When matter is struck by electromagnetic waves with relatively short wavelengths, such as ultraviolet light or visible light, its atoms can emit electrons. This process is known as the photoelectric effect or, less commonly, the Hertz effect, and it occurs because electromagnetic waves possess energy that is capable of dislodging the electrons in an atom. The observation of the photoelectric effect helped to clarify some issues about the nature of light and about the nature of atoms. Light, it was discovered, can act both as a wave and as a particle; light travels in wave motion but can physically impact surfaces and even cause mechanical change by dislodging electrons from atoms.
The photoelectric effect is usually observed when light is shined on metallic surfaces. The beam of light that is shone in a metal surface is referred to as the photocathode, and the electrons that it ejects from an atom are called photoelectrons. Shining light on a conductive metal surface can actually cause an electrical current, called a photocurrent , to form. A material that is sensitive to light, such as the metals which can carry an electrical current because of light, are referred to as photosensitive substances.
The number of electrons ejected as a result of the photoelectric effect is closely related to the frequency and the intensity of the light shone on the metal surface. Low frequency light, which has a long wavelength, tends to dislodge few, if any, electrons from a metallic surface. This is true if the light is of high intensity or low intensity. At high frequency, however, light tends to dislodge far more electrons, especially if the light is particularly intense. This basically means that, at any intensity, red light will release very few electrons but blue light will dislodge many.
The observation of the photoelectric effect presented strong evidence for the quantum nature of light, which had previously not had much support. It also supported the wave-particle duality theory of light at a time when most scientists believed that light behaved as either a particle or a wave, not both.
Light exists in discrete particles known as photons, which are scientifically described as quanta of light. One photon is one quantum of light; it is the smallest unit of light that can possibly interact with anything else. Quanta of light strike and dislodge electrons when light is shone on a metallic surface; this is the photoelectric effect.