Sunday, November 7, 2010



To determine the focal length of a convex lens using a vertical pin and a plane mirror by parallax removal method.


Convex lens, plane mirror, vertical pin, stand for mounting and a scale.


If an object is kept at the focus of a convex lens, the light rays incident from the object on the lens become parallel to the principal axis after refraction. The refracted rays fall normally on the plane mirror and retrace their path to form an image on the object itself.

1. Mount the pin, convex lens and the plane mirror on their respective stands as shown in the figure.
2. Adjust the height of the pin such that the tip of the pin and the optical centre of the lens are at the same horizontal level.
3. Adjust the position of the needle so that on looking through the lens at the image of the needle, there is no parallex between the object and the image.
4. Note the values of x1 and x2 from the scale.
5. Repeat the experiment for different positions of the convex lens.


1.The tip of the pin and the optical centre of the lens should be at the same horizontal level.
2.The parallax should be properly removed.


The focal length of the lens is found to be --------

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