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. Alex_Neof

    Determining the focal length of a compound lens

    Homework Statement A biconvex lens, L_1, with focal length of magnitude \left| f_1 \right| = 15 cm is used with a second biconvex lens, L_2, with focal length of magnitude \left| f_2 \right| = 25 cm to form a compound lens system. If the two lenses are placed in mutual contact what is...
  2. D

    Finding the Focal Length: A Homework Challenge

    Homework Statement Homework Equations 1/do+1/di=1/f The Attempt at a Solution I tried finding the distance of image at 27m and then at 30.5m and taking the difference but that didn't work.
  3. W

    Deriving back focal length using Fermat's principle

    Homework Statement The plano-convex lens has a diameter of 200 mm and the central thickness of 20 mm. The edge thickness is zero and the refractive index of glass is 1.5. Find the following parameters of the lens: (i) the back focal length Homework Equations N/A The Attempt at a Solution I've...
  4. B

    Small focal spot in X-ray imaging

    Unfortunatley I cannot post this question in the Homework section (not sure why) so I hope its OK if I post here and show work as appropriate < Mentor Note -- thread moved to HH from the technical physics forums, so no HH Template is shown > QUESTRION State two design features in an X-ray tube...
  5. F

    Thick Lens focal length and surface facing the object

    Hello Forum, In the case of an ideal, thin lens (free from any aberration), it does not matter which face of the lens is facing the object. The results will be identical. What if the lens was an ideal (no aberration) thick less with the H planes, etc...? Would it matter which lens surface is...
  6. NaukowiecGirl

    Infinity Focal Points of Plane Mirror

    Hello! I've read on several pages that plane mirrors have an infinite amount of focal points. I don't understand? I thought plane mirrors have no focal points because the rays are parallel and don't focus in the first place. Why does a plane mirror have infinity focal points and what does it mean?
  7. vetgirl1990

    What is the relationship between focal length and focal points?

    Homework Statement This is a silly question... but I know that a lens has two focal points but one focal length. If each focal point is at 5cm from the lens, would the focal length be 5cm, or 10cm (adding the position of each focal point together)? Homework Equations N/A The Attempt at a...
  8. Alettix

    Measurement of Focal length with Laser beam

    Homework Statement Hi! I have lately come across some tricky experimental physics tasks, where no solution is given. Some of them involved parts where the focal length of a convex lens had to be measured with a laser. How do you do this? Homework Equations 1/l + 1/d = 1/f (1) where f is the...
  9. K

    Can you find lens magnification from focal length?

    If you know the focal length of a lens and use the lens equation 1/do + 1/di = 1/f by assigning some arbitrary do and solve for di, can you then find the magnification just from the focal length from -di/do?
  10. M

    Focal length of diffraction grating for recip. dispersion

    Homework Statement What would the focal length be of a diffraction grating if the set up is a laser placed 1 m from the 0 order diffraction? Would it be the distance from the grating to the laser or the distance from the grating to the image? Homework Equations 1/f = 1/o + 1/i Rd = [Df]-1...
  11. K

    Find Focal Length of Diverging Lens | Physics Homework Help

    Homework Statement When an object is placed at the proper distance to the left of a converging lens, the image is focused on a screen 30.0 cm to the right of the lens. A diverging lens is now placed 15.0 cm to the right of the converging lens, and it is found that the screen must be moved 19.2...
  12. A

    System of lenses with a light source at first focal point

    Homework Statement So I have this system of lenses with this problem: First lens: focal length ##f_1=50 mm## Second lens : focal length ##f_2=400 mm## Distance between them ##d=600mm## A small light source is placed at the focal point for the first lens. At the focal point for the second...
  13. Kompewt

    Focal length of thin converging lens

    Homework Statement An outdoor security camera has a simple lens with focal object length fo = 6.00 cm and perfectly focuses the image of a bird onto its detector located 9.00 cm away from the lens. The animal then flies away from the lens and the new focal distance is 8.00 cm. (a) How far did...
  14. P

    Equivalent focal length of two lenses

    Homework Statement Two thin lenses have focal lengths of -5cm and +20cm. Determine their equivalent focal lengths when (a) cemented together and (b) separated by 10cm. Homework Equations ##\frac{1}{f_{eq}}=\sum_{n=1}^k \frac{1}{f_n}## The Attempt at a Solution So the first part was easy since...
  15. Max Matiauda

    Image formed from object at focal point of concave mirror

    The question posed: "An object is located to the right of a mirror that in concave in its [the object's] direction. If the object sits on the focal point of the mirror, what answer best describes the image formed?" Our class is united in the notion that as the rays would be parallel...
  16. D

    Focal length of magnifying glass for VR?

