Key Terms

Key Terms

aberration

distortion in an image caused by departures from the small-angle approximation

accommodation

use of the ciliary muscles to adjust the shape of the eye lens for focusing on near or far objects

angular magnification

ratio of the angle subtended by an object observed with a magnifier to that observed by the naked eye

apparent depth

depth at which an object is perceived to be located with respect to an interface between two media

Cassegrain design

arrangement of an objective and eyepiece such that the light-gathering concave mirror has a hole in the middle, and light then is incident on an eyepiece lens

charge-coupled device (CCD)

semiconductor chip that converts a light image into tiny pixels that can be converted into electronic signals of color and intensity

coma

similar to spherical aberration, but arises when the incoming rays are not parallel to the optical axis

compound microscope

microscope constructed from two convex lenses, the first serving as the eyepiece and the second serving as the objective lens

concave mirror

spherical mirror with its reflecting surface on the inner side of the sphere; the mirror forms a “cave”

converging (or convex) lens

lens in which light rays that enter it parallel converge into a single point on the opposite side

convex mirror

spherical mirror with its reflecting surface on the outer side of the sphere

curved mirror

mirror formed by a curved surface, such as spherical, elliptical, or parabolic

diverging (or concave) lens

lens that causes light rays to bend away from its optical axis

eyepiece

lens or combination of lenses in an optical instrument nearest to the eye of the observer

far point

furthest point an eye can see in focus

farsightedness (or hyperopia)

visual defect in which near objects appear blurred because their images are focused behind the retina rather than on the retina; a farsighted person can see far objects clearly but near objects appear blurred

first focus or object focus

object located at this point will result in an image created at infinity on the opposite side of a spherical interface between two media

focal length

distance along the optical axis from the focal point to the optical element that focuses the light rays

focal plane

plane that contains the focal point and is perpendicular to the optical axis

focal point

for a converging lens or mirror, the point at which converging light rays cross; for a diverging lens or mirror, the point from which diverging light rays appear to originate

image distance

distance of the image from the central axis of the optical element that produces the image

linear magnification

ratio of image height to object height

magnification

ratio of image size to object size

near point

closest point an eye can see in focus

nearsightedness (or myopia)

visual defect in which far objects appear blurred because their images are focused in front of the retina rather than on the retina; a nearsighted person can see near objects clearly but far objects appear blurred

net magnification

($M_{\mathrm{net}}$) of the compound microscope is the product of the linear magnification of the objective and the angular magnification of the eyepiece

Newtonian design

arrangement of an objective and eyepiece such that the focused light from the concave mirror was reflected to one side of the tube into an eyepiece

object distance

distance of the object from the central axis of the optical element that produces its image

objective

lens nearest to the object being examined.

optical axis

axis about which the mirror is rotationally symmetric; you can rotate the mirror about this axis without changing anything

optical power

(P) inverse of the focal length of a lens, with the focal length expressed in meters. The optical power P of a lens is expressed in units of diopters D; that is, $1\text{D}=1\text{/m}=1{\text{m}}^{\mathrm{-1}}$

plane mirror

plane (flat) reflecting surface

ray tracing

technique that uses geometric constructions to find and characterize the image formed by an optical system

real image

image that can be projected onto a screen because the rays physically go through the image

second focus or image focus

for a converging interface, the point where a bundle of parallel rays refracting at a spherical interface; for a diverging interface, the point at which the backward continuation of the refracted rays will converge between two media will focus

simple magnifier (or magnifying glass)

converging lens that produces a virtual image of an object that is within the focal length of the lens

small-angle approximation

approximation that is valid when the size of a spherical mirror is significantly smaller than the mirror’s radius; in this approximation, spherical aberration is negligible and the mirror has a well-defined focal point

spherical aberration

distortion in the image formed by a spherical mirror when rays are not all focused at the same point

thin-lens approximation

assumption that the lens is very thin compared to the first image distance

vertex

point where the mirror’s surface intersects with the optical axis

virtual image

image that cannot be projected on a screen because the rays do not physically go through the image, they only appear to originate from the image