Focal length is the most important characteristic of any lens. However, this parameter is usually not indicated on the magnifying glass itself. In most cases, only the magnification is indicated on them, and on rimless lenses there is often no marking at all.
It is necessary
- Light source
- Screen
- Ruler
- Pencil
Instructions
Step 1
The simplest way to determine the focal length of a lens is experimental. Place the light source at some distance from the screen, obviously greater than twice the focal length of the lens. Attach a ruler parallel to the imaginary line connecting the light source to the screen. Lean the lens against a light source. Move it slowly towards the screen until the light source appears clearly on the screen. Mark the location of the lens on a ruler with a pencil.
Step 2
Continue to move the lens towards the screen. At some point, a clear image of the light source will appear on the screen again. Also mark this lens position on the ruler.
Step 3
Measure the distance between the light source and the screen. Square it.
Step 4
Measure the distance between the first and second lens positions and also square.
Step 5
Subtract the second squared from the first.
Step 6
Divide the resulting number by four times the distance between the light source and the screen to get the focal length of the lens. It will be expressed in the same units in which the measurements were made. If this does not suit you, convert it to units convenient for you.
Step 7
It is not possible to directly determine the focal length of a diffusing lens. To do this, you will need an additional lens - collecting, and its focal length may not be known.
Step 8
Position the light source, screen and ruler in the same way as in the previous experiment. Gradually move the collecting lens away from the light source to achieve a clear image of the light source on the screen. Lock the lens in this position.
Step 9
Place a diffuser, the focal length of which you want to measure, between the screen and the collecting lens. The image will become blurry, but do not pay attention to this for now. Measure how far this lens is from the screen.
Step 10
Move the screen away from the lens until the image is focused again. Measure the new distance from the screen to the diffuser lens.
Step 11
Multiply the first distance by the second.
Step 12
Subtract the second distance from the first.
Step 13
Divide the result of multiplication by the result of subtraction, and you get the focal length of the diverging lens.
