Optics

Optics is a branch of physics that studies light: how it is produced, travels, appears in different media, and interacts with objects. Light is an electromagnetic wave, and the portion visible to the human eye has wavelengths between 380 and 700 nanometres. Optics also studies radiation close to visible light, such as infrared and ultraviolet, which follow the same principles.

Optics is fundamental to optical instruments: eyeglasses, cameras, microscopes, telescopes, and fibre optic cables used in data communication. It also relates to biological optics, including how the eye works and how the brain interprets visual information.

Major divisions

Optics is commonly divided into several branches. Geometrical optics treats light as straight rays and deals with the laws of reflection and refraction; it applies to mirrors and lenses. Wave optics (or physical optics) considers the wave nature of light and explains interference, diffraction, and polarisation. Quantum optics views light as particles called photons and describes phenomena such as the photoelectric effect and laser function.

Basic laws

When light strikes a reflective surface, the angle of incidence equals the angle of reflection—this is the law of reflection. When light passes from one medium to another, such as from air to water, its speed changes and its path bends; this is described by Snell's law, which depends on a quantity called the refractive index. Because this index varies with wavelength, white light passing through a prism spreads into colours—the basis of the rainbow.

Brief history

The laws of optics were investigated by ancient Greek scholars and Islamic scientists; Ibn al-Haytham (Alhazen) is credited with systematically explaining how vision occurs when light from objects enters the eye rather than emanating from it. In later centuries, Snell, Newton, Huygens, Young, Fresnel, and Maxwell advanced understanding of light. Maxwell is noted for showing that light is an electromagnetic wave, while twentieth-century quantum theory introduced the concept of the photon. This led to light being recognised as having both wave and particle properties.

Modern applications

Optical technology is now present in internet communications through fibre optics, scientific measurement, medical devices such as endoscopes and laser eye surgery, data storage on discs, and sensors in digital cameras. Optics is also fundamental to astronomy, with telescopes on Earth and in space collecting light from distant objects for study.