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Science in Focus: Shedding Light: Glossary

This glossary contains important terms for understanding the content in Shedding Light on Science. Definitions are of two types, formal as in a dictionary and informal as in conversation. Some words have definitions of both types. The informal definitions are intended to help teachers present the meaning of these words to children in a more concrete way.

To retain (sunlight or sound, for example) wholly, without reflection or transmission.

To transfer energy from light to matter. Photons can be absorbed by electrons in matter then emitted as visible light photons (this is reflection) or re-radiated as infrared photons (heat). A photon has energy and momentum, but no rest mass. If a photon comes to rest by being absorbed, it ceases to exist. Its energy has been converted to some other form of energy.

When an object absorbs all light, we see black.

When sunlight is absorbed by the ground, it feels warm.

When light energy is absorbed by an object, the object might feel warmer because light energy is transformed to heat energy. See transform.

air mass
A large body of air with only small horizontal variations of temperature, pressure, and moisture.

Although an air mass has distinct boundaries, it may extend hundreds or even thousands of miles. Air masses are found when the atmosphere is in contact with a uniform land or ocean surface long enough for it to acquire the temperature and moisture of that surface. On the Earth, major air masses originate in polar or subtropical latitudes.

We are usually within an air mass. When there is a change in temperature, a different air mass is moving in.

angle of incidence
The angle between the incoming photons and the normal line.

See reflection.

angle of reflection
the angle between the reflected outgoing photons and the normal line.

See reflection.

Antarctic Circle
The parallel of latitude approximately 66°30' south. This parallel marks the northern limit of the area within which for at least one day each year the Sun does not set or rise (about June 21st).

It forms the boundary between the South Temperate and South Frigid zones.

In a camera, the aperture is the opening through which light enters. It is analogous to the pupil of a human eye.

Arctic Circle
The parallel of latitude approximately 66°30' north. It forms the boundary between the North Temperate and North Frigid zones.

This parallel marks the southern limit of the area within which for at least one day each year the Sun does not set (about June 21st) or rise (about December 21st).

atmospheric pressure
Pressure (force per unit volume) caused by the weight of the atmosphere. At sea level it has a mean value of one atmosphere but reduces with increasing altitude. The variations in pressure largely determine wind and storm patterns. Also referred to as barometric pressure, it is often measured with a mercury barometer that indicates the height of a column of mercury that balances the weight of the column of atmosphere. At sea level, the standard pressure is 29.92 inches of mercury.

When you rise quickly in an elevator or air plane, you feel a change in atmospheric pressure in your ears.

A straight line about which a body or geometric object rotates.

The Earth rotates around an imaginary axis from the North Pole to the South Pole.


An instrument that measures the pressure of the atmosphere in its environment. All of the air above a certain area is pressing down on that area because the force of gravity is acting between that air and the Earth. The barometric pressure is greater at sea level than at the top of a mountain because there is less air pressing down at the altitude of a mountaintop than at sea level.

Beaufort scale
The scale used to measure wind velocities by assigning code numbers from 0 (calm) to 12 (hurricane), corresponding to wind speeds of less than one mile per hour (0-1 kilometer per hour) to over 74 miles per hour (over 117 kilometers per hour).

See also breeze and wind.

Any of five winds with speeds from 4 to 31 miles per hour (6 to 50 kilometers per hour), according to the Beaufort scale.

See also wind.


A molecule containing the atoms carbon, hydrogen and oxygen. There can be different numbers of each of these atoms arranged together to make different carbohydrate molecules.

For instance, the sugar we use to sweeten food and beverages is generally sucrose and has twelve units of carbon, twenty-two units of hydrogen and eleven units of oxygen in each molecule (C12H22O11)

The aggregate weather conditions in a place or region, including temperature, precipitation, and wind.

The summer climate of Central Indiana is warm and humid even though some summer days may be cool.

1. A visible body of very fine water droplets or ice particles suspended in the atmosphere at altitudes ranging up to several miles above sea level.
2. A mass, as of dust, smoke, or steam, suspended in the atmosphere or in outer space.

Every atmospheric cloud (e.g. not dust kicked up by the wind) is made up of water droplets and/or ice crystals.

cold front
The leading portion of a cold atmospheric air mass moving against and eventually replacing a warm air mass. A front is the area of contact between air masses.

concave mirror
a mirror that is curved inwards like the inside of a spoon

The process where water as a gas (water vapor) forms as a liquid on a particle in the air (condensation nuclei) or surface.

Water forms on a mirror during a shower because molecules of water collect on the glass. Water collects around particles in the air. These particles may fall as rain or form fog or clouds.

condensation nuclei
Particles on which water molecules, which are energetic enough to be a gas, collect to form liquid water.

The detectors in the retina which discriminate between photon energy levels and perceive color. The three types of cones are especially sensitive to high, medium and low energy levels (blue, green, and red light).

convex mirror
a mirror that is curved outward like the outside of a spoon

convection current
The transfer of heat from a warm object to a cooler object through a fluid (or gas). Convection occurs when objects with different temperatures are in contact with a fluid (or gas). The fluid in contact with the warm object gains energy (through heat conduction) and usually expands. Because it is less dense than the surrounding fluid, it rises, carrying the energy it has gained with it. If the fluid encounters a cool object, energy from the fluid will be transferred. The fluid will become more dense and sink.

See also heat conduction.

primary, secondary and tertiary

A consumer is an organism that gains its energy by eating other organisms. A primary consumer is a herbivore; it feeds on plant material (e.g. aphids, caterpillars, deer, sheep). A secondary consumer is a carnivore because it eats an animal that has eaten a plant (e.g. lion eating a zebra, bird eating a caterpillar). Other carnivores may prey upon the secondary consumers; they are known as tertiary consumers (e.g. cat eats a bird that has eaten a caterpillar).

Coriolis effect
The observed effect of the Coriolis force, especially the apparent deflection of an object moving in a north-south path above the earth, rightward in the northern hemisphere and leftward in the southern hemisphere. The object does not deviate from its path, but appears to because of the motion of the Earth. Named after Gustave-Gaspard Coriolis (1792-1843), French mathematician and engineer.

Coriolis force
An apparent force used mathematically to describe motion, as of aircraft or cloud formations, relative to a non-inertial, uniformly rotating frame of reference such as Earth. Named after Gustave-Gaspard Coriolis (1792-1843), French mathematician and engineer.

Water flowing toward the drain of a bath tub is similar to air flowing from a high pressure toward a low pressure. The rotation of the earth causes the water to rotate. In the context of a bathtub or toilet, this is a very weak force and can easily be affected by other forces.

The cornea is the transparent curved front of the eyeball. As the first optical component of the eye, it refracts light into the eyeball.

1. Meteorology. An atmospheric system characterized by the rapid, inward circulation of air masses about a low-pressure center, usually accompanied by stormy, often destructive, weather. Cyclones circulate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
2. A violent, rotating windstorm.

A general term for strong, rotating low pressure systems. See hurricane and typhoon.


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