What does wave-particle duality mean? Wave-particle duality is the scientific theory that electromagnetic (EM) radiation exists as both a particle and a wave. But how can radiation be both particle-like and wave-like? The first prevailing theory was that EM radiation exists only as a wave because of its ability to transfer energy, sound, and light through mediums while having both a frequency and a wavelength. The evidence that EM radiation also exhibited qualities such as refraction and interference also supported the wave theory. However, it wasn’t until Albert Einstein proposed the wave-particle theory that the idea of EM radiation existing as a particle truly surfaced. After this, the double slit experiment (an experiment where light was passed through two slits) proved that EM radiation can exist as both a wave and particle because of its showcase of light and dark bands (wave crests and troughs) as well as discrete particles. The photoelectric effect in which Einstein earned his Nobel prize for, was also utilized in order to prove EM radiation’s existence as a particle by bouncing certain wavelengths off a metal plate and in turn releasing electrons.