Qualitative Analysis

Qualitative analysis is the process by which a chemist determines what chemical elements are present in a given sample of material. For instance, many people are now concerned about the presence of lead in our environment.

Quantitative Analysis

The term quantitative analysis is used to described any procedure by which the percentage composition of any compound or mixture is determined. For example, chemists might want to know the exact composition of some new compound that has been discovered.

Quantum Mechanics

Quantum mechanics is a method of studying the natural world based on the concept that waves of energy also have certain properties normally associated with matter, and that matter sometimes has properties that we usually associate with energy. For example, physicists normally talk about light as if it were some form of wave traveling through space.


Quasars are compact objects located far outside of our galaxy. They are so bright they shine more intensely than 100 galaxies combined, but they are so distant their light takes several billion years to reach Earth.


Radar (a contraction of radio detection and ranging) is an electronic system that measures the position, speed, or other characteristics of a far-off object by means of radio waves bounced off the surface of that object. It can pierce any atmospheric disturbance, such as a storm, all the way to the horizon.

Radial Keratotomy

Radial keratotomy is a type of eye surgery that is used to correct permanently myopia (pronounced my-O-pee-ah) or nearsightedness. In this surgery, a physician typically cuts slits into the cornea (pronounced KOR-nee-ah) with a tiny diamond scalpel, changing the shape of the cornea.


The word radiation comes from a Latin term that means "ray of light." It is used in a general sense to cover all forms of energy that travel through space from one place to another as "rays." Radiation may occur in the form of a spray of subatomic particles, like miniature bullets from a machine gun, or in the form of electromagnetic waves. Subatomic particles are the basic units of matter and energy (electrons, neutrons, protons, neutrinos, and positrons), which are even smaller than atoms.

Radiation Exposure

The term radiation exposure refers to any occasion on which a human or other animal or a plant has been placed in the presence of radiation from a radioactive source. For example, scientists have learned that the radioactive element radon is present in the basements of some homes and office buildings.


Radio is the technology that allows information to be transmitted and received over radio waves. Radio makes it possible to establish wireless two-way communication between individual pairs of transmitters/receivers and it is used for one-way broadcasts to many receivers.

Radioactive Tracers

Radioactive tracers are substances that contain a radioactive atom to allow easier detection and measurement. (Radioactivity is the property possessed by some elements of spontaneously emitting energy in the form of particles or waves by disintegration of their atomic nuclei.) For example, it is possible to make a molecule of water in which one of the two hydrogen atoms is a radioactive tritium (hydrogen-3) atom.


Radioactivity is the emission of radiation by unstable nuclei. That radiation may exist in the form of subatomic particles (primarily alpha and beta particles) or in the form of energy (primarily gamma rays).

Radio Astronomy

Matter in the universe emits radiation (energy) from all parts of the electromagnetic spectrum, the range of wavelengths produced by the interaction of electricity and magnetism. The electromagnetic spectrum includes light waves, radio waves, infrared radiation, ultraviolet radiation, X rays, and gamma rays.


Radiology is a branch of medical science in which various forms of radiant energy are used to diagnose and treat disorders and diseases. For nearly 80 years, radiology was based primarily on the use of X rays.

Rain Forest

Rain forests are ecosystems characterized by high annual precipitation and an abundance of many large trees, generally of very old age. (An ecosystem is an ecological community, or the plants, animals, and microorganisms in a region considered together with their environment.) Rain forests can be found in both tropical and temperate regions.

Reaction, Chemical

When a chemical reaction occurs, at least one product is formed that is different from the substances present before the change occurred. As an example, it is possible to pass an electric current through a sample of water and obtain a mixture of oxygen and hydrogen gases.


Recycling is a method of reusing materials that would otherwise be disposed of in a landfill or incinerator. Household products that contain glass, aluminum, paper, and plastic are used for recycling and to make new products.

Red Giant

A red giant is a star that has exhausted the primary supply of hydrogen fuel at its core. An average-sized star like our Sun will spend the final 10 percent of its life as a red giant.


In astronomy, when matter moves away from an observation point, its light spectrum displays a redshift. A redshift is one type of Doppler effect.

Reinforcement, Positive and Negative

In psychology, reinforcement refers to the procedure of presenting or removing a stimulus to maintain or increase the likelihood of a behavioral response. (A stimulus is something that causes a response.) Reinforcement is usually divided into two types: positive and negative.

Relativity, Theory of

The theory of relativity is an approach for studying the nature of the universe. It was devised by German-born American physicist Albert Einstein (1879–1955) in the first quarter of the twentieth century.


Reproduction is the process by which plants and animals produce offspring. Reproduction may be asexual or sexual.

Reproductive System

The reproductive system is a group of organized structures that make possible the creation, or reproduction, of new life for continuation of a species. Human reproduction is sexual, meaning that both a male and a female contribute genetic material in the creation of a new individual.


A reptile is an organism in the kingdom Animalia and the class Reptilia. (Kingdoms are the main divisions into which scientists classify all living things on Earth; kingdoms are further subdivided into phylums [or divisions], classes, and orders.) The reptiles include more than 6,000 species grouped into four orders: the turtles (Chelonia), the snakes and lizards (Squamata), the crocodiles and alligators (Crocodilia), and the tuataras (Sphenodonta), large lizardlike animals found only on islands off the coast of New Zealand.


The term respiration has two relatively distinct meanings in biology. First, respiration is the process by which an organism takes oxygen into its body and then releases carbon dioxide from its body.

Respiratory System

Respiration is the process by which living organisms take in oxygen and release carbon dioxide. The human respiratory system, working in conjunction with the circulatory system, supplies oxygen to the body's cells, removing carbon dioxide in the process.

Rh Factor

Rh factor is a protein called an antigen that is found on the red blood cells of most people. (An antigen is any substance that the body considers "foreign" and thus stimulates the body to produce antibodies against it.) Rh factor, like the blood types A, B, and O, is inherited from one's parents.


A river is a natural stream of freshwater that is larger than a brook or creek. Rivers are normally the main channels or largest tributaries of drainage systems.


Robotics is the science of designing and building machines (robots) that are directed by computers to perform tasks traditionally carried out by humans. The word robot comes from a play written in 1920 by the Czech author Karel Capek.