Focal lengths and focal points

Lenses can focus light and make images in a very similar way to mirrors. Lenses are either concave or convex. A convex lens will focus light from a distant source to a point, the focal point. Magnifying glasses are made with convex lenses and can be used to start a fire. The focusing is illustrated at the right.
Concave lenses spread light out rather than bring rays together. The image from a concave lens is therefore always virtual. The focusing is illustrated to the right. To an observer on the right side of the lens, the light appears to come from the focal point on the left side of the lens. The focal length of a concave lens is assigned a negative number. Both concave and convex lenses are used in a variety of optical instruments, the subject of the next lecture.

Examples     Lenses' index