What is light?

Light is electromagnetic radiation, although everyday usage usually means the visible range detected by human eyes. Physics also examines invisible radiation such as infrared and ultraviolet within the same framework. Light carries energy and momentum, and explaining it requires both wave and quantum properties.

Basic properties of light

1. Speed and propagation Light’s vacuum speed is exactly 299,792,458 meters per second. Propagation in matter depends on interactions with the medium. Slower propagation through a transparent material does not mean the fundamental vacuum speed changes arbitrarily. Light needs no material medium such as air. 2. Wavelength and frequency Wavelength is the separation between successive points of equal phase; frequency counts oscillations per second. Their product in vacuum is light speed. Visible colors relate to wavelength ranges, but perceived color also involves the visual system and the light reaching it. 3. Photons and energy In quantum descriptions, light interacts with matter in quanta called photons. Photon energy increases with frequency. A beam’s total power also depends on the number of photons it carries. Brightness and individual photon energy are different quantities.

Optical phenomena and observation

4. Reflection and refraction Reflection from a mirror differs from light changing direction upon entering water. Refraction relates to differing propagation in media. Lenses use it to form images. Magnification is different from resolving fine detail. 5. Interference and diffraction Overlapping waves can form bright and dark patterns. Spreading through small openings is studied as diffraction. Such phenomena reveal limits of describing light only as straight rays. 6. Scientific instruments Microscopes, telescopes and spectrometers exploit different optical properties. A star’s spectrum can reveal composition and motion. Scientific image colors may be assigned to represent invisible wavelengths rather than the colors an unaided eye would see.