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. M

    Converging Lenses and Magnification

    I'm in the middle of studying for the MCAT, and just want to get all the concepts instead of memorizing how to solve problems. I'm having a hard time understanding converging lenses. From what I understand, converging lenses create real images that are inverted. This property of inversion...
  2. J

    Luneburg Lenses: Understanding Diffraction, Spill-over and Focusing

    Hi there! I wonder if anyone could explain to me exactly how the optics of Luneburg lenses work? Are there any major losses due to diffraction, spill-over etc? Is it possible to focus just one image using this lens? Thanks, Jean
  3. S

    What factors determine the focal length of a lens?

    Let's say that there are three lenses A (all of which are semicircles: flat on one side, curved on the other). One lense is a short and curved D, another lense is a very elongated and narrow D (longer than the first lense), and the third lense is as long as the second lense but has a sharper...
  4. S

    Exploring the Relationship between Lens Shape and Focal Lengths

    Let's say that there are three lenses A (all of which are semicircles: flat on one side, curved on the other). One lense is a short and curved D, another lense is a very elongated and narrow D (longer than the first lense), and the third lense is as long as the second lense but has a sharper...
  5. G

    Calculating Focal Length of Human Eye When Focusing Closely

    The focal length of a relaxed human eye is approximately 1.78 cm. When we focus our eyes on a close-up object, we can change the refractive power of the eye by about 16 diopters. Calculate the focal length of the eye when we focus closely. 1/di+1/do=1/f im not really sure what to...
  6. R

    Question about optics and thin lenses

    Homework Statement Hey, I was just doing some questions on thin lenses, and thought of a question I really want answered just can't find anything on in the text or anywhere. So the question involves a lens with a focal length of f = 18.18 cm now the question asks if the lens would be...
  7. A

    What are some real-life applications of lenses?

    1. Are all convex lenses also converging lenses? The attempt at a solution Convex refers to shape. Converge refers to action. My answer is yes, provided that the material the lens is made of is denser than air, or denser than some other surrounding medium. Am i correct? OR are there other...
  8. U

    Finding the Correct Position of a Converging Lens for a Sharp Image

    Homework Statement a candle is placed 36cm from a screen. where between the candle and the screen should a converging lens with a focal length of 8.0cm be placed to produce a sharp image on the screen? Homework Equations 1/focal length = 1/distance of object from lens + 1/distance of...
  9. T

    Combination of lenses and mirrors.

    Homework Statement The lens and mirror in the figure below have focal lengths of +74.0 cm and -59.1 cm, respectively. An object is placed 1.00 m to the left of the lens as shown. http://www.jmlproductions.net/jmlproductions/p36-54.gif I need to find the final image location. Homework...
  10. K

    Studying without glasses/ with under-corrected lenses

    Hi everyone, You can call me kwen & I've got a question which has been on my mind for some time now. ~ All my life I've been studying with full-powered glasses (perfect vision) and I understand that in the long run, that's not very good for the eyes due to the strain it puts on them...
  11. D

    Contact lenses, focal length and uncorrected far point distance

    1. A nearsighted person wears contacts with a focal length of -7.6cm. If this person's far point distance with her glasses is 8.7m, what is her uncorrected far point distance? 2. 1/F=1/do+1/di 3. 1/-7.6=1/do+1/870 =-7.534 I am completely lost here...please help. Thanks!
  12. E

    Combining Lenses: Focal Length & Transfer Matrix

    Homework Statement 2 thin lenses separated by 10cm. First has a focal length of 20cm, the second has a focal length of 30cm.Homework Equations Lens transfer matrix for each lens: \begin{bmatrix} 1-\frac{D_2d_{21}}{n_{t1}} &-D_1-D_2+\frac{D_1D_2d_{21}}{n_{t1}} \\ \frac{d_{21}}{n_{t1}} &...
  13. grooveactiva

    Two thin converging lenses and intermediate image.

