Refracted ray

TM 9-258

the normal. See angle of reflection. Regular reflection

objects or points close together. No lens or optical

will form a sharp image. Diffuse reflection (fig 2-16)

system can form a perfect image of a point; it will appear

as a small disk surrounded by concentric circles. If two

occurs when the surface is irregular and the reflected

points are so close together that the disks overlap, the

light diverges from each point as if it were a separate

points cannot be distinguished separately; they are not

reflecting surface. Diffused rays are scattered, go in

resolved. The measure of the resolving power is the

many directions, and will not form a distinct image.

Refracted ray - The ray of light passing through and

angle subtended at the optical center of the lens by two

points which are just far enough apart to permit

leaving a refracting surface representing the path of light

resolution into two separate images. This angle is

after refraction (fig 2-24).

termed the limiting angle of resolution.

Refracting power - The power of a lens or lens

Retaining ring - A thin ring threaded on the outside

system to converge or diverge light. See diopter.

Refraction - The bending of light which occurs when

surface. It is screwed into a tube, cell, or other body

member of an optical instrument to retain or hold a lens

a ray of light passes obliquely from one medium to

or other part in fixed position.

another of different optical density.

See angle of

Reticle - Marks or patterns placed in the focal plane

refraction and angle of deviation.

Refractive index - See index of refraction.

of the objective of an optical instrument which appear to

the observer to be superimposed upon the field of view

Regular reflection - See reflection.

(fig B-27). They are used as a reference point for

Relief - Effect of stereoscopic or three dimensional

sighting or aiming; to measure angular distance between

vision; solidity or depth; sharpness of outline due to

two points; to determine the center of the field; or to

contrast of the object standing out, from a background.

assist in the gaging of distance, determining leads, or

Resolution - In optics, the ability of a lens system to

measurement. The reticle may be a pair of crosslines

reproduce an image in its true sense. Forming separate

composed of fine wire or may be etched on a glass plate

images of two objects or points very close together.

with plane parallel surfaces. If it is etched on glass, the

Resolving power - A measure of the ability of a lens

entire piece of glass is referred to as the reticle. (Also,

or optical system to form separate images of two

see figs 4-24 and 4-25).

Figure B-27. Reticle patterns superimposed on image of object .

Retina - The light-sensitive inner coat or tunic of the

right becomes left and vice versa. It is the effect

produced by a vertical mirror in reflecting an image.

eyeball upon which the image is formed by the lenses of

Reverted erect - See erect image.

the eye. It contains the visual cells called the cones and

Reverted image - An image, the right side of which

rods (figs 3-2 and 3-5).

Reversion - Turned the opposite way so that

appears to be the left side of the object and vice versa.

B-27