Wave–particle duality describes how particles such as electrons and photons exhibit both wave-like and particle-like behaviour depending on the situation. In this topic you study experiments that reveal this dual nature: electron diffraction shows that particles can produce interference patterns like waves, while the photoelectric effect demonstrates that light can behave as individual energy packets, or photons. You examine the de Broglie relationship, which links a particle’s momentum to its wavelength, and you explore how this idea helps explain atomic structure and electron behaviour in materials. Wave–particle duality forms a core part of early quantum theory, highlighting that classical descriptions of waves and particles are not sufficient on their own to describe behaviour at very small scales.

Special relativity describes how measurements of space and time change for observers moving at constant high speeds relative to one another. In this topic you examine two key postulates: that the laws of physics are the same in all inertial frames, and that the speed of light in a vacuum is constant for all observers. From these, you explore consequences such as time dilation, where moving clocks run more slowly; length contraction, where moving objects appear shorter along the direction of motion; and relativity of simultaneity, which shows that events judged simultaneous in one frame may not be in another. You also work with Lorentz transformations to relate measurements between frames and examine how mass, energy, and momentum behave at relativistic speeds. Special relativity provides the theoretical basis for modern particle physics, high-energy astrophysics, and technologies such as GPS.