    Homework Statement Here is the layout of 2 lenses: -----------------(1)------------(2)-------------I-------------- Two thin lenses, numbered 1 and 2, with the intermediate image "I" is to the right of one of the lenses. The distance between the lenses is lettered d. Both lenses have...
  14. R

    Lenses and Autocollimation: Altering Focal Length and the Lensmaker's Equation

    Homework Statement a) In the autocollimation experiment of Sec. 3.5, a thin convex lens is placed 25 cm from a pinhole that acts as a point source of light. A plane mirror placed 20 cm behind the lens reflects the light back through the lens, and the reflected light forms a sharp spot beside...
  15. R

    Help with thin lenses and uncertanty

    Heyy this is my first time on here and I'm only doing first year physics so my understanding might not be up to scratch =/ I'm doing a practical on thin lenses where we have to do a various amount of measurements and calculations to do with convex lenses. However I'm not very confident with...
  16. X

    Cleaning Rod lenses in optics equipment

    Hi, We have an optical equipment that has a narrow entrance slit, on the other end of the narrow entrance slit two rod lenses are exposed. They are getting dirty for various reasons. Does anybody know of any handheld air/vacuum cleaning with thin nozzle that can penetrate the slit and even...
  17. 0

    Mysterious Ball Lenses: What Are They & What Are They Used For?

    any idea what these ball lenses are.. ? any specific application?
  18. I

    Understanding Concave Lenses: A Homework Statement

    Homework Statement the question is attached The Attempt at a Solution ok so a and b i can do but c), i donts know whether my answer is right and I am a little confused about the conceptual idea... so first thing i did was found the radius of curvature using the lens maker equ...
  19. I

    Finding object distance with converging lenses.

    A converging lens with f=33.7 cm yields an image with magnification M=−2. Find the object distance. Give your answer ... * in terms of centimeters. Do not include units in your answer. They will be assumed to be cm...
  20. Q

    Calculating Focal Length Range for Projector with Varying Distances

    Hi. There's a question that has given some variables which am not sure how to compute to some previous lens/optics equations. These are not the actual variable values by the way. If a projector (square pixel SXGA) has a diagonal size of 5 mm which uses a screen of 1 m in which the distances...
  21. I

    Lenses and optics: solving for focal point.

    Lenses and optics: solving for focal point. (SOLVED) (2 points, penalty of 0.5) When a flower pot is placed 1.2 meters on the left of a magnifying glass (converging lens), an image is formed on a screen 0.37 meters to the right of the lens. Find the focal length of the lens. Give your answer...
  22. W

    Lenses and Optical Instruments

    1. A diverging lens with a focal length of -14 cm is placed 12 cm to the right of a converging lens with a focal length of 18 cm. An object is placed 33cm to the left of the converging lens. Where will the final image be located. 2. 1/f = 1/di + 1/do dob = L - dia 3. After solve...
  23. P

    Focal length of her contact lenses

    Homework Statement Anne is farsighted; the nearest object she can see clearly without corrective lenses is 2.4 m away. It is 1.8 cm from the lens of her eye to the retina. What should the focal length of her contact lenses be so that she can see clearly objects as close as 24.0 cm from...
  24. S

    What does a negative distance indicate in a combination of lenses?

    Homework Statement As shown in the diagram below (not to scale), a 3.0 cm high object is placed 50.0 cm from thin converging lens A that has a focal length of 10.0 cm. Converging lens B with a focal length of 15.0 cm is placed 25.0 cm to the right of lens A. Find and describe the final...
  25. L

    Understand Near & Far Points of the Eye: Lenses

    Homework Statement Could someone explain to me about the near point and the far point in the eye because I'm confused about the 25 cm for the near point and infinity for the far point. Also, like in problems like this: What eyeglasses would you prescribe for persons with the following...
  26. L