    Hi the standard focal distance seems to be 45mm for vr headsets such as google cardboard. In other words there's a pair of convex lenses in front of your eyes with the screen being 45mm away from the lenses ... I am not sure why you can't use different lenses to reduce the focal distance to the...
  17. W

    Optics - Imaging from focal plane

    Hi! Assume paraxial rays. If I have a lens with a focal length ##f## and I place an object at the focal length to the left of the lens, the image will be at infinity. Correct? But will it be imaged in infinity to the left or right of the lens? If I am looking into the lens from the right I...
  18. B

    Understanding the Human Eye: Exploring Focal Length and Retinal Distance

    I never could understand this: the lens focuses light onto the retina so shouldn't the distance between the lens and the retina be the focal distance? It does not seem to be the case while I was trying to look around for the answer on the internet. And since light is focused on the retina, the...
  19. M

    Focal length of lens at border of two liquids

    Homework Statement I need to find optical power (reciprocal focal length) of this system with thin lens Homework Equations I tried to solve this using spherical diopter equation n1/a+n2/b=(n2-n1)/R where a is object distance and b is image distance The Attempt at a Solution equation for...
  20. S

    Calculating Focal Length of 1D Fresnel Lens

    Homework Statement Calculate the focal length of 1D Fresnel lens, whose transmittance is given as $$T(\xi)=\frac 1 2(1+\cos(\alpha \xi ^2)).$$ Homework Equations Anything you wish The Attempt at a Solution I have no idea. I tried to use the equation for diffraction image $$u_p=C\int _0...
  21. M

    Determining the focal length of a gradient index lens

    Homework Statement There are three subquestions in this question, all marked bold. Let's consider a gradient index lens of thickness ##d##, whose refractive index changes with the distance from the axis with the following formula $$ n(r) = n_1 + a r^2 $$. Determine the lens's focal length...
  22. DrewHizzy

    Diverging and Converging lenses in a system

    Homework Statement A)Find the final position of the image (from the object, I assume?) B)Find the size of the final image of the object. Homework Equations 1/f = 1/do + 1/di The Attempt at a Solution 1) Solved first distance: 1/5 - 1/4 = 1/di --> 4/20 - 5/20, di = -20cm 2) Use di1 for do...
  23. B

    Distance from the object to objective lens and focal length

    Homework Statement 1) The medium-power objective lens in a laboratory microscope has a focal length fobjective = 3.70mm . If this lens produces a lateral magnification of -42.5, what is its "working distance"; that is, what is the distance from the object to the objective lens? 2) What is the...
  24. S

    Calculating the distance between the optical center and focal length

    -Hello everyone. At school, I studied about convex lense and concave lense. They are very interesting, we learned about how lights would be after going through the lense. But what I want to ask is how we can calculate the distance between optical center and focal (I means focal length). Focal...
  25. brianeyes88677

    Understanding the Focal Plan of Parallel Light Through a Thin Lens

    Why does parallel light focus on the focal plan when enters a thin lens at an angle θ? How to proof that mathematically?
  26. R

    Focal length of 2 lenses to reduce laser beam diameter

    I need to reduce the beam diameter of a laser I am using. I plan to do this by first passing the beam through a positive spherical lens with focal length f1 and then a negative spherical lens with focal length f2 and f2 < f1. By placing the lenses so that the two focal points overlap I will have...
  27. Robsta

    Calculate focal length of lens by diffraction.

    Homework Statement A collimated beam from a white-light source is incident normally on a transmission grating with 500 lines per mm. The transmitted light then passes through a lens which is used to project the visible (380–780 nm) spectrum of the light source on to a strip of photographic film...
  28. A

    Focal length of a distorted window

    Homework Statement What is the focal length of a dewar window distorted by a pressure difference? As a result of the pressure difference, the radius of the first surface of the window is R1 = 100 meters. The window is 3 mm thick, with an index n = 4 at l = 4 um. Homework Equations I think I...
  29. F

    Interference viewed in the focal plane

    I'm doing a problem where we have plane monochromatic waves incident on finite double slits (each of width a, separation d), and the transmitted light is observed in the focal plane of a lens of focal length f. The position of the lens isn't specified. So I have worked out that the intensity...
  30. R

    Focal length of the optical system

    I understand this that when an object is kept at focus of convex lens, then image is formed at infinity. So when we keep a convex lens above plain mirror and move object needle on the principal axis of convex lens and see a coinciding image with needle on lens and measure distance from lens then...
  31. S

    Focal length to get clear image

    Homework Statement Harris stands at point A and wants to take a picture of bird located at point B. Because there is wall, he has to direct his camera to mirror located 3.7 m to the north from his position. What is the focal length of his camera so that he can take a clear picture of the...
  32. M

