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. |