16.3 Lenses

• The convex lens has been shaped so that all light rays that enter it parallel to its central axis cross one another at a single point on the opposite side of the lens.
• The central axis, or axis, is defined to be a line normal to the lens at its center. Such a lens is called a converging lens because of the converging effect it has on light rays.
• A concave lens and the effect it has on rays of light that enter it parallel to its axis (the path taken by ray 2 in the figure is the axis of the lens).
• The concave lens is a diverging lens because it causes the light rays to bend away (diverge) from its axis.
• In standard microscopes, the objectives are mounted such that when you switch between them, the sample remains in focus.
• Objectives arranged in this way are described as parfocal. The second lens, the eyepiece, also referred to as the ocular, has several lenses that slide inside a cylindrical barrel.
• An aberration is a distortion in an image. There
• One common type of aberration is chromatic aberration, which is related to color.