This chapter extends the study of reflection to curved mirrors and introduces lenses. A spherical mirror has a reflecting surface shaped like part of a hollow sphere: a concave mirror curves inward, while a convex mirror curves outward. Using a shiny spoon and real mirrors, students observe that a concave mirror can form an enlarged, erect image when the object is close and an inverted, smaller image when the object is far, while a convex mirror always forms an erect, diminished image. Concave mirrors are used in torch and headlight reflectors and dental mirrors, while convex mirrors serve as side-view and road safety mirrors and store surveillance mirrors. The two laws of reflection are established: the angle of incidence equals the angle of reflection, and the incident ray, the normal, and the reflected ray all lie in the same plane; these laws hold for all mirrors. When parallel rays strike mirrors, a plane mirror keeps them parallel, a concave mirror converges them, and a convex mirror diverges them, so a concave mirror can focus sunlight to ignite paper. A lens is transparent and curved: a convex lens is thicker in the middle and converges light, while a concave lens is thicker at the edges and diverges light. Convex lenses form enlarged or inverted images depending on distance, and concave lenses always give erect, diminished images. Lenses are used in spectacles, cameras, microscopes, telescopes, and the human eye.
Concave and convex mirrorsImages formed by spherical mirrorsLaws of reflectionConvex and concave lensesConverging and diverging of light
Key terms
Concave mirror
A spherical mirror whose reflecting surface curves inward and which converges parallel light rays.
Convex mirror
A spherical mirror whose reflecting surface curves outward, always forming an erect, diminished image, and diverging light.
Angle of incidence
The angle between the incident ray and the normal at the point where light strikes a mirror.
Laws of reflection
The rules that the angle of incidence equals the angle of reflection and that the incident ray, normal, and reflected ray lie in one plane.
Convex lens
A lens thicker at the middle than at the edges that converges light; also called a converging lens.
Concave lens
A lens thicker at the edges than at the middle that diverges light and always forms an erect, diminished image.
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