What is Focal: Definition and 292 Discussions

The HP-41C series are programmable, expandable, continuous memory handheld RPN calculators made by Hewlett-Packard from 1979 to 1990. The original model, HP-41C, was the first of its kind to offer alphanumeric display capabilities. Later came the HP-41CV and HP-41CX, offering more memory and functionality.

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  1. F

    Calculating Focal Length and Image Height of a Lens

    Homework Statement A lens forms an image of an object. The object is 16 cm from the lens. The image is 12 cm from the lens on the same side as the object. (a) What is the focal length of the lens? Is the lens converging or diverging? (b) If the object is 8.5 mm tall, how tall is the image...
  2. F

    Does Focal Length Differ for Convex Lenses?

    I know that for concave lenses, the focal length is one half the length of the radius of the circle. Is this also true for convex lenses? Thanks
  3. I

    Can spherical aberration be avoided in lenses with non-spherical curves?

    I've been doing some calculations on lenses, and I'm a little confused about some of the diagrams I've seen. Looking at a ray diagrams, I see the light passing through the lens, at one angle, and then converging on a focal point from there. Using snells law to calculate refraction through the...
  4. H

    Software to calculate diffraction figure at focal point of concave mirror?

    Classic paraboloidal mirror. Incoming flat wavefront of known wavelength (500 nm, green light), propagating parallel to the major axis of the mirror. The wave hits the mirror and is being sent back towards focus. At the focal plane, there's a flat CCD or some other light detector. Upon hitting...
  5. A

    Why Does Tube Length Affect Microscope Focal Length?

    I am trying to design a microscope and I am confused about some concepts. I plan to buy a Olympus 4x infinite-corrected objective lens and 6 inch long tube and a tube lens (f= 75mm) and then a CCD camera. By chance I found a tutorial online...
  6. M

    Determination of microlens focal length

    Hi guys! I face some difficulty in finding the focal length of a microlens. Assuming that the microlens is spherical, and the medium of the surrounding is vacuum. I really need an equation to get me going for my research task. I actually found the lensmaker's equation which is /=(−)(/−/+(−)/...
  7. D

    Finding Convex Lens Focal Length

    Homework Statement Following data recorded: Di (distance image to lens) 6.0 8.0 10.0 12.0 14.0 Magnification 0.2 0.6 1.0 1.4 1.8 Graph magnification against di and use the graph to find the focal length of the lens. Homework Equations...
  8. V

    Calculate Focal Length to Reduce Viewing Distance to 25cm

    Homework Statement A patient is unable to see objects closer than 1m from the eye. Calculate the focal length of the converging lens which is required to reduce this to 25cm. Homework Equations Lensmaker's equation: 1/u + 1/v = 1/f u= distance from object to lens v= distance of...
  9. T

    Find Image & Object Distances from h, h', f

    length. I was asked to find the image and the object distance. I didnt think it was possible till i found this M=h'/h=-q/p : I knew this q=-ph'/h : I understood how they did this 1/p+1/q=1/f : I knew this 1/p+1(-ph'/h)=-1/f : This i understood also, but how they came up with...
  10. A

    System focal length for thin lenses

    Homework Statement Three “thin” lenses, each with focal length 25 mm, are placed 5 cm from each other. What is system focal length? Homework Equations Lens equation: (1/oxd) + (1/ixd) = i/fx Oxd = object distance Ixd = image distance fx = focal The Attempt at a Solution...
  11. edpell

    The Longest Focal Length for Lenses & Mirrors

    What is the longest focal length practical for a lens and/or shaped mirror? I ask because it would be good if we could put a mirror at GEO and shine sunlight at one spot on the Earth 24 hours per day for energy production. The problem is sunlight is dispersing 1% so from 36,000km it would...
  12. W

    Calculate Lens Focal Length - Convex & Concave Lenses

    Homework Statement I have an actually interesting practical homework. I need to calculate the optical strength (hope I translated it right, D=1/F), to do I need to find the lens focal length. I need to find my self a concave and a convex lens. I don't have a concave lens (I think), but I do...
  13. J

    Find focal length for meniscus convex

    Homework Statement Find focal length for meniscus convex (3) Given r = 15, and n = 1.5 Homework Equations Lens maker's equation The Attempt at a Solution I can find the rest except #3 and #6 (they are opposite sign of each other) I had (1/r1 - 1/r2) gives zero... which is wrong... I...
  14. K

    Finding object distance with only the focal length and magnification?

    A magnifying glass has a diverging lens with a 13.0 cm focal length. At what distance from a toothpick should you hold this lens to form an image with a magnification of +5.00? I simply have no idea what to do. I tied moving around the equations to make them work, but I can't figure it out...
  15. D

    What is the focal length of the mirror?

