angstrom
A unit of distance corresponding to 10-10 m. One angstrom (Å) is 0.1 nm so that 200 nm corresponds to 2000 Å.
antisolar point
The point where an imaginary ray connecting the sun and yourself meets the sky. If the sun is above the horizon, the antisolar point is below the horizon. If the sun is setting in the west, the antisolar point is on the eastern horizon. If the sun is below the horizon, the antisolar point is above the horizon on the other side of the sky.
atomic number
The number of protons in an atomic nucleus. For an neutral atom, this also corresponds to the number of electrons orbiting the nucleus. This number also denotes an atom's place on the Periodic Table of the Elements and the Chart of Nuclides. Carbon has an atomic number of 6: 6 protons and 6 electrons.
charged
Having acquired a net electric charge. Objects can become charged via a transfer of electrons from one object to another. An example of this transfer is when you shuffle across a carpet or when a plastic rod is rubbed with a piece of fur.
concave
Bending inward. A concave lens or mirror has at least one side that bulges inward. A concave lens produces diverging light rays.
See also reflect, refract, focal length, focal point
converge
To move in the same direction or towards a common point. Light converges when it passes through a convex lens. The opposite of converge is diverge

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convex
Bending outward. A convex lens or mirror has at least one side that bulges outward. A convex lens produces converging light rays. A convex mirror produces diverging rays.
See also reflect, refract, focal length, focal point
diffraction
A phenomenon that occurs whenever a wave is obstructed in any way. Often diffraction fringes can be seen when a small aperature or object blocks light waves. Scientists use diffraction gratings to break up light into many wavelengths.
diffraction grating
A device used to break light into its component wavelengths. It is usually composed of a material with tiny grooves cut into it which disperses the light as it passes through or bounces off the grating (depending on the type of grating). Physicists and astronomers often use diffraction gratings to study the nature of light.
disperse/dispersion
Breaking light into its various wavelengths. All transparent materials disperse light because their refractive indexes change with wavelength: red light (long wavelengths) will be refracted less than blue light (shorter wavelengths).

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diverge
To move in different directions from a common point or from each other.
from Webster's New World Dictionary. The opposite of diverge is converge.
double concave
A concave lens which has both edges bulging inward.
double convex
A convex lens which has both edges bulging outward.
electromagnetic force
One of the four fundamental forces in nature. Refers to the combination of two separate forces -- the electric foce and the magnetic force. EM forces act on objects with electric charges and/or magnetic fields.
electromagnetic spectrum
The complete range of energies of electromagnetic waves from the lowest to the highest (from largest to smallest wavelengths), including, in order, radio, infared, visible light, ultraviolet, X-ray, and gamma ray waves.
from Webster's New World Dictionary
electron
One of the elementary particles of nature. The electron has a charge of -1.6 x 10-19 Coulombs and a mass of 9.1 x 10-31 kg. Electrons orbit the nuclei of atoms and are the principle tool of molecular bonding.

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electroscope
A device that enables you to determine if two objects have opposite or like charges.
focal length
The distance from a lens or mirror to its focal point. The focal length, f, is the defining parameter of most lenses and mirrors.
focal point
The point at which parallel light rays, incident on a lens (or mirror), are focused after refracting (or reflecting). For concave lenses and mirrors, the focal point is on the same side of the lens (or mirror) as the source. For convex lenses and mirrors, the focal point is on the opposite side of the lense (or mirror) as the source.
incident
Falling or shining upon. When light is incident on something, the light may be reflected, refracted, absorbed, or any combination of these.
index of refraction
A number signifying how well a material can refract light. Usually specified as n by scientists, the index of refraction of a material depends on its composition and density. Water has an index of refraction of 1.333; n of air is nearly 1.0; and n of a vacuum is exactly 1.0.
infrared
A region of the electromagnetic spectrum that is of lower energy and longer wavelength than visible light. Typical wavelengths of infrared radiation range from 1.0 mm to 780 nm (780 x 10-9 m).
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.

