The focal length of a concave lens using a convex lens

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.    Parameter, Theory and Final law of the experiment.

6.    Medical application and advantages of the experiment.

1. Introduction of the experiment:

The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system’s optical power. A positive focal length indicates that a system converges light, while a negative focal length indicates that the system diverges light

2. Aim of the experiment:

-To determine the focal length of a concave lens using a convex lens.

3-Tools of The experiment

·         Concave lens and holder.

·         Convex lens and holder.

·         Two mounted pins.

·         Optical Bench.

·         Illuminated object.

·         Screen.



4. Steps and methods of the experiment:

1-Find the focal length f1 of the convex lens.

2-Combine the two lenses together by placing them in the same lens holder and find the focal length F of the convex combination by the same method.

3-Deduce the focal length f2 of the concave lens from the following equation

1/F= 1/f1 + 1/f2 ………..(1)


5-ParametersFinal Law of The experiment


v1: Distance of first image from convex lens (cm)

v2: Distance of second image from concave lens (cm)

d: Distance apart of lenses (cm)

(v1-d): Virtual object distance from concave lens (cm)

f: Focal length of concave lens (cm)

Final Law:

the focal length of concave lens


6-Table of The readings:

7-Medical application

lensometer is an instrument measures the power of an unknown lens by moving an illuminated object unit a sharp image is seen in a viewing lens, the viewing lens is focused for light.

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