Statement Now consider a diverging lens with focal length f=−15cm, producing an upright image that is 5/9 as tall as the object. What is the object distance? You will need to use the magnification equation to find a relationship between s and s′s′. Then substitute into the thin lens...
  13. D

    MHB Convex and concave lens and paraxial rays

    What effect would a convex (concave) lens have on paraxial rays? For a convex lens, the rays would pass through and converge on the focus on the other side. For a concave lens, the rays would reflect back to the focus on the same side. Is this correct?
  14. B

    Thin Lens Equation: Nearsighted

    Homework Statement For those fortunate souls who do not need glasses, the lens of the eye adjusts its focal length in order to form a proper image on the retina. This typically means that very distant objects as well as objects as close as 25 cm can be seen clearly. Many of us need...
  15. G

    How to Calculate Image Position and Magnification for a Camera Lens?

    Homework Statement A camera lens has focal length 55mm and an object is 500mm away. Calculate the position of the image and the magnification (or reduction) Homework Equations 1/u + 1/v = 1/f u = Object Distance v = Image Distance f = focal length m = magnification The...
  16. N

    Explaining the Lens Equation: Why h' & h Positive, s' Negative & s Positive?

    The lens equation is given as M = h'/h = -s'/s It's not explained in the book as to why h' and h are positive and s' is negative while s is positive. Could someone explain?
  17. D

    4 Lens optical system/fourier transform

    Question on my study guide: An optical systems consists of 4 lenses spaced apart. Each lens has a focal length f. Each lens is located a distance "z" away from each plane as shown. The total length of the system is 8z. Find the distance z needed to satisfy a FOURIER TRANSFORM condition...
  18. S

    Fermat's principle to derive lens formula

    1. For the past 1 hour, I'm trying to derive lens maker's equation using fermat's principle, which of course is our homework. Any help would be great regarding the topic. 2. According to Fermat's Principle, optical path length OPL = PA + AQ, here PA and AQ are two rays. Now using this I...
  19. Spinnor

    Neutrino beam passes through optics lens, focus the neutrinos?

    If a neutrino beam passes through an optics lens in principle does the matter in the lens focus the neutrino beam granted it may be ridiculously small amount? If the lens were instead made of compressed matter of nuclear densities would the answer change much? Thanks for any help!
  20. H

    Lens Reflection: Does Light Bounce Off?

    When a passing through a lens,will there any reflection on the surface of it?
  21. U

    Diffraction Grating, focal length of lens

    Homework Statement Light from infinity strikes a diffracting grating normally with 600 lines per mm. The emitted light then passes through a lens to project visible light (400-800 nm) spectrum onto a screen that is 50mm, just enough to cover it. Find the focal length of the lens. Homework...
  22. D

    [College Electrical Engineering/Physics] Waves & Lens

    Homework Statement Show that when a plane wave is transmitted through a thin lens of focal length f in a direction parallel to the axis of the lens, it is converted into a paraboloidal wave (the Fresnel approximation of a spherical wave) centered about a point at a distance f from the lens...
  23. G

    Lens of Constant Thickness

    Homework Statement The index of refraction of glass can be increased by diffusing in impurities. It is then possible to make a lens of constant thickness. Given a disk of radius a and thickness d, find the radial variation of the index of refraction, n(r), which will produce a lens with...
  24. U

    What is the maximum refractive index for lens numerical aperture?

    Hi, I have a question about resolution as defined by rayleigh's equation r = 0.61 x lambda/numerical aperture The maximum half angle of acceptance of a lens is 90 degrees and the sine of this is 1. At this point the only thing that can increase the value of the denominator in the equation...
  25. L

    Orientation of Image Produced by Double Convex Lens

    This is not really a problem, more just a fact-based question. I have been using the Google, but I have been unable to find an answer. Is there a way to induce the answer that I am missing? :confused: Question: What range of distance does a double convex lens have to be from an object to...
  26. N

    Optical components - lens and mirror

    Homework Statement a) Despite the diverging lens with focal length f we observed object which is perpendicular to the optical axis (Figure 6 in the article ,,5. Rozptylka a zrcátko"). To what distance from the diverging lens we have to put the subject to be scaled twice? (cross-magnification Z...
  27. M

    Fresnel lens project with my dad

    Hello, So my dad and I took apart our old TV that had a Fresnel lens inside. Unfortunately there is a crack near the edge due to a billiard ball accidentally being dropped on it. I was wondering how much this crack would affect the lens' ability to focus the sunlight into a point. When we...
  28. Q

    Optics question: converging lens with virtual object

    Hi everyone, I was thinking about an optics question in a physics textbook. I think the solutions I saw in the solution manual may be wrong. The question is: When you do the calculations using 1/p + 1/q = 1/f you find that the lens first creates an image 400 cm to the...
  29. V

    Images produced by a simple two lens microscope?

    Homework Statement The image formed by a simple two lens microscope is (a) real and inverted. (b) real and upright. (c) virtual and upright. (d) virtual and inverted. (e) none of these. Homework Equations N/A The Attempt at a Solution I was confused on how to understand how...
  30. Z

    Why can photonic crystals be used to create a super lens?

    Firstly, sorry if this doesn't go here please feel free to move it as I was a little confused where to put it. TL:DR Doing a project question interests me, "Why can photonic crystals be used to create a super lens?" Okay so I am doing an undergraduate project for my honours year that I...
  31. G

    Using diffraction slit as a lens

    Homework Statement "A TV tube manufacturer is attempting to improve the picture resolution, while keeping costs down, by designing an electron gun that produces an electron beam which will make the smallest possible spot on the face of the tube, using only an electron emitting cathode followed...
  32. M

    Angle of Incidence on a spherical lens

    Firstly, I'm sorry if this is incorrect or if there is a specific place for such questions but as this is neither a problem posed to me, nor something that has been taught - I have little background with which to work with but it is something I need to do for my ERT and 2 maths teachers have...
  33. S

    From what point is the focal length of a thick lens measured?