    Diverging Lens: Find Object Given Real Image & Size

    Homework Statement The focal length of a diverging lens is -20 cm. Locate the object, given that the image is real, erect, and 150% of the size of the object. Homework Equations (1/f)=(1/do)+(1/di) M=-(di/do) The Attempt at a Solution i'm not really sure how a diverging lens forms a...
  27. R

    Optics, lenses, and light reflection

    Hi, I'm new to this place, and I'm not sure if my problem fits with this forum, but I'll give it a try. Right now, I'm building a laser range finder, but my biggest concern will be focusing the reflected laser beam onto a small silicon photodiode. And to add to that, I have barely any...
  28. G

    Solve for di: Negative Effect on Object Distance?

    When I'm given do, ho, and hi. How would i solve for di Normally i would do -di/do = hi/ho and cross-multiply but I'm not sure how the negative comes into effect. do = object distance di = image distance ho = object height hi = image height If... ho = 3cm hi = 4cm do = 5cm...
  29. J

    Optics - combination of thin lenses

    What if two converging or diverging lenses were placed side by side, in contact with each other. Where would be the image formed for the converging lens, would it be to the right of the lens combined or to the left? Why? Also, where would it be if both lens were diverging? thanks!:)
  30. S

    Understanding Path Difference in Convex Lenses

    Path Difference in Lenses... A convex lens will converge all rays parallel to the optical axis to the focus. Now obviously different rays at different distances from the optical axis will travel different distances in reaching the focus. So why does this not create any path difference between...
  31. J

    Basic optics question - lenses

    Why if two concave and convex lenses are nested together, will there be no resulting convergence or divergence if their focal lengths are the same, but if the concave lens has a larger focal length the rays will converge slightly? What is (are) the mathematical equation(s)/reason(s)?
  32. G

    Does Light Focus When Passing Through Converging and Diverging Lenses?

    Homework Statement A converging lens has a focal length f, and a diverging lens has a focal length –f, which has the same magnitude as the converging lens. They are separated by a distance D which is greater than f, as shown. Parallel light enters from the left. Will the light be brought...
  33. F

    Thin lens equation with multiple lenses

    Homework Statement Heres the problem exactly as written: A system of lenses is composed of three lenses. Calculate the maximum magnification these three lenses will achieve if the lenses are located 35.0 cm apart. The lenses have focal lengths of 15.0, 20.0, and 25.0 cm.Homework Equations 1/p...
  34. G

    How does the f-number of camera lenses affect image area?

    I was reading on the f-number of lenses for cameras...somewhere in the text, they stated that the area of the image is proportional to q2, in which q is image distance. I figured the area to be proportional to q, but I don't really see it as q2...so how does this work out?
  35. G

    Need simple help with: Duality of Light and Gravity Lenses

    hello members of www.physicsforums.com ! primarily i want to apologize for the double thread post; i posted another thread at this link: https://www.physicsforums.com/showthread.php?p=1950906#post1950906 and i am afraid it was homework and i did not see the homework section so please forgive...
  36. G

    How do virtual and real objects affect the image formed by multiple lenses?

    The statement is that the image formed by the 1st lens acts as the object for the 2nd lens. For 2 thin lenses aligned along a common axis, would this statement be true for when the image formed by the 1st lens is real and past the 2nd lens (while ignoring the effect of the 2nd lens in this...
  37. C

    How do I determine the distance between two lenses in a telephoto lens system?

    Homework Statement A simple camera telephoto lens consists of two lenses. The objective lens has a focal length f1 = +36.0 cm. Precisely 33.0 cm behind this lens is a concave lens with a focal length f2 = -10.0 cm. The object to be photographed is 3.67 m in front of the objective lens. (a)...
  38. G

    Thin Lenses: Lens Makers' Equation Proved

    I was reading on thin lenses, in which the lens makers' equation was proved. The proof first gave the lens (a biconvex lens) a thickness (which it would later consider negligible) so that the light would pass through one surface and out the other (so it comes into contact with 2 surfaces). The...
  39. C

    Where is the image due to lens 1 and is it on the right or left?

