Light exhibits both wave like and particle like behaviour, so the scientifically appropriate answer is that neither description alone is sufficient. In classical physics, light behaves as an electromagnetic wave consisting of oscillating electric and magnetic fields. This description successfully accounts for interference, diffraction, polarisation, reflection, refraction and many other optical effects. In quantum physics, light is quantised and its interactions with matter occur through discrete units called photons.
Photon based descriptions explain phenomena such as the photoelectric effect, Compton scattering and the quantisation of electromagnetic energy. The phrase wave particle duality describes this experimentally observed combination of properties. It does not mean that a photon is literally a tiny classical ball that sometimes turns into a classical wave. Quantum mechanics provides a more fundamental description in which the mathematical state of a quantum system can produce wave like probability amplitudes and discrete measurement outcomes. Which aspects are observed depends on the experimental arrangement and what quantity is measured.
The wave description of light is supported by numerous experiments. When coherent light passes through two slits, the resulting intensity pattern can contain alternating bright and dark regions caused by constructive and destructive interference. Diffraction causes light to spread when it passes through narrow openings or around obstacles, particularly when their dimensions are comparable to the wavelength. Polarisation demonstrates that light has transverse electromagnetic field properties. Refraction and dispersion can also be described naturally using wave propagation through matter.
These phenomena were difficult to explain using a simple corpuscular picture in which light consisted only of independent particles following trajectories. Thomas Young's double slit experiment provided especially influential evidence for interference, while later experiments involving diffraction and polarisation established wave optics in increasing detail. Maxwell's electromagnetic theory gave a deeper explanation by identifying light as electromagnetic radiation and predicting its propagation speed from electrical and magnetic constants.
The particle description became necessary because some experiments revealed that electromagnetic energy is exchanged in discrete amounts. The photoelectric effect showed that illuminating certain materials with light can eject electrons, but the effect depends strongly on the frequency of the radiation. Increasing intensity does not compensate indefinitely for radiation whose frequency is below the relevant threshold. Albert Einstein explained this behaviour by proposing that electromagnetic radiation is absorbed in individual quanta with energy E equals hf.
Each photon therefore has an energy determined by its frequency, while the number of photons affects quantities such as total energy and intensity. Compton scattering provided further evidence that electromagnetic radiation transfers energy and momentum in interactions resembling collisions between particles. The photon concept also explains why electromagnetic radiation can produce discrete microscopic transitions in atoms and molecules. These observations cannot be fully represented by treating light solely as a continuous classical wave.
Quantum mechanics resolves the apparent contradiction by replacing the classical categories of wave and particle with a quantum description. A photon is an excitation of the electromagnetic field, and its quantum state can exhibit interference even when photons are sent through an apparatus individually. In a double slit experiment performed at sufficiently low intensity, individual detection events appear as separate localised spots, but repeated events build an interference pattern.
This demonstrates that the probability distribution associated with quantum propagation has wave like behaviour while measurements produce discrete detection events. The experiment does not require one photon to be divided into two classical pieces. Instead, the quantum state includes alternatives associated with different paths, and their probability amplitudes can interfere. Measurement changes which information is available and therefore changes the observable pattern. Quantum field theory provides the modern framework for understanding these effects, with photons representing quantised excitations rather than ordinary macroscopic particles.
The wave and particle descriptions also differ in what they emphasise. Classical wave optics treats electromagnetic radiation through continuously varying fields and is particularly effective for macroscopic propagation, interference, diffraction and polarisation. Quantum optics treats the electromagnetic field as quantised and becomes essential when photon number, quantum correlations, individual detection events or interactions with individual atoms are important. A classical electromagnetic wave can carry energy continuously in the mathematical description, whereas quantum theory describes energy exchange in discrete photon quanta.
The classical description nevertheless emerges as an excellent approximation when many photons are involved and quantum fluctuations are relatively small. Thus, wave optics is not simply incorrect; it is a limiting description of quantum electromagnetic phenomena under appropriate conditions. Similarly, saying that light is a particle is incomplete because photons do not behave like classical particles with ordinary trajectories and simultaneously definite positions and momenta.
Wave particle duality is part of a broader feature of quantum physics in which microscopic systems cannot always be assigned classical properties before measurement. Electrons, neutrons, atoms and other quantum objects also display interference and diffraction while producing localised detection events. The relevant concept is therefore not unique to light. Quantum theory describes these systems using states and operators, with measurable quantities represented by observables and outcomes governed by probability amplitudes.
The uncertainty principle places limits on simultaneous knowledge of certain pairs of physical quantities, although this should not be interpreted merely as an imperfection of measurement instruments. For photons, position is particularly subtle because photons are massless relativistic quantum fields rather than nonrelativistic particles with a straightforward position operator. This is one reason that simplistic statements about photons following definite paths can be misleading.
Different experimental arrangements reveal different aspects of light. A spectrometer can resolve wavelength and demonstrate the frequency structure of radiation. Interferometers exploit phase relationships and are extremely sensitive to path differences. Photodetectors register individual absorption events and can measure photon statistics. Beam splitters, polarising filters and single photon sources are used in quantum optics to investigate superposition, interference, entanglement and measurement.
Lasers can operate in regimes where classical electromagnetic descriptions are extremely accurate, while carefully controlled experiments can reveal nonclassical photon statistics and quantum correlations. Quantum communication protocols exploit properties of individual photons, while conventional optical communications can often be analysed using classical wave theory supplemented by quantum noise considerations. The appropriate model therefore depends on the scale, precision, experimental purpose and physical process being investigated.
The most accurate modern statement is that light is a quantum electromagnetic phenomenon that has both wave like and particle like manifestations. Calling light a wave is useful for describing propagation, interference, diffraction and polarisation. Calling light a particle is useful for describing quantised energy and momentum transfer, detection events and interactions with matter. Neither word should be interpreted as claiming that light is literally a classical water wave or a microscopic solid object.
The deeper concept is the quantised electromagnetic field, whose states produce phenomena that have no exact classical equivalent. Wave particle duality is therefore not a temporary uncertainty that future experiments are expected to resolve by choosing one classical category. It is a consequence of quantum theory and the way physical systems produce observable outcomes.