    Homework Statement A 2.0 cm tall object is placed in front of a mirror. A 1.0 cm tall upright image is formed behind the mirror, 165 from the object Homework Equations 1/s + 1/s' = 1/focal length h' / h = magnification -s' / s = magnification h' / h = s' / s (i found this...
  16. O

    Deriving Focal Length for a Thick Lens

    Homework Statement Derive 1/f = (n-1)*( (1/r1)-(1/r2)+(((n-1)t))/(nr1r2))) using the ray matrix method. Homework Equations Matrix for curved dielectric interface: M1 = [1, 0] [(1/r)(n1/n2 - 1), n1/n2 ] Matrix for translation through homogeneous medium: M2 = [1, d] [0...
  17. S

    Focal length based on light color

    Homework Statement A thin lens has radii of curvature R1=-8.00 cm and R2=-6.00 cm, indices of refraction n(red)=1.50 and n(violet)=1.53 for red and violet light, and is surrounded by air. An object at distance s=20.0 cm from the lens has an image at distance s' from the lens. Is there a second...
  18. fluidistic

    What is the mathematical expression for the index of refraction in a GRIN lens?

    Homework Statement I must calculate the focal length of a GRIN lens. I.e. a lens which radii of curvature are both infinite. I will consider the refractive index to be varying in function of y (the height in the x-y plane). Say I'm given that at y=0, n(0)=n_0. I'm not sure how to write...
  19. G

    Simple Optics Question : Back focal length

    Homework Statement http://img256.imageshack.us/img256/6330/35727193.jpg Homework Equations [PLAIN]http://upload.wikimedia.org/math/2/0/a/20aa1e3d192ecb3164ac4f2095c86cd3.png The Attempt at a Solution i have used triangle from up side of h1 to the b. and by using sinß and...
  20. T

    Calculating Focal Length of a Drop of Water

    Homework Statement What is the focal length of a drop of water in the form of a sphere with radius 4mm given that the refractive index of water = 4/3 Homework Equations The Attempt at a Solution Here, i think i will need to assume that the light rays are coming from infinity...
  21. C

    Magnification; What's focal point

    After using the equation 1f = 1÷do + 1÷di I found what the focal point / f was. Now, what would the focal point represent? Is the focal point the image height or object height in the equation m = hi÷ho = -di÷do
  22. A

    How Does Bisecting a Lens Affect Its Focal Length?

    1)Why when a convex lens having radius of curvature R, focal length f and refractive index μ is bisected horizontally along the principal axis its focal length remains the same, whereas when it is bisected vertically focal length becomes 2f? Does the same thing happens in the case of...
  23. C

    Exploring the Difference in Focal Lengths of Telephoto & Convex Lenses

    Why is a telephoto lens considered to have a longer focal length than that of single convex lens?
  24. C

    Telephoto lens and focal length being longer that single lens.

    1. Why is a telephoto lens considered to have a longer focal length than that of a single convex lens? A lens combination, such as a diverging and a convex lens, is referred to as a telephoto lens. Suggest why a telephoto lens is considered to have a longer focal length than that of a single...
  25. C

    Why is a telephoto lens considered to have a longer focal length than that of single?

    1. Why is a telephoto lens considered to have a longer focal length than that of a single convex lens? A lens combination, such as a diverging and a convex lens, is referred to as a telephoto lens. Suggest why a telephoto lens is considered to have a longer focal length than that of a single...
  26. S

    Focal Length Calculation for Concave Mirror with Multiple Image Distances

    Homework Statement An object is placed in front of a concave mirror, 15.0 cm from the mirror's focal point. The image formed by the mirror is five times farther away from the focal point. Calculate the focal length of the mirror. There are actually two possible image distances di that...
  27. F

    Fresnel Lens Array - Focal Point Question

    Hello, all, I am using 9 fresnel lenses (~1x1' per lens) and 9 water blocks for a solar water heater. Each fresnel lens is positioned so that a water block below is heated. A pump is used to pump the water through the system. However, I have one question... I have all of my fresnel...
  28. K

    HELP Finding the focal length of a lens, I'm totally stuck

    Homework Statement A slide projector needs to create a 81 cm high image of a 2.0 cm tall slide. The screen is 360 cm from the slide. Assume that it is a thin lens. What focal length does the lens need? How far should you place the lens from the slide? Homework Equations 1/f = 1/s +...
  29. E

    Index of refraction and focal length question

    Homework Statement A Lens maker desires to make a lens of focal length 30 cm. in air. The equipment in the shop is set to make lens faces of 20 cm. a. What index of refraction should the lens material have to produce this focal length? b. What would the focal length of this lens be...
  30. M

    Combination/Convergence of Laser Beams On A Singular Focal Point

    Hey Guys, first of all I apologise if this question is posted in the wrong forum section and a mod has to move it to its correct location. I am trying to find information on combining the paths of several laser beams. I have found much complex information pertaining to the combination of the...
  31. P

    Lenses- magnification adn focal length

    Homework Statement Hey,, I was wondering if anyone could help me with some physics work i am doing as i am completely stuck. Is there supposed to be any relationship with the magnification and focal length of lenses and if so what is it? I did an experiment and measure the magnification of...
  32. J

    Can a Magnetic Lens Have Multiple Focal Points?

