1. The Photoelectric Effect:
* Observation: When light shines on a metal surface, electrons are emitted. The energy of these electrons depends on the *frequency* of the light, not its *intensity*. This is contrary to classical wave theory, which predicts that the electron's energy should depend on the intensity of the light wave.
* Explanation: Albert Einstein explained this by proposing that light is quantized into packets of energy called photons. The energy of a photon is directly proportional to the frequency of the light. An electron absorbs the entire energy of a single photon, which is enough to eject it from the metal if the photon's energy exceeds the metal's work function.
2. Blackbody Radiation:
* Observation: A blackbody is a hypothetical object that absorbs all electromagnetic radiation that falls on it. Classically, the blackbody spectrum should have an energy distribution that increases without bound at higher frequencies, leading to the "ultraviolet catastrophe." However, experimentally, the spectrum peaks at a specific frequency that depends on the temperature of the blackbody.
* Explanation: Max Planck successfully explained the observed spectrum by assuming that light energy is quantized. He proposed that light is emitted and absorbed in discrete packets, later called photons, with energy proportional to frequency.
3. Compton Scattering:
* Observation: When X-rays scatter off electrons, they lose energy and change wavelength. This energy loss cannot be explained by classical wave scattering, which predicts only a change in direction.
* Explanation: This experiment provides further evidence for the particle nature of light. The change in wavelength can be explained by assuming that the X-ray photon collides with the electron like two billiard balls, transferring some of its energy and momentum.
4. Double-Slit Experiment:
* Observation: While the double-slit experiment demonstrates wave interference, it also shows that light behaves like particles when interacting with the detector. Individual photons arrive at the screen in discrete locations, but the pattern of photons over time shows an interference pattern.
* Explanation: This experiment highlights the wave-particle duality of light. Even though light propagates as a wave, it interacts with matter as individual particles (photons).
5. Single-Photon Experiments:
* Observation: Experiments have been performed where a single photon is sent through a double slit. Despite the lack of another photon to "interfere" with, the photon still creates an interference pattern on the detector.
* Explanation: This demonstrates that the photon somehow "interferes with itself," further blurring the lines between wave and particle behavior.
These observations and experiments provide strong evidence that light exhibits properties of both waves and particles. The classical wave description of light fails to explain these phenomena, leading to the development of quantum mechanics, which provides a more complete picture of the nature of light.