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interference
The result of waves impinging on one another. 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.
ionization
The gaining or losing of electrons by an atom. If an atom loses an electron(s), it becomes a positive ion. If an atom gains an electron(s), it becomes a negative ion.
lightning
Most simply defined as the discharge of electricity from one cloud to another, or from a cloud to the ground.
magnify
To cause an image to appear larger or smaller than the corresponding object. When we look through a lens, we often see things magnified: convex lenses often magnify objects larger, while concave lenses always magnify objects smaller. Mirrors can also magnify objects.
microscopy
The use of a microscope. Or, more specifically, investigation by means of a microscope.
from Webster's New World Dictionary
negative charge
Something has a negative charge if it has an excess number of electrons.
normal
Perpendicular. If one line is normal to another, then they are at right angles to each other.
photoelectric effect
A phenomenon discovered by Einstein in the early 20th century in which electrons are ejected from a solid when impinged upon by electromagnetic radiation. This led to the understanding of light as particles, or photons. The energy required to strip an electron from an atom is called the ionization energy.
photon
A quantum of electromagnetic energy having both particle and wave properties. A photon has no charge or mass but possesses momentum and energy.
from Webster's New World Dictionary
pinhole camera
A type of camera in which the "lens" is simply a very small aperature, or pinhole. This camera uses the principle of straight-traveling lightpaths and reversibility of lightpaths to form images on a screen or film.

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polarize
To align something in one direction or another. Natural light is a combination of two polarization states. A polarizer can be used to block out one of the polarization states and allowing the other to pass. Polaroid sunglasses are one example of this phenomenon.
prism
A device used to break light into its component wavelengths. This is usually a triangular piece of glass through which light can pass and get dispersed. Isaac Newton first used a prism to see a spectrum of colors in 1672.
proton
One of the elementary particles of nature. The proton has a charge of 1.6 x 10-19 Coulombs and a mass of 1.67 x 10-27 kg, much higher than an electron. The proton resides in the nucleus of an atom, sharing the space with neutrons, neutrally charged particles.
ray tracing
A process used in optics by which the position and orientation of an image can be established if one knows the focal length of the lens and the position of the object. This process uses strategic rays from the object passing through the lens to locate the image.
real image
A type of image created by converging light rays. This type of image would form on the image side of a lens, where it could be projected. The opposite of a real image is a virtual image.
recharge
To charge again.
reference charge
....
reflect
When light bounces off an object. Usually we associate reflection with a smooth, polished surface, however the surface doesn't have to be smooth. Light reflects off your shirt, therefore others can see that you're wearing one.
refract
When light bends at an interface between two transparent materials. We find many things in our everyday experience refracting light: glass, water, air, plastic. The illusion of a broken leg dangling in the pool is due to the refraction of light.

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refractive index
See index of refraction
semiconductor
A substance that exhibits properties of a conductor but with higher resitivities. If a semiconductor has a small amount of a foreign substance added to it (doping), the original material can become a normal conductor. Silicon and germanium are natural semiconductors.
spectroscope
An instrument which disperses white light and allows the user to distinguish and measure the wavelength or energy of the resulting colored light.
spectrum
The result of breaking up light into its various wavelengths. In the visual regime, we see this as a display of the "rainbow" of colors. A prism is a good tool to use to see the spectrum of colors.
static electricity
....
types of charges
There are two types of charges: positive and negative.
total internal reflection
A phenomenon occuring when light is incident on an interface between two transparent materials at a very shallow angle. The light must be traveling through a medium with a higher index of refraction than the neighbouring medium. At the interface, all of the light is reflected back into the surrounding material and none of the light is refracted out into the neighbouring material.
ultraviolet
A region of the electromagnetic spectrum that is of higher energy and shorter wavelength than visible light. Typical wavelengths of ultraviolet radiation range from 375 nm to 12.5 nm (12.5 x 10-9 m).
virtual image
A type of image created by diverging light rays. This type of image would form on the object side of a lens, if it formed at all. The opposite of a virtual image is a real image.