6A60.49 - Thin Lenses - Focal Points and Focal Lengths

Code Number:
Demo Title:
Thin Lenses - Focal Points and Focal Lengths
Lenses and Image Formation
Area of Study:
Slide Projector, lenses of various focal lengths, screen, video camera.

Set the slide projector so that it is at least 25 ft. away from the lens.  Make sure the camera is focused well onto the screen.  Turn on the projector and use the lens to bring the light to a small point as seen by the camera.  Measure the distance from the lens to the screen.  This is your focal length.  Negative lenses may be used to show that no focal point can be produced.

  • Yoong Sheng Phang, Yiping Zhao, "Determining the Focal Length of Converging and Diverging Lenses Using a Smartphone", TPT, Vol. 60, #8, Nov. 2022, p. 703.
  • Jun Wang, and Wenqing Sun, "Measuring the Focal Length of a Camera Lens in a Smart-Phone with a Ruler", TPT, Vol. 57, #1, Jan. 2019, p. 54.
  • Jair Lucio Prados Ribeiro, "A Glass of Wine a Day Does Not Keep Optics Away! Reflection and Refraction Images in Wine Glasses", TPT, Vol. 51, #8, Nov. 2013, p. 506.
  • Leos Dvorak, "A Do-It-Yourself Optical Bench", TPT, Vol. 49, #7, Oct. 2011, p. 452. 
  • Donald Russell Trummel, "Error in Determining Focal Length", TPT, Vol. 42, #2, Feb. 2004, p. 123.
  • Steve Brown, "Finding the Focal Length of a Diverging Lens", TPT, Vol. 35, #8, Nov. 1997, p. 452.
  • D.A. Crandles and R.P. Kauffman, "Locating Images Formed by Diverging Lenses", TPT, Vol. 35, #6, Sept. 1997, p. 369.
  • Werner B. Schneider, "A Lens with an Adjustable Focal Length", TPT, Vol. 31, #2, Feb. 1993, p. 118.
  • Simon George, "Focal Length of a Thin Lens Under Water", TPT, Vol. 16, #6, Sept. 1978, p. 379.
  • Donald Russel Trummel, "Error in Determining Focal Length", TPT, Vol. 15, #2, Feb. 1977, p. 112.
  • William S. Heaps, "How Not to Focus a Small Source with a Single Lens", AJP, Vol. 55, #10, Oct. 1987, p. 888.
  • Fred M. Goldberg and Lillian C. McDermott, "An Investigation of Student Understanding of the Real Image Formed by Converging Lens or Concave Mirror", AJP, Vol. 55, #2, Feb. 1987, p. 108.
  • M. M. Michaelis, "Laser Beam Guidance by Flames", AJP, Vol. 48, #11, Nov. 1980, p. 990.
  • Og-5, 6, 12: Freier and Anderson, A Demonstration Handbook for Physics.
  • Pat Murphy, Ellen Macaulay, and the staff of the Exploratorium, "Finding the Focal Length", Exploratopia, p. 296.
  • David Kutliroff, "13, Make an Air Lens", 101 Classroom Demonstrations and Experiments For Teaching Physics, p. 38.
  • Jearl Walker, "6.93, Starting A Fire With Ice", The Flying Circus of Physics Ed. 2, p. 278.
  • "Nodal Slide: A Study of the Properties of Thick Lenses", Selective Experiments in Physics, CENCO, 1962.
  • Julius Sumner Miller, Q226 & A226, Millergrams II – Some More Enchanting Questions for Enquiring Minds, p. 70 & 117.
  • Julius Sumner Miller, Q230 & A230, Millergrams II – Some More Enchanting Questions for Enquiring Minds, p. 72 & 118.

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