Focal Length of A Convex Lens By Plane Mirror Method

Content of this page :

1.    Introduction of the experiment.

2.    Aim of the experiment.

3.    Tools of the experiment.

4.  Steps and methods of the experiment.

5.    Table of The Readings.

6.    Medical application and advantages of the experiment.


1. Introduction of the experiment:

The focal length of a convex lens, a fundamental optical property, can be accurately determined using the plane mirror method. This method leverages the principles of reflection and image formation to find the focal point of the lens.

2. Aim of the experiment:

To find the focal length of a convex lens by the plane mirror method.

3-Tools of The experiment

  •    Optical bench
  •    plane mirror
  •    object pin
  •    meter scale and holders

4. Steps and methods of the experiment:

1.Arrange the plane mirror, convex lens and object pin with help of holder on the optical bench as shown in the figure and align them properly with the help of a meter scale.

2. Fix the position of the plane mirror at one end of the optical bench. Now put the convex lens at 20cm distance from the plane mirror and locate the position of image behind the convex lens in a way to have no parallax between the image and object pin.

3.Record the position of the plane mirror, convex lens and the object pin. Keep the distance between the plane mirror and convex lens as 30cm, 40cm… for other set of the readings 

4.The distance between the convex lens and object pin is the focal length of the convex lens.


5. Table of the Readings:

6. Medical application

  • Eyeglass Prescription:

    • Application: Optometrists and ophthalmologists use convex lenses to correct refractive errors such as hyperopia (farsightedness) or presbyopia (age-related farsightedness).
    • Use of Focal Length: Determining the accurate focal length of lenses ensures precise prescription of corrective eyeglasses. The plane mirror method provides a straightforward way to verify or calibrate lens focal lengths, ensuring optimal visual correction for patients.
  • Contact Lens Design:

    • Application: Contact lenses are designed based on specific optical parameters, including focal length, to provide vision correction without the need for eyeglasses.
    • Use of Focal Length: The focal length of convex lenses, determined accurately through methods like the plane mirror experiment, guides the design and manufacturing of contact lenses. This ensures comfort and effectiveness in correcting refractive errors while maintaining ocular health.
  • Ophthalmic Instrument Calibration:

    • Application: Instruments such as ophthalmoscopes, slit lamps, and fundus cameras rely on lenses for magnification and imaging of ocular structures.
    • Use of Focal Length: Knowing the focal length of lenses used in diagnostic and surgical instruments ensures precise calibration. This enhances the accuracy of diagnostic imaging, surgical procedures, and measurements performed during ophthalmic examinations and treatments.
  • Vision Testing Equipment:

    • Application: Vision testing equipment, including phoropters and autorefractors, use lenses to assess visual acuity and refractive errors in patients.
    • Use of Focal Length: Calibrating lenses accurately using methods like the plane mirror experiment ensures reliable results in vision testing. This is crucial for diagnosing and monitoring conditions such as myopia (nearsightedness) and astigmatism, guiding treatment decisions, and evaluating the effectiveness of corrective measures.
  • Research and Development:

    • Application: Research in optics and vision science involves studying the properties and behavior of lenses for various applications in medicine and healthcare.
    • Use of Focal Length: Experimental methods like determining focal lengths using the plane mirror provide foundational data for developing new optical technologies, improving existing devices, and advancing understanding of optical principles relevant to medical optics.

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