tion:To determine the FOCAL length by an auxillary plane mirror method :If we are given a vertical stand , a plane mirror , a lens and a pin WE CAN PROCEED AS FOLLOWS :i. Place the lens L on the plane mirror M M' Kept on the horizontal surface of the vertical stand and arrange the pin P horizontally in the clamp so that its tip is vertically above the centre O of the lens L as shown in figure.ii. Adjust the height of the pin till it has no parallax with its inverted image as seen from vertically above the pin .To check for parallax , keep the eye vertically above the tip of the pin P at the distance NEARLY 25 cm from it and move it sideways . If the pin and its image shift together then there is no parallax.iii Measure the distance x of the Pin P from the lens and the distance Y of the pin from the mirror using a metre rule and a plumb line .Calculate the average of the two distance . This gives the focal length of the lens f = X + Y / 2iv. Take THREE difference observation after removing the parallax each time and FIND the mean value of focal length of the lens .THANK YOU