Method for Finding The Focal Length of a Lens

The focal length is the distance from the optical centre of the lens to the focal point, which is the point where all reflected rays intersect (in a converging lens) or all the extensions of the reflected rays (in a diverging lens) intersect.

The focal length can be measured in different lenses.

1. Draw a line down the centre of the sheet of paper.

2. Lay the lens down so that the centre of the lens is over with the centre of the sheet.

3. Trace the outline of the lens.

4. Place the ray box in front of the lens.

5. Using a three or five beam filter, project parallel rays onto the lens.

6. Draw a few dotted lines on the refracted rays’ paths to identify the position of each light ray.

7. Remove the ray box and the lens from the paper.

8. Using the draft created in the previous step, draw a line starting from the lens through the dotted lines.

9. At the intersection point between the drawn rays, add an F indicating the position of the focal point.

10. Put away the equipment.

By knowing the position of the focal point, it is possible to determine the focal length of the lens used. For this purpose, mark where the centre of the lens is on the sheet of paper. Then simply measure the distance between the centre of the lens and the focal point, which is the focal length of the lens. In the example below, the focal length of the converging lens is |\text {12.45 cm}.|

1. Draw a line in the centre of the sheet.

2. Lay the lens down so that the centre of the lens is over the centre of the sheet.

3. Trace the outline of the lens.

4. Place the ray box in front of the lens.

5. Using a three or five beam filter, project parallel rays onto the lens.

6. Draw a few dotted lines in the refracted rays to identify the position of each light ray.

7. Remove the ray box and the lens from the paper.

8. Starting from the lens, draw a line through the dotted lines and extend each ray behind the lens. 

9. At the intersection point of the extension lines of the drawn rays, add an F indicating the position of the focal point.

10. Put away the equipment.

By knowing the position of the focal point, it is possible to determine the focal length of the lens used. For this purpose, mark where the centre of the lens is. Then simply measure the distance between the centre of the lens and the focal point, which is the focal length of the lens. In the example below, the focal length of the diverging lens is |\text {13.7 cm}|.

To find the focal point of a converging spherical lens, it is necessary to work with an optical bench, which is a meter stick that is supported by optical bench supports.

1. Prepare the optical bench by placing the metre stick into the bench supports.

2. Put the lens into the lens support, and place it at one end of the metre stick.

3. Place a screen in a screen holder, and position it behind the lens.

4. Position the light source as far as possible from the lens, in front of the lens.

5. Move the screen forward or backward until you get a clear, crisp image. The resulting image should be bright and very small.

6. Measure the distance between the tip of the lens support and the tip of the screen support.

7. Put away the equipment.

The distance obtained between the two supports represents the focal length. In the example above, the focal length of the converging lens is |\text {12.3 cm}.|

By using this method, it is possible to calculate the focal length without having to draw anything. However, ensure that the image obtained on the screen is optimal. It may, therefore, be practical to redo the procedure a second time in order to validate the experimental results.