Experiment of determination of focal length of lens
combination by nodal points
Trisha Banerjee @ 2010
Whole setup of the experiment:-
Light source
Illuminated slit wit cross wires
Nodal slide
Vertical plane mirror
Trisha Banerjee @ 2010
Directions of rays:-
Trisha Banerjee @ 2010
Lens keeping stand
Position of the lens on slide
Screw to move the stand
Apparatus :- Nodal slide assembly and two convex lens.
Procedure in brief:- The nodal slide is clamped in front of a screen with an illuminated slit fitted with cross wires .on the other sides remote from the screen, is mounted a vertical plane mirror. The distance between the screen and the lens system is adjusted in such a way that a well defined image of the slit is obtained on the screen adjusted to it. obviously the , centre of the slit is at the first principal focus of the system. The carriage is now rotated through a small angle and it will be found that the image shifts sideward's to the right or the left. The carriage and the upright ( stand) carrying the nodal slide are then adjusted such that the direction of rotation of the image changes its sign and finally the image remains stationary for a slight rotation of the carriage. The distance between the screen and the axis of the rotation for no shift in the image measures the first principal focal length of the lens system. The other focal length can be determined by the turning the nodal slide through 180 degree and repeating the experiment.
Trisha Banerjee @ 2010
Screw to move the carriage longitudinally
Trisha Banerjee @ 2010
Nodal slide:-
Setting of the nodal slide:-
To locate the Nodal points , the lens system is mounted with its axis horizontal on a carriage which can be rotated about the vertical axis and can be moved longitudinally with the help of a rack and pinion arrangement . This apparatus is known as Nodal slide.
Note:-
parallax should be removed very carefully and the stationary points also.
Trisha Banerjee @ 2010
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