Photonics is the study and use of light. The word photonics is based on a “photon”, which is a single particle of light. This is similar to electronics where the electrons are single particles of charge that make up electric current.
In photonics, the photons are single particles of energy that make up light. The amount of energy provided by a photon depends on the color (wavelength). For example, a laser pointer that outputs 1 mW of red (640 nm) light provides 3 x 1015 photons/s. Comparing to electronics, a power supply that provides 1 Amp of current provides 6 x 1018 electrons/s.
Light is generated by a variety of sources. Some come from nature, like the sun, fire, or bioluminescence (lightning bugs). Manufactured sources include light bulbs, LEDs, and lasers.
Much like wires are used to transport electric current, photonics uses optical fiber to transport light from one location to another.
Similar to electronics using resistors and capacitors to modify the current flow through a circuit, photonics uses optics like lenses, mirrors, and prisms to direct and modify light paths.
Almost all analysis of light is done with the same measurement equipment used in electronics, but a device is required to first convert the photons into electrical current.
Common uses for photonics are to measure distance (laser radar), transmit/receive information (telecommunications), image objects that are difficult to see by the eye alone (microscopes / endoscopes / borescopes), and create sensors such as the amount of oxygen in the blood (pulse oximeters) and the quality of the air around us (particle size and trace gas detection).