Three Ways To Find Focal Length

1、convex lens:

direct use of sunlight through the convex lens, there is a smallest and brightest point on the paper, is the focus. Then measure the distance from the focal point to the center of light, which is the focal length.
2、concave lens:

with several laser Pointers, shot from different places, when the refraction light is parallel, from the incident light can be extended to a point, that is, the focus, and then measure the focus to the center of light distance, is the focal length.

3、focal length：

Is the measurement of the aggregation or divergence of light in the optical system, refers to the distance from the optical center of the lens to the focus of light aggregation when the parallel light incident. Optical systems with short focal lengths have a better ability to concentrate light than optical systems with long focal lengths. Simply put, the focal length is the distance between the focal point and the center of the mirror. Camera focal length f< Image distance <2f to image.
4、In thin lenses in air,

The focal length is the distance from the center of the lens to the main focus. For a converging lens (such as a convex lens), the focal length is positive, and a parallel beam of light will converge on a single point.

For a divergent lens (such as a concave lens), the focal length is negative, and a parallel beam of light will spread out after passing through the lens.