Figure B-3. Angle of elevation, angle of site, and quadrant angle.

TM 9-258

Angle of incidence - The angle between the normal

Angstrom unit - A unit of measure equaling one ten-

(perpendicular) to a reflecting or refracting surface and

millionth part of a millimeter.

Angle of reflection - The angle between the normal

Angular - Composed of or measured by angles.

to a reflecting surface and the reflected ray (fig 2-13).

Antiglare diaphragm - See diaphragm.

The incident ray, reflected ray, and normal all lie in the

Aperture - An opening or hole through which light or

same plane.

matter may pass. In an optical system, it is equal to the

Angle of refraction - The angle between the normal

diameter of the largest entering beam of light which can

to the refracting surface and the refracted ray (fig 2-24).

travel completely through the system. This may or may

not be equal to the aperture of the objective (fig B-4).

The incident ray, refracted ray, and normal all lie in the

same plane.

Angle of site - The vertical angle between the

horizontal and the line of site (line from gun to

B-3