Half-life is the time required for half of the radioactive atoms in a sample to decay, a key concept in nuclear physics.

Proper time is the time interval measured by a clock that is at rest relative to the events being timed, typically experienced by an observer in their own frame of reference.

Planck's constant (h) is a fundamental constant that relates the energy of a photon to its frequency, valued at approximately 6.626 x 10^-34 J·s.

The SI unit of force is the Newton (N), defined as the force required to accelerate a one-kilogram mass by one meter per second squared.

The speed of light in a vacuum is approximately 3.00 x 10^8 m/s, which is a fundamental constant in physics.

The photoelectric effect demonstrates wave-particle duality, showing that light can behave as both a wave and a particle, as it can eject electrons from a material when it strikes it.

Luminous intensity is measured in candelas, which quantifies the amount of light emitted in a particular direction.

The Reynolds number is a dimensionless quantity used in fluid mechanics to predict flow patterns in different fluid flow situations.