Index of refraction

TM 9-258

Infinite - Without limit, as opposed to finite, meaning

calibration, graduation, or scale is set to indicate the

extent to which a mechanism is adjusted; something that

with limits.

Infinity - Distance having no end or limits. Greater than

points out.

b. The ratio of one dimension or quantity to

any assignable distance or quantity. In optics, the term

another.

is used to denote a distance sufficiently great so that light

Index of refraction - A number applied to a

rays coming from that distance are practically parallel to

one another. Infinity is indicated by the symbol ∞

transparent substance which denotes how much faster

light will travel in a vacuum than through that particular

Infrared radiation - Electromagnetic radiation whose

substance. It is equal to the ratio between the speed of

wavelength lies just beyond the red end of the visible

light 'in a vacuum to the speed of light in a substance. It

spectrum, and the beginning of the region that can be

determines the relation between the angle of incidence

detected by microwave radio techniques.

(i) and the angle of refraction (r) when light passes from

Interpupillary - Between the pupils of the eyes.

one medium to another (figs 2-24 and B-18). The index

Interpupillary adjustment - The adjustment of the

between two media is called the relative index, while the

distance between the eyepieces of a binocular

index when the first medium is a vacuum is called the

instrument to correspond with the distance between the

absolute index of the second medium. The relative index

pupils of the eyes of an individual.

is the ratio of the sine of the angle of incidence to the

Interpupillary distance - The distance between the

sine of the angle of refraction, or the speed of light in the

centers of the pupils of the eyes. It is generally stated in

first medium to the speed of light in the second medium.

millimeters. It varies with the individual.

If the first medium is air, this relation is expressed as -

Inversion - Turned over; upside down so that top

= sin i = Speed of light in air

becomes bottom and vice versa.

It is the effect

n sin r Speed of light in other medium

produced by a horizontal mirror in reflecting an image.

(Snell's law)

Invert-type Image - The type of image observed in

where n equals very nearly the absolute index of

certain coincidence range finders. When in coincidence,

refraction, i equals the angle of incidence, and r equals

the upper half image appears to be the mirrored

the angle of refraction.

reflection of the lower half image.

Inverted image - Turned over; upside down. An

image, the top of which appears to be the bottom of the

object and vice versa. Usually refers to the effect of a

mirror, a prism, or lens upon the image.

Inversion is the effect of turning upside down. A plane

mirror held under an object will produce an inverted

image. The normal inverted image is seen upside down,

with the right side on the right. The reverted inverted

image is seen upside down with the right side on the left.

Iris -

The colored part of the eye about the pupil.

Depending upon the intensity of light, the iris causes the

pupil to dilate or contract.

Jack screw - A screw threaded through one part and

pressing against another. A jack (portable machine) in

which a screw is used for lifting or for exerting pressure.

Laser - Acronym for light amplification by stimulated

emission of radiation.

Law of reflection - The angle of reflection is equal to

Figure B-18. Index of refraction

the angle of incidence; the incident ray, reflected ray, and

normal all lie in the same plane (fig 2-13).

NOTE

Law of refraction - When light is passing from an

The index of refraction expressed in tables is the

optically lighter medium to an optically denser medium,

absolute index, that is, vacuum to substance at a

its path is deviated, toward the normal; when passing

certain temperature, with light of a certain

into an optically less dense medium,

wavelength.

Examples: vacuum, 1.00; air,

1,000293; water 1.333; ordinary crown glass,

1.517. Since the index of air is very close to that

of vacuum, the two are often used

interchangeably as being practically the same.

B-18