General Science:

* Radiation: The emission of energy as electromagnetic waves or as moving subatomic particles.

* Radioactive: Emitting or relating to radiation.

* Reaction: A process that results in the formation of new chemical substances.

* Research: Systematic investigation into a subject in order to establish facts and reach new conclusions.

* Revolution: A complete or dramatic change in something.

* Refraction: The bending of a wave as it passes from one medium to another.

* Reflection: The bouncing back of a wave when it hits a surface.

* Reproduction: The biological process of creating new organisms.

* Respiration: The process of breathing and using oxygen to release energy from food.

* Rotation: The spinning of an object around an axis.

* Reversible: A reaction that can proceed in both directions.

* Rate: The speed at which a process occurs.

Biology:

* Ribonucleic acid (RNA): A nucleic acid that plays an important role in protein synthesis.

* Ribosome: A cellular structure that is responsible for protein synthesis.

* Respiration: The process of breathing and using oxygen to release energy from food.

* Reproduction: The biological process of creating new organisms.

* Regulation: The control of biological processes.

* Ruminant: A mammal that chews cud.

Chemistry:

* Reaction: A process that results in the formation of new chemical substances.

* Rate: The speed at which a chemical reaction occurs.

* Reagent: A substance that is used in a chemical reaction.

* Radioactive: Emitting or relating to radiation.

* Redox: A chemical reaction that involves the transfer of electrons.

Physics:

* Radiation: The emission of energy as electromagnetic waves or as moving subatomic particles.

* Reflection: The bouncing back of a wave when it hits a surface.

* Refraction: The bending of a wave as it passes from one medium to another.

* Rotation: The spinning of an object around an axis.

* Resistance: The opposition to the flow of electric current.

This is just a small sample. There are many more science words that begin with "r."