The document describes an experiment to determine the focal length of a lens combination using nodal points. The experiment involves using a nodal slide with two convex lenses, an illuminated slit, and a screen. By rotating the nodal slide and adjusting its position until the image on the screen remains stationary, the distance from the screen to the axis of rotation measures the first principal focal length. Repeating the experiment with the nodal slide rotated 180 degrees determines the other focal length.

1. Experiment of determination of focal length of lens combination by nodal points Trisha Banerjee @ 2010

2. Whole setup of the experiment:- Light source Illuminated slit wit cross wires Vertical plane mirror Nodal slide Trisha Banerjee @ 2010

3. Directions of rays:- Trisha Banerjee @ 2010

4. Lens keeping stand Position of the lens on slide Screw to move the stand

5. 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

6. Nodal slide:- Screw to move the carriage longitudinally Trisha Banerjee @ 2010

7. 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