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How can you dertermine focal length of convex lens experimentally ? |
| Answer» Determination of Focal Length of a Convex Lens by Distant Object Method\tMount the convex lens and an object needle on their respective uprights. place the upright carrying the object needle in front of the lens on an optical bench.\tAdjust a plane mirror at the back of the lens, such that its plane is vertical and perpendicular to the axis of the lens.\tMove the needle forward and backward and adjust its position to get real and inverted image of the needle formed by the lens, after reflection from the plane mirror.\tWhen the tip of the image in the plane mirror coincides with the tip of the optical needle, i.e., focus of the lens. The distance between the optical needle and the lens gives the focal length of the lens.\tThe rays proceeding from the object at the principal focus after refraction through the lens are rendered parallel. These parallel rays are reflected from the plane mirror and after refraction through the lens, meet at the focus.Determination of Focal Length of a Convex Lens by Auxillary Method\tPlace a convex lens on a plane mirror kept on the horizontal surface of the vertical stand and arrange the a pin horizontally in the clamp such that its tip is vertically above the centre of the lens.\tAdjust the height of the pin until it has no parallax with its inverted image when seen from vertically above the pin. To check for parallax, the eye is moved side wards. If the pin and its image shift together, then parallax is moved.\tMeasure the distance x of the pin from the lens and the distance y of the pin from the mirror, using the plumb line and a\xa0meter scale.\tCalculate the average of these two distances i.e., x and y, that gives the focal length of the lens.\xa0\tFocal length of the convex lens, f = (x + y) / 2\xa0\tTake three different observations and find the mean focal length of the convex lens. | |