Figure B-15. Focal length of lens and mirror.

TM 9-258

Fluorescence - Phenomenon whereby light of one

to the observation and calculations necessary for

determining the correct aiming of a weapon system.

wavelength is absorbed by a material, and then

Fixed focus - The term applied to instruments which

reemitted as light of a different wavelength. Fluorescent

are not provided with means for focusing.

Such

light sources require generally external white light to

instruments, generally, have a wide range of

cause them to become luminous and glow with a colored

accommodation which permits them to be used by the

hue. They cease to become luminous when the esciting

majority of observers. Collimator is an example.

agent is removed.

Flint glass - One of the two principal types of optical

Focal length - The distance from the principal focus

glass, the other being crown glass. Flint glass is softer

(focus of parallel rays of light) to the surface of a mirror

than crown glass, has a higher index of refraction, and

or the optical center of a lens (figs 2-44, 2-45, 2-46 and

higher dispersion. See compound lens.

B-15)

Figure B-15. Focal length of lens and mirror.

Focal plane - A plane through the focal point

lens of a camera so that a sharp, distinct image is seen

on the ground glass.

perpendicular to the principal axis of a lens or mirror (fig

b. The process of adjusting the distances between

Focal point - The point to which rays of light

optical elements to obtain the desired optical effect.

c. Same as focal point.

converge or from which they diverge when they have

Focusing nut - A threaded nut in the center of which

been acted upon by a lens or mirror (figs 2-46 and 2-62).

A lens has many focal points, depending upon the

the eyepiece of a telescope is attached to permit the

distance of the object from the lens.

eyepiece to be moved in or out to accommodate the

Focus -

instrument to eyesight variations. It

a .To adjust the eyepiece of a telescope so that

the image is clearly seen by the eye or to adjust the

B-15