In the double-slit experiment, light exhibits both particle-like and wave-like behavior. This experiment is often used to illustrate the wave-particle duality of light and other quantum particles.
When light is passed through two closely spaced slits onto a screen, an interference pattern is observed, similar to what is observed when two water waves or sound waves interfere with each other. This interference pattern is a characteristic of wave behavior.
However, the double-slit experiment can also be performed using very low intensities of light, such that only one photon (a particle of light) is present in the apparatus at a time. Surprisingly, even when single photons are used, they still produce an interference pattern over time, as if they were interfering with themselves.
This phenomenon led to the conclusion that light behaves both as a particle and a wave. When a single photon is emitted from a source, it can be thought of as a localized particle-like entity. However, as it propagates through space, it exhibits wave-like properties described by a mathematical function called a wave function or probability amplitude.
The wave function describes the probability distribution of where the photon is likely to be detected when it interacts with the screen. The interference pattern arises due to the overlapping and interference of the wave functions associated with the photon passing through each of the two slits. This interference pattern suggests the existence of wave-like behavior.
The double-slit experiment demonstrates that particles such as photons can exhibit wave-like behavior and interfere with themselves. This phenomenon is a fundamental aspect of quantum mechanics and illustrates the wave-particle duality of quantum particles. It shows that the behavior of particles at the microscopic level is inherently different from our classical, everyday experience, where we tend to associate particles strictly with localized entities.