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R

radiation
The transfer of energy through free space by photons (see Light Links on electromagnetic radiation). The type of material and its temperature determine the amount and energy of the radiation. In general a hot object will emit much more energy than a cool object. A cool object near a warm object may absorb more energy from the warm object than the cool object radiates. Since electromagnetic radiation travels through empty space, the transfer of heat through radiation does not require physical contact.The Earth receives energy from the Sun through radiation.

rain
Water condensed from atmospheric vapor and falling in drops.

The water we experience as rain is condensation around dust particles that occur in the atmosphere.

When enough water condenses in droplets in the atmosphere, they fall as rain.

rainbow
1. An arc of spectral colors appearing in the sky opposite the sun as a result of the refractive dispersion of sunlight in drops of rain or mist.
2. A similar arc or band as one produced by a prism or by iridescence.

If the sun is shining while it is raining, turn your back to the sun and look for a rainbow.

reflection
The re-emission of light from an object that has been struck by light. The law of reflection states that the incoming angle of light as it hits the object is equal to the outgoing angle (the angle of incidence equals of angle of reflection).

refraction
The change in speed and direction of light when it crosses the boundary between two transparent objects of different densities, such as when light passes from air to water, or when light passes through a glass of water.

retina
The inner layer of tissue that lines the back of the eyeball. It contains the light detectors -- rods and cones -- attached to the optic nerve.

rods
The detectors in the retina which are sensitive to low levels of light. They sense brightness and darkness (like black and white film) but not color.

S

satellite
1. In astronomy. A celestial body that orbits a planet; a moon.
2. In aerospace. An object launched to orbit Earth or another celestial body.

scattering
The reflection of light from a rough surface in many directions with each individual photon following the law of reflection. This is also known as diffuse reflection.

shadow
1. A region that is not illuminated or is only partially illuminated because of the interception of light by an opaque object blocking the source of radiation.
2. The slope cast by an object blocking light.

snow
Frozen precipitation in the form of white or translucent hexagonal ice crystals that fall in soft, white flakes.

See rain.

South Pole
The southern end of Earth's axis of rotation at -90° latitude and located at a point in Antarctica. The South Pole has six months of sunlight follow by six months of darkness each year.

See North Pole.

Southern Hemisphere
The half of the Earth south of the equator.

See Northern Hemisphere.

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