Focal Length of A Convex Lens By Plane Mirror Method

Introduction Of The Focal Length Of A Convex Lens

The focal length of a convex lens by plane mirror method is a classic optics technique used to accurately determine this fundamental optical property. This method leverages the principles of reflection and image formation to locate the focal point of the lens without needing a separate screen or image-detection setup.

The plane mirror method works because light behaves predictably when it strikes a reflective surface at a perpendicular angle — it simply retraces its own path. By placing an object pin at the exact position where its reflected image shows no parallax, the lens is effectively forcing incoming light to travel back the way it came, which only happens when the pin sits precisely at the lens’s focal point. This makes the technique both simple to set up and reliably accurate, which is why it remains a standard exercise in optics and medical physics laboratories.

Aim Of The Focal Length Of A Convex Lens By Plane Mirror Method

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

Tools Of The Focal Length Of A Convex Lens By Plane Mirror Method

Optical bench
Plane mirror
Object pin
Meter scale
Holders

Steps And Method 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.

Table of The Readings Of The Focal Length Of A Convex Lens By Plane Mirror Method

Sr. No.	Position of Mirror (cm)	Position of Convex Lens (cm)	Position of Object Pin (cm)	Focal Length (cm)	Mean Focal Length (cm)
1.
2.
3.
4.
5.

Medical Application

Eyeglasses: Focal length helps determine the correct lens power for treating refractive errors such as hyperopia and presbyopia.
Contact Lenses: Accurate focal length measurements assist in designing lenses that provide effective vision correction.
Ophthalmic Instruments: Lenses with known focal lengths are essential for proper calibration of instruments such as ophthalmoscopes and slit lamps.
Vision Testing: Accurate lens focal lengths improve the reliability of equipment used to measure visual acuity and refractive errors.
Medical Research: Focal length measurements support the development and improvement of optical technologies used in medicine.

Frequently Asked Questions About Capillary Tube Experiment

Why is there no parallax between the object and its image at the correct position?

When the pin sits exactly at the lens's focal point, light rays leave the lens parallel, strike the mirror perpendicularly, and retrace their path exactly — making the image coincide with the object pin at every viewing angle, which is what "no parallax" means.

Why take multiple readings at different mirror-lens distances?

Repeating the measurement at several distances and averaging reduces the effect of small setup or reading errors, giving a more reliable value for the mean focal length.

Why use a plane mirror instead of just measuring where the image forms directly?

The mirror reflects the light back through the lens, letting you locate the focal point using only the object pin's position — no separate screen or image-detection setup is needed, which simplifies the apparatus.