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The focal length of a convex lens.

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Physics Full Laboratory Report (3) Title: The focal length of a convex lens. Aim: *To learn the technique of locating real image by no-parallax method. *To study the real image formed by a convex lens. *To determine the focal length of a convex lens by graphical method. *To determine the focal length of a convex lens by conjugate foci method. Apparatus: 1 convex lens on a holder (with focal length not greater than 15 cm) 1 1-meter ruler, 1 translucent screen, 1 plane mirror, 1 optical pin on a cork, 1 illuminated object (a lamp house with a letter 'F' on it) For further investigation: 1 more convex lens of the same focal length as above. Principles: A convex lens (converging lens) is a circular glass plate convex on both surfaces. The non-uniform thickness causes bending of light towards the principle axis. In particular, a convex lens converges light from infinity parallel to the principal axis to a point, called the focus (F). Light rays from infinity not parallel to the principal axis are converged to the focal plane as shown in Fig. a. When an object in placed at the placed at the focus F of the convex lens, the emerging light would be parallel to the principal axis.


The mirror and the lens were made to be parallel. 3. The pin was moved away from the lens along the principal axis until the separation was about the focal length. The position of the pin was adjusted until it coincides exactly with its image. In this case, there was no relative motion between the image and the object pin when we moved our head sideways, i.e. they do not separate. 4. The pin was then at the focus of the lens. The focal length of the lens was then measured. 5. The plane mirror was moved to vary the separation between the mirror and the lens. Experiment 3: Graphical method using an illuminated object 1. The lamp house was placed beginning with an object distance greater than twice the focal length (beyond 2F) of the convex lens. The position of the screen on the opposite side was adjusted so that a sharp diminished image was formed. The object distance u and the image distance v were measured. 2. Step 1 was repeated to capture 3 other diminished images and 4 other magnified images on the screen. The object distance u and image distance v were recorded in each case.


Adjust the vertical position of the pin so that there is no gap between the object and the image. In experiment 4, the displacement of the lens and the separation between the object and the image needed to be correctly measured. Sources of error: 1. There will be error when we plot the graph that will affect the calculated result. 2. The sharp image of bulb instead of the object letter F is form on the screen may affect the measurement of the focal length. Discussions: Actually the graphical methods are more reliable, as the experiment can repeat for many times and more data can be obtained. If the size of sample data increases, the result is more accurate. In using graphical method, we can also discard those obviously discrepant data. Theoretically, experiment 3 is more reliable than experiment 4. Since there is only one personal judgment in each trial in experiment 3, but there are 2 of that in experiment 4. If the lower half of the convex lens is covered, the image will be dimmer but still visible. In further investigation, we found that if 2 lens of similar focal length are placed in contact with each other, the new focal length will be shortened. If the original focal lengths of 2 lenses are the same, then the resulted focal length will be half of that of 1 lens.

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