Lenses made with surfaces of small radius have the shorter focal lengths. A lens with two convex surfaces will always refract rays parallel to the optic axis so that they converge to a focus on the side of the lens opposite to the object. Concave lenses form only virtual, erect, and diminished images. If the object distance is smaller than the focal length of this lens, the image is virtual, erect, and larger than the object. Observer is then using the lens as a magnifier or simple microscope. The angle subtended at the eye by this virtual enlarged image is greater than would be the angle subtended by the object if it were at the normal viewing distance. The ratio of these two angles is the magnifying power of the lens.
- lens [lenz] n.镜头
- radius [ˈreidjəs] n.半径(plural: radii [ˈreidiai])
- focal length [ˈfəukəl - leŋθ] n.焦距
- convex [ˈkɔnveks]adj.凸的, 凸面的
- refract [riˈfrækt]v.使折射
- parallel [ˈpærəlel]adj.平行的
- optic axis [ˈɔptik - ˈæksis]n.光轴
- converge [kənˈvə:dʒ]v.会聚
- concave lens [kɔnˈkeiv - lenz] n.凹透镜
- erect [iˈrekt]adj.竖立的
- diminish [diˈminiʃ]v.减少
- virtual [ˈvə:tjuəl]adj.虚的
- magnifier [ˈmæɡnifaiə]n.放大镜
- microscope [ˈmaikrəskəup]n.显微镜
- subtend [səbˈtend]v.对向
- ratio [ˈreiʃiəu]n.比, 比率
- angle [ˈæŋɡl] n.角，角度
