R value – ratio of a material’s thickness to its thermal conductivity

Radioactivity – radiation emitted from atomic nuclei

Radian – the ratio of the arc length on a circle to the radius of the circle

Radiation – (thermal) the transfer of thermal energy via electromagnetic waves

Ray – straight line path of light, perpendicular to the wave fronts

Rayleigh's criterion – the minimum angular separation required to separate two sources occurs when the first minimum of the diffraction pattern of one source falls on top of the central maximum of the diffraction pattern of the other source (see figure 24.27)

Rayleigh scattering – the type of scattering that occurs when the particles that cause the scattering are smaller than the wavelength of the incident light

Real image – an image formed when the light rays actually intersect at the image

Rectifier – a device that converts alternating current to direct current; a diode

Reference frame – a system of coordinate axes fixed to a particular body

Refracting astronomical telescope – a telescope with an objective lens with a relatively long focal length (see figure 23.9)

Refrigerator – a system designed use work to transfers heat from a low temperature environment to a high environment

Relative velocity – the difference between the velocities of two objects

Resistance– the ratio of the voltage to the current

Resistivity – the resistance of a unit length of a conductor of unit cross sectional area (see Table 18.1)

Resistors– electric components manufactured especially for their resistance

Resonance – a condition in which energy is fed into a system at the natural frequency of the system

Resonant frequency – (natural frequency) the frequency of oscillation that, once begun, will continue without a driving force if no damping forces are present

Rest mass – (proper mass) the mass of an object when measured by an observer at rest with respect to the object

Restoring force – a force which acts in a direction opposite the direction of the displacement from the equilibrium position

Resultant – the sum of two or more vectors

Reversible process – a process in which the system is near equilibrium at all times (see figure 13.4)

Reynold’s number – the ratio of the inertial force to the viscous force (see equation 10.14)

Rigid body – a body that rotates with no deformation, so that all parts remain at constant distances from every other part

rms current – the square root of the time average of the square of the current

rms voltage – the square root of the time average of the square of the voltage (see equation 21.13)

Rotational equilibrium – that state in which the sum of the external torques on an object is zero

Rotational kinetic energy – energy due to the rotation of an object

Rydberg formula – a relationship between wavelengths of emitted light and the initial and final energy states of the atom which emitted the light (see equation 27.2)