| Radiant energy: | The form of energy that can travel through space; for example, visible light and other parts of the electromagnetic spectrum. |
| Radiation: | The emission and propagation of waves transmitting energy through space or through some medium. |
| Radioactive decay: | The natural spontaneous disintegration or decomposition of a nucleus. |
| Radioactive decay constant: | A specific constant for a particular isotope which is the ratio of the rate of nuclear disintegration per unit time to the total number of radioactive nuclei. |
| Radioactive decay series: | Series of decay reactions that begin with one radioactive nucleus that decays to a second nucleus that decays to a third nucleus and so on until a stable nucleus is reached. |
| Radioactive decay law: | The rate of disintegration of a radioactive substance is directly proportional to the number of undecayed nuclei. |
| Rarefaction: | A part of a longitudinal wave in which the density of the particles of the medium is less than the normal density. |
| Real image: | An image which can be projected on a screen. |
| Rectilinear Motion: | The motion of a body in a straight line. |
| Reflection: | The bouncing back of a wave from a boundary. |
| Refraction: | The bending of light from its straight line path when it travels from one medium to another. |
| Refractive index: | The ratio of speed of light in vacuum to that in the medium. |
| Relative density: | The ratio of density of a substance to the density of water at 4 . |
| Relative humidity: | The percentage of the amount of water vapor actually present in a certain volume of the air to the amount of water vapor needed to saturate it. |
| Resolving power: | A quantitative measure of the ability of an optical instrument to produce separable images of different points of an object. |
| Resonance: | When the frequency of an external force matches the natural frequency of the body then the body oscillates with large amplitude. |
| Restoring force: | The force which tends to bring an oscillating body towards its mean position whenever it is displaced from the mean position. |
| Resultant Force: | A single force, which acts on a body to produce the same effect in it as, done by all other forces collectively. |
| Reverberation: | The prolongation of sound at a given point after direct reception from the source has ceased, it is due to reflections from the boundary surfaces. |
| Rigid body: | An idealized extended body whose size and shape is fixed and remains unaltered when forces are applied. |
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