You will need
- The light source
The simplest way to determine the focal length of the lens is experimental. Position the light source at some distance from the screen, obviously exceeding the double of the focal distance of the lens. Parallel to the imaginary segment connecting the light source with the screen, attach a line. Hold the lens to the light source. Slowly moving it in the direction of the screen, achieve the appearance of a clear image of the light source. Mark on the ruler with a pencil the place where the lens.
Continue to move the lens toward the screen. At a certain point on the screen will appear again a clear image of the light source. Also note on the line is the position of the lens.
Measure the distance between the light source and the screen. Erect it in the square.
Measure the distance between the first and second provisions of the lenses and also erect in the square.
Subtract the first result of the squaring by the second.
Resulting from subtracting the number of divide by four times the distance between the light source and the screen, and you get the focal distance of the lens. It will be expressed in the same units in which measurements were taken. If this does not suit you, put it in a convenient unit.
To determine the focal distance of the dispersing lens directly. This would require an additional lens - collecting, and its focal distance may be unknown.
Position the light source, the screen and the line the same as in previous experience. Gradually pushing back the collecting lens from the light source, obtain a clear image of the light source on the screen. Lock the lens into position.
Between the screen and the collecting lens place of scattering, the focal distance you want to measure. The image will become vague, but it is not necessary to pay attention. Measure at what distance from the screen is the lens.
Remove the screen from the lensuntil the image becomes focused. Measure the new distance from the screen to the scattering lens.
Multiply the first distance to the second.
Subtract the second distance from the first.
The result of the multiplication divide by the result of the subtraction, and you get the focal distance of the scattering lens.