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.

Detection of Light

For each way of producing light there is a corresponding way of detecting it. Just as heat produces incandescent light, for example, light produces measurable heat when it is absorbed by a material.

1. Photoelectric Effect

The photoelectric effect is a process in which an atom absorbs a photon that has so much energy that the photon sets one of the atom’s electrons free to move outside the atom. Part of the photon’s energy goes toward releasing the electron from the atom. This energy is called the activation energy of the electron. The rest of the photon’s energy is transferred to the released electron in the form of motion, or kinetic energy. Since the photon energy is proportional to frequency, the released electron, or photoelectron, moves faster when it has absorbed high-frequency light.

Metals with low activation energies are used to make photodetectors and photoelectric cells whose electrical properties change in the presence of light. Solar cells use the photoelectric effect to convert sunlight into electricity. Solar cells are used in place of electric batteries in remote applications like space satellites or roadside emergency telephones (see Solar Energy). Hand-held calculators and watches often use solar cells so that battery replacement is unnecessary.

2. Photochemical Detection

The change induced in photographic film exposed to light is an example of photochemical detection of photons. Light induces a chemical change in photosensitive chemicals on film. The film is then processed to convert the chemical change into a permanent image and to remove the photosensitive chemicals from the film so it will not continue to change when it is viewed in full light.

Human vision works on a similar principle. Light of different frequencies causes different chemical changes in the eye. The chemical action generates nerve impulses that our brains interpret as color, shape, and location of objects.