Object image – the distance from the object to the center of the lens

Objective lens – in a compound microscope or a telescope, the lens closest to the object being viewed

Ocular – in a compound microscope or a telescope , the lens through which one looks (see eyepiece)

Ohm – a unit of resistance; a volt per ampere

Ohm’s law – defines the relationship among potential difference, current and resistance (see equation 18.3)

Open circuit – a circuit in which the path for the current is broken or interrupted

Optical resonator – a structure made of mirrors, with the same optical axis, that reflect light back and forth

Orbital quantum number – characterizes the orbital angular momentum and has integer values from zero to one less than the principal quantum number

Ordinate – vertical axis of a graph

Overtones – any resonant frequency higher than the fundamental frequency of a system