    Hello, I wanted to ask, is it possible for a magnetic lens to have many focal points, when the magnetic field of the lens is sufficiently large? if so what is the reason for this? how does this give better resolution of images compared to a simple optical lens. I know for a normal optical lens...
  33. S

    Focal Length of Lens: Solving for f with Object and Image Distances

    The homework problem is "An object 26.0 cm in front of a certain lens is imaged 1.60 cm in front of that lens (on the same side as the object). What is the focal length (in centimeters) of this lens" 1/f = 1/di + 1/do I tried solving based on having distance of object (do) and...
  34. A

    What is the actual focal length?

    I understand completely how to go about doing this problem... I know that you would have to plot the points and draw a line of best fit and that the x-intercepts and y-intercepts would equal 1/f... but what would the actual focal length be? and how do you calculate the gradient? This is actually...
  35. M

    What Is the Focal Length of a Flat Glass Lens?

    What is the focal length of a thin, flat piece of glass with no curvature? I found that the power is 0 diopters, but I don't know what the focal length is.
  36. Z

    Optical System: Two Identical Lenses w/2fo Focal Length

    Homework Statement An optical system consists of two identical lenses; the focal length of each lens is fo (where fo) is positive). The lenses are separted by a distance d. Denote the focal lenths of the lens assembly by f and f'. a) Show that for any separation of the lenses f = f'...
  37. T

    The Focal Length of a Lens?

    Homework Statement An object is located 26.0cm from a certain lens. The lens forms a real image that is twice as high as the object. (i) What is the focal length of this lens? (ii) Now replace the lens used in Part 1 with another lens. The new lens is a diverging lens whose focal...
  38. V

    Calculating Focal Length of Eye-Lens System

    Homework Statement a) What is the focal length of the eye-lens system when viewing an object at infinity? Assume that the lens-retina distance is 1.8cm. b) What is the focal length of the eye-lens system when viewing an object 35cm from the eye? Assume that the lens-retina distance is 1.8cm...
  39. M

    How Do You Choose the Right Lens for Photographing a Building at 490m?

    Homework Statement A building will be photographed with a camera. The distance is 490 m to the building. The building height is 85m and width 40m. You can choose between 35mm, 50mm, 135mm and 250mm lenses (focal length of lenses). Select the lens that gives the largest image of the building...
  40. P

    The Magnification of a Microscope Given Distances and Focal Lengths

    Alright, I was hoping that someone could help me out with this... The problem is: "The distance between the eyepiece and the objective lens in a certain compound microscope is 21.0 cm. The focal length of the eyepiece is 2.60 cm, and that of the objective is 0.400 cm. What is the overall...
  41. V

    Focal Length - please check my work and tell me what's wrong.

    Homework Statement A small object is 25.0 cm from a diverging lens as shown in the figure . A converging lens with a focal length of 11.4 cm is 30.0cm to the right of the diverging lens. The two-lens system forms a real inverted image 17.0 cm to the right of the converging lens. The...
  42. Z

    Spectacles, Focal length and Lens Formula

    Homework Statement When using the lens formula: 1/f = 1/u + 1/v for someone who has short sightedness and cannot focus on objects on objects more than 2.5m away, I get: 1/f = 1/infinity + 1/-2.5 f = -2.5 mIf using the formula for someone who had long sightedness and could not focus on...
  43. M

    What is the image height formed by the new diverging lens?

    Homework Statement An object is located 27.5 cm from a certain lens. The lens forms a real image that is twice as high as the object. b) Now replace the lens used in Part A with another lens. The new lens is a diverging lens whose focal points are at the same distance from the lens as the...
  44. G

    Focal point of image in magnifying glass

    focal point of image in magnifying glass -------------------------------------------------------------------------------- I have been given a statement and have to decide if it is true or false and then justify my answer. I have already established that this statement is false by the fact...
  45. G

    Focal point of image in magnifying glass

    I have been given a statement and have to decide if it is true or false and then justify my answer. I have already established that this statement is false by the fact that a magnifying glass creates a 'virtual image' not a 'real' one but i am having trouble working out what else is false...
  46. S

    Finding focal length given magnification and object distance?

    The distance between an object and its upright image is 15.5 cm. If the magnification is 0.7, what is the focal length of the lens being used to form the image? Answer in units of cm. So I tried combining the equations p+q=15.5 and -q/p=.7 and still was unable to come up with a correct...
  47. D

    Focal Length of Lens: Object 5 cm Tall, 10 cm Away

    An object 5 cm tall is 10 cm from a convace lens. The resulting image is one fifth as large as the object. What is the focal length of the lens? I have been doing questions involving focal lenght, but not when the resulting image is a fraction of the original image. i do not know where to...
  48. W

    Thickness of lens and its focal length

    I'd like to ask does thinner convex lens give a shorter focal length? If so, why is it? thanks very much~~
  49. 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...
  50. 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...
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