    I'm designing an eyepiece for fun and want to be accurate. I just realized my physics book equations are approximated for thin lenses only. Suppose I have a convex convex lens or a plano convex lens, lateral parallel rays coming from one side or the other, and a focal point on the opposite...
  34. M

    How to Simulate a Camera Using a Modulation Transfer Function?

    Hello, Homework Statement and Homework Equations I need to simulate a camera given it's Modulation Transfer Function (MTF), which is a polynomial f(x). The expression for the MTF is: y = 2E-08x4 + 0.0008x2 - 0.464x + 1.0076 (with x the spatial frequency in number of cycles/mm)...
  35. L

    Imaging the surface of a half ball lens

    Hi, I'm new to this forum and I have an application where I want to image an object that is in contact with the round side of a half ball lens. I've attached a brief project description. I think this topic on optics is suitable for the Differential Geometry forum, but please let me know if...
  36. V

    Expression of an object and its image onto a screen with two lens?

    Homework Statement Formulate an expression that gives the focal length of a corrective lens, to be used in conjunction with a fixed lens. You may assume that both lenses are on the same plane, i.e. that they are very close together. The corrective lens should allow an object at a specified...
  37. W

    Understanding Virtual Images in Lenses and Mirrors

    1. A stamp collector uses a converging lens with focal length 32 cm to view a stamp 17 cm in front of the lens. Where is the image located? a. in front of the lens (real image) b. behind the lens (virtual image) 2. 1/f = 1/do + 1/di 3. 1/32 = 1/17 + 1/di di = -32.267 cm...
  38. M

    Aberrations at Lens Elements as Unsharp Filter

    I want to learn do aberrations and compensation actions used as unsharp filter for final image ? I read a Leica paper. There was symmetrical lens Summicron as a subject. I found that at the exit of the lens there were no aberrations without corrected. But when you look inside , some lens...
  39. S

    Question about diverging lens for correcting myopia

    Homework Statement The problem and its solution are attached as (the leftmost and middle) jpg files. (Given that the problem depends on the drawing, I think it's more convenient for the reader to view the text in the image as well.) Homework Equations N/A The Attempt at a Solution...
  40. U

    Elementary lens system, microscope with negative ocular

    Homework Statement Surgeons often use a type of "binocular microscope" [that's a word by word translation]. It consists of a 50mm lens close to the patient and a -15mm right in front of the doctor's eye. The distance between the patient and the 50 mm lens is 400mm and the negative lens is...
  41. S

    Diverging lens problem -image should be between focus & optical centre

    Homework Statement Problem: An object 2.0 cm high is placed in front of a diverging lens of focal length 1.8 cm. The object is 2.7 cm from the optical centre of the lens. Questions: 1) Where is the image located with respect to the focus and the optical centre of the lens, qualitatively? 2)...
  42. A

    Galaxies Rotate Fast: Gravitational Lens Effect

    Hypothesis: Galaxies appear to rotate too fast because their own gravitational lens let's them appear bigger than they really are. Just an idea, do you think it could work ?
  43. K

    Real 3D Image produced by concave lens

    Homework Statement Find the correct image produced by the concave lens. F is focal length. Homework Equations If an object is placed between F and 2F, the image will be inverted, real, larger and beyond 2F. The Attempt at a Solution I choose the answer A. Is it correct? *Please...
  44. G

    Focal distance of lens submerged in water

    Homework Statement A biconvex lens with radius of curvature 15 cm has a focal length of 15 cm in air. What is its focal length if it is submerged in water (n=1.33)? Homework Equations 1/f = (n-1)(1/R1 - 1/R2) The Attempt at a Solution 1/f = 1/0.15 = (n-1)(2/0.15) Solving for n...
  45. A

    Optics: Can a magnified real image be larger than the lens diameter?

    Hi all, This is probably a silly questions, but I want to be sure :). I'm wondering if a real image, created by a convex lens, can appear larger than the lens diameter itself. As an example, I'm thinking about the following: - I have a an object of height h_0=7.76" (display size of...
  46. O

    Optics Question, where is the lens

    Homework Statement A 3 cm tall object is placed 11 m from a wall. If the image is to be projected upside down with a 20 cm focal lense, what is: A} The type of lens used? B] Where is the lens placed? C] What is the height of the image? D] What is the magnification of the...
  47. R

    Electromagnetic lens for an SEM

    Does anyone know how the machining tolerance in the construction of an electromagnetic lens affects resolution in a scanning electron microscope?
  48. A

    Calculating distance of image from lens

    Homework Statement An object is placed on the optic axis and in front of a converging lens of focal length 38 cm such that its image size is 1.5 times the object size. Calculate the distance of the image from the lens. Express your answer in centimeter. Homework Equations 1/f=1/di +...
  49. S

    Engraving laser machine with a lens

    Hi I have a laser beam(engraving laser machine) and then use a lens for focus the beam but my lens has a crack on the center of lens and when i use my laser in high power the beam diameter will bigger and burning area zone is bigger so: 1- Does the crack doesn't let the maximum focus?in law or...
  50. H

    Choosing the Right Fresnel Lens for Automotive Headlamp Projection?

    Hello, I'm working with an automotive headlamp and I was asked to simulate a projection of this lamp onto a wall 25 feet apart. I know I need to use a Fresnel lens as a collector to achieve this, however I have many questions: 1. How should I choose the right Fresnel lens? 2. I need to...
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