    Focal length from thin lens to object distance

    Hello, I've come across an problem that requires the lens to be at least about 350 mm away from the object. Object to be examined is 80 mm tall and imaging plane is either 11 mm or 25 mm tall. I've tried to calculate required focal length, but have not succeeded. Distance from lens to imaging...
  33. C

    Focal Length Problem: -30cm Converging/Diverging Lens

    Homework Statement A lens made of glass has one concave side with a radius of curvature of 5 cm, and one convex side with a raduis of curvature of 7.5 cm. What is the focal length of the lens? Is this a converging or diverging lens? Homework Equations nglass=1.5 nair=1.0...
  34. P

    How is the focal length of an objective calculated?

    Hi, I am using an objective lens to illuminate my object by a collimated beam. The magnification and NA is available on the objective. How can I calculate the focal length from this information? The objective is not infinity corrected. Thank you
  35. S

    Laser focal point manipulation

    Is it possible with modern optic technologies to achieve the following effect: A relatively thing laser beam passes 30-50 cm and after that completely defocuses? It should defocus under very broad angle. What I exactly mean is creation of a 2D screenless images just in a thin air with help of a...
  36. H

    Focal Length of a Plane Glass: How to Measure and Understand Its Infinite Power

    Homework Statement what is the focal length of a plane glass ? Homework EquationsThe Attempt at a Solution place a plane glass in a plane and allow light to pass through it measure its focal length (i think its focal length is infinite )
  37. K

    Finding the focal length of a negative lens without another lens.

    Homework Statement A simple method for determining the focal length of a negative lens is shown below. Prove this is true. Homework Equations The Attempt at a Solution The linear magnification formula relates magnification, the distance from the lens to the image (i), the distance from...
  38. D

    How Do You Calculate the Focal Length of a Telescope's Objective?

    Homework Statement An optical telescope with a 12mm eyepiece makes the Moon appear to have an angular diameter of 30 degrees. What is the focal length of the telescope's objective? Homework Equations magnification=focal length of objective/focal length of eyepiece The Attempt at a...
  39. D

    Focal length of combined glass lens and water lens

    Homework Statement please refer to the photo, Homework Equations The Attempt at a Solution why can't I directly use the lens maker formula to get the ans ? my working is 1/f = ( (1.5-1.33) /1.33) x ( (1/20) + 0 ) ...finally , i get my f = -160cm. the radius of curvature of...
  40. E

    Focal length of converging meniscus lens filled with a liquid

    Homework Statement A converging meniscus lens is made of glass with index of refraction n = 1.55, and its sides have radii of curvature of 4.5 cm and 9 cm. The concave surface is placed upward and filled with carbon tetrachloride which has index of refraction n' = 1.46. Determine the focal...
  41. A

    Measuring the focal length of a deformable lens

    Hello all, My lab recently purchased a deformable lens, which can change its focal length from 140 mm to 40 mm and anywhere in between, based on current applied to it. I'm wondering how to use a beam profiling camera to measure the focal length of this lens at various current levels. We...
  42. M

    Focal length of a sphere with refractive index gradient

    Homework Statement We take a sphere (1mm) which has a parabolically changing refractive index, which is given in a function. Homework Equations Depending on the gradient of the refractive in the sphere, how does it correlates with the focal length. The Attempt at a Solution I...
  43. T

    Focal length short and long. Which to choose?

    I'm building a telescope and one of the more important choices I'm making is what focal length to make the mirror, my understanding is the longer the focal length the closer you see far away stuff, but the field of view diminishes a bunch. I've seen both long tube dobsonians and short tube ones...
  44. 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.
  45. 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?
  46. 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...
  47. X

    Optics: Focal length of Koenig eyepiece

    Homework Statement A Koenig eyepiece with a focal length of 100mm is constructed in the following way: The first lens is made of 755276 glass which is 10 mm thick on axis. The radius of curvature of its left hand surface is -225mm, the radius of curvature of it right hand surface is 83.6...
  48. L

    Analyzing Two Converging Lenses of the Same Focal Length

    Homework Statement Two converging lenses of the same focal length f are separated by distance 2f. The axis of the second lens is inclined at angle θ = 60º with respect to the axis of the first lens. A parallel paraxial beam of light is incident from left side of the lens. Then: (A)...
  49. 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...
  50. A

    Image from object at a focal point of mirror

    Hi all, If I place an object far away from a converging mirror or lens and start to move it closer, I note that I get an inverted, real image that steadily grows in size. As the object crosses the focal point the image becomes incredibly large and blurry and then flips upright and begins to...
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