    Homework Statement There are 2 lenses separated by 5 cm. Very far to the left of the first lens is an object (the distance of the object from lens one is NOT given). The focal length of lens 1 is 20 cm and the focal length of lens 2 is 10 cm. Where is the image due to the first lens? State the...
  40. G

    Real Images and Thin Lenses: Can You Explain the Postulation?

    I was reading a bit on thin lenses when a postulation was made that the image formed from the first surface would be the object for the second. The postulation made sense for when the first image is virtual, but I don't see how it works when the first image is real? Can anyone provide me the...
  41. F

    Lenses and Infinity: Exploring Image Formation at Infinity

    Hello, here is my question: if an object position can only tend to infinity, without ever reaching it (since infinity is just an abstraction), its image will tend to appear on the back focal plane of a converging lens, without ever forming there. The image will always be created on the image...
  42. P

    Object 18cm in Front of Lens to Reduce Image by 2.0

    Homework Statement An object is 18cm in front of a diverging lens that has a focal length of -12cm. How far in front of the lens should the object be placed so that the size of its image is reduced by a factor of 2.0? Homework Equations 1/do + 1/di = 1/f m= -di/do The Attempt at a Solution...
  43. M

    Medical Questions About Contact Lenses and Laser Eye Surgery

    have absolutly no idea where to put this but ill put it here My eyesight is something like +6.25/+6.50 Now the eye man (optomotrist?) said that they only make contact lenses that go up to +6. I was wondering why this is? Also why can't long sighted people have laser eye sugery? Is there...
  44. N

    Two identical convergent lenses

    Homework Statement the figure shows a combination of two identical lenses. There is a 2cm tall object that is 36 cm away from the first lens (f=9 cm). The second lens is 15 cm away from the first lens. It also has a focal length of 9 cm. So, the first question was "find the position of the...
  45. K

    Geometrical optics LENSES Refraction

    Homework Statement A lens is made of glass for which the refractive index is n[r]=1.5 for red beams and n[v]=1.52 for violet beams. radii of both surfaces are the same and equal R=1m. Find the distance between foci for red and violet beams Homework Equations The Attempt at a Solution
  46. K

    Solving Two Lenses In A Row Homework Problem

    Homework Statement Two lenses, one converging with focal length 20.0cm and one diverging with focal length -10.0cm, are placed 25.cm apart. An object is placed 60.0cm in front of the converging lense. Determine the position of the final image. Homework Equations \frac{1}{s_i} +...
  47. B

    Practical Applications of Converging Lenses: Object Positions and Uses

    Homework Statement For each of the following object positions in front of a converging lens, suggest at least one practical application. (E.g. overhead projector, camera etc.) a) Between the lens and F' b) At F' c) Between F' and 2F' d) At 2F' e) Beyond 2F' The Attempt at a...
  48. D

    Building a Telescope for Physics Class: Choosing Lenses & Mirrors

    Hi, I need to build a telescope for my physics class. I need to use any 3 lenses(converging or diverging) and 1 mirror. I was thinking of using an achromatic telescope design for that. However I am not sure what kind of lenses to use and whether I should make the eyepiece from two lenses or just...
  49. L

    How Does Lens Focal Length Affect Image Distance?

    1. An object is placed in front of a converging lens in such a position that the lens (f = 11.0 cm) creates a real image located 27.0 cm from the lens. Then, with the object remaining in place, the lens is replaced with another converging lens (f = 19.0 cm). A new, real image is formed. What is...
  50. N

    Angle deflection from gravitational lenses

    Hi all The expression for the angle deflection on the following page: http://relativity.livingreviews.org/open?pubNo=lrr-1998-12&page=node2.html Is the impact parameter r in the expression the distance to the surface of the objects that deflects the light or the the center of mass of the...
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