Q
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Quark
The building blocks of all matter, quarks are the constituent parts of protons, neutrons, and mesons.
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R

Radian
A unit for measuring angles; also called a “rad.”?2π?rad = 360o.
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Radiation
Heat transfer via electromagnetic waves.
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Radioactive decay
The process by which unstable nuclei spontaneously release particles and/or energy so as to come to a more stable arrangement. The most common forms of radioactive decay are alpha decay, beta decay, and gamma decay.
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Radioactivity
An object is called radioactive if it undergoes radioactive decay.
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Radius of curvature
With spherical mirrors, the radius of the sphere of which the mirror is a part.
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Rarefaction
An area of high air pressure that acts as the wave trough for sound waves. The spacing between successive rarefactions is the wavelength of sound, and the number of successive areas of rarefaction that arrive at the ear per second is the frequency, or pitch, of the sound.
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Real image
An image created by a mirror or lens in such a way that light does actually come from where the image appears to be. If you place a screen in front of a real image, the image will be projected onto the screen.
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Reflect
A wave on a string that is tied to a pole at one end will reflect back toward its source, producing a wave that is the mirror-image of the original and which travels in the opposite direction.
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Reflected ray
The ray of light that is reflected from a mirror or other reflecting surface.
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Reflection
The phenomenon of light bouncing off a surface, such as a mirror.
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Refracted ray
The ray of light that is refracted through a surface into a different medium.
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Refraction
The bending of light as it passes from one medium to another. Light refracts toward the normal when going from a less dense medium into a denser medium and away from the normal when going from a denser medium into a less dense medium.
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Restoring force
The force that causes simple harmonic motion. The restoring force is always directed toward an object’s equilibrium position.
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Right-hand rule
A means of defining the direction of the cross product vector. To define the direction of the vector?, position your right hand so that your fingers point in the direction of?A, and then curl them around so that they point in the direction of?B. The direction of your thumb shows the direction of the cross product vector.
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Rigid body
An object that retains its overall shape, meaning that the particles that make up the rigid body stay in the same position relative to one another.
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Rotational kinetic energy
The energy of a particle rotating around an axis.
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Rotational motion
Occurs when every point in the rigid body moves in a circular path around a line called the axis of rotation.
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Rutherford nuclear model
The model of the atom according to which negatively charged electrons orbit a positively charged nucleus. This model was developed by Ernest Rutherford in light of the results from his gold foil experiment.
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