Matter & Energy


Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Photoelectric Effect


Photoelectric Effect, formation and liberation of electrically charged particles in matter when it is irradiated by light or other electromagnetic radiation. The term photoelectric effect designates several types of related interactions. In the external photoelectric effect, electrons are liberated from the surface of a metallic conductor by absorbing energy from light shining on the metal's surface. The effect is applied in the photoelectric cell, in which the electrons liberated from one pole of the cell, the photocathode, migrate to the other pole, the anode, under the influence of an electric field.

Study of the external photoelectric effect played an important role in the development of modern physics. Experiments beginning in 1887 showed that the external photoelectric effect had certain qualities that could not be explained by the theories of that time, in which light and all other electromagnetic radiation was considered to behave like waves. For example, as the light shining on a metal becomes increasingly intense, the classical wave theory of light suggests that the electrons that absorb the light will be liberated from the metal with more and more energy. However, experiments showed that the maximum possible energy of the ejected electrons depends only on the frequency of the incident light, and is independent of the light's intensity.