Focal length
2024-6-2
Definition:
A measure of how well an optical system focuses or defocuses light.
Many types of optical systems (e.g., microscope objectives and curved laser mirrors) can focus or defocus light, and focal length is a quantity that quantifies these effects.
The simplest case is a thin lens (Figure 1a). If the light entering the lens is a collimated beam, the beam will be focused after passing through the lens, and the focal length is the distance from the lens to the focal point (assuming that the prism is in a vacuum or air, rather than in a material with a high refractive index).
For a defocusing lens (Figure 1b), the focal length is the distance from the lens to the virtual focus (represented by the dotted line), which is a negative value.
Figure 1: Focal lengths of focusing and defocusing lenses. A defocusing lens has a negative focal length.
A measure of how well an optical system focuses or defocuses light.
Many types of optical systems (e.g., microscope objectives and curved laser mirrors) can focus or defocus light, and focal length is a quantity that quantifies these effects.
The simplest case is a thin lens (Figure 1a). If the light entering the lens is a collimated beam, the beam will be focused after passing through the lens, and the focal length is the distance from the lens to the focal point (assuming that the prism is in a vacuum or air, rather than in a material with a high refractive index).
For a defocusing lens (Figure 1b), the focal length is the distance from the lens to the virtual focus (represented by the dotted line), which is a negative value.
Figure 1: Focal lengths of focusing and defocusing lenses. A defocusing lens has a negative focal length.
