#### Iconometer

An instrument used to measure the distance or size of an object utilizing the image of the object produced by a lens of known focal length. If the distance is being measured, then the object’s size must be known beforehand. If the size is being measured, then the distance from the lens must be known in advance.

from L. illuminationem (nom. illuminatio), from illuminare "to throw into light," from in- "in" (with assimilation of -n- to the following consonant) + lumen (gen. luminis) "light." Meaning "the action of lighting" is from 1563. Illuminate (M.E. enlumyen) originally meant "decorate written material with gold, silver, bright colors; sense of "shining light on" first recorded 1563. (Illumine in this sense is from 1375.)

#### Illumination

The application of light to something.

from L. illuminationem (nom. illuminatio), from illuminare "to throw into light," from in- "in" (with assimilation of -n- to the following consonant) + lumen (gen. luminis) "light." Meaning "the action of lighting" is from 1563. Illuminate (M.E. enlumyen) originally meant "decorate written material with gold, silver, bright colors; sense of "shining light on" first recorded 1563. (Illumine in this sense is from 1375.)

#### Illusion

Illusion is a distortion of the senses, that most people sense. Optical illusions deal with the sense of sight. Optical Illusions can use color, light, and patterns to create images that can be deceptive or misleading to our brains. The information gathered by the eye is processed by the brain, which creates a perception that does not match the true image. Perception refers to the interpretation of what we take in through our eyes. Optical illusions occur because our brain is trying to interpret what we see and make sense of the world around us. Optical illusions simply trick our brains into seeing things that may or may not be real.

For more on optical illusions, click on the link to Optical Illusions at the Optics for Kids website.

c.1340, "act of deception," from O.Fr. Sense of "deceptive appearance" developed in Eng. c.1374. Illusionist "conjurer, magic act performer" is from 1850.

#### Image

The reproduction of an object produced by light rays. If the beam converges to a point, a real image is formed. If the beam diverges, a virtual image is formed located where the divergent rays, if extended back, would converge to.

c.1225, "artificial representation that looks like a person or thing," from O.Fr. image, earlier imagene (11c.), from L. imaginem (nom. imago) "copy, statue, picture, idea, appearance," from stem of imitari "to copy, imitate" (see imitate). Meaning "reflection in a mirror" is c.1315. The mental sense was in L., and appears in Eng. c.1374.

#### Impulse

The integral of an applied force on an object with respect to time, producing a change in momentum of the object. If the force is constant and applied over a period of time then the impulse is equal to the force multiplied by the time it is applied for.

1432 (implied in impulsion) "an act of impelling, a thrust, push," from L. impulsus "a push against, pressure, shock," also "incitement, instigation," pp. of impellere.

#### Incident

Falling or shining upon. When light is incident on something, the light may be reflected, refracted, absorbed, or any combination of these.

#### Incoherent

The lack of a fixed phase relationship between two waves.

#### Index of refraction

A number signifying how well a material can refract light. Usually specified with the letter “n” by scientists, the index of refraction of a material depends on its composition and density. It is the ratio of the velocity of light in a vacuum to the velocity of light in a refractive material for a given wavelength.

#### Infrared (IR)

Electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 μm (read as micrometer, 1 μm=10-6 m), and extending conventionally to 300 μm. These wavelengths correspond to a frequency range of approximately 1 to 400 THz. (1 THz=1012 Hz).

#### Infrared camera

A camera that can detect infrared light and produce images.

#### Integrating sphere

Also known as an Ulbricht sphere. A hollow sphere coated inside with a white diffusing material. It has openings for an incident beam, a specimen, and a detector used for measuring the diffuse reflectance or transmittance by objects.

#### Interaction

Any action and reaction that takes place between two objects. In physics, interactions (or forces) fall into four main categories. These categories are gravitational interactions, electromagnetic interactions (electric and magnetic forces), and the strong and weak interactions (which affect atomic nuclei and fundamental particles).

#### Interface

The boundary between two different materials or media.

1962 (n.), 1967 (v.), from inter- + face.

#### Interference

The result of waves impinging on one another, leading to the additive process where the amplitude of the two or more overlapping waves are systematically attenuated or reinforced. Constructive interference occurs when the waves are nearly in phase or when their “peaks” combine; destructive interference occurs when the waves are nearly 180˚ out of phase or when the “peaks” cancel out the troughs of the waves. (A) Constructive interference showing $\mathrm{sin}\left(x\right)+\mathrm{sin}\left(x\right)=2\text{\hspace{0.17em}}\mathrm{sin}\left(x\right)$ (B) Destructive interference showing .

1440, "to strike against," from M.Fr. enterferer "to strike each other," from entre- "between" + ferir "to strike," from L. ferire "to knock, strike," related to L. forare "to bore, pierce," and cognate with O.E. borian "to bore" (cf. punch (v.), which has both the sense "to hit" and "to make a hole in").

#### Interferometer

An instrument that employs the interference of lightwaves to measure the accuracy of optical surfaces. It can measure a length in terms of the length of a wave of light.

#### Interferometry

The study and utilization of interference phenomena.

#### Ion

An atom or molecule with an overall positive or negative charge.

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#### Ionization

The physical process of converting an atom or molecule into an ion by changing the difference between the number of protons and electrons. An ion is an atom or molecule that has lost or gained one or more electrons, making it negatively or positively charged. If an atom loses an electron(s), it becomes a positive ion. If an atom gains an electron(s), it becomes a negative ion.

#### Ionize

To change the number of electrons of an atom or molecule, causing it to become an ion.

#### Iris

A mechanical device designated to smoothly vary the effective diameter of a lens by controlling the amount of light allowed through.

From M.E., iris (the plant); from L. iris, irid-, "rainbow, iris (the plant)"; from Gk., rainbow, brightly-colored gemstone, iris of the eye. It regulates the amount of light entering the eye.