Spectrum

A spectrum in a general sense is a way of showing the fluid transition from one extreme to another. Within physics, it typically refers to the band of colors created when light is separated into its components based on their wavelengths. All electromagnetic waves in the universe are technically on the same spectrum, but we can only see a small part of it, which is appropriately named the visible light spectrum. This section shows the the progression from the tight-waved ultraviolet light to the more loosely-waved infrared light.

Emission and absorption
When talking about electromagnetic radiation, also known as light, the terms emission and absorption are very relevant. Emission is the act of emitting, or sending out electromagnetic waves, while absorption is absorbing, or receiving them. A light source, such as a light bulb, emits light, and everything else that we see in the world around us absorbs certain wavelengths of light.

Different types of spectra
There are various ways in which we can see and categorize spectra based on their different uses. Often, spectra are a continuous band, known as the continuum, but they can also show lines of color at specific points, known as spectral lines. These lines can help distinguish between and identify gases that are present, as each gas will reveal different spectral lines. Three important types for our purposes are the continuous spectrum, the emission spectrum and the absorption spectrum. As evident in their names, emission and absorption spectra show the light that is emitted and absorbed respectively.

To explain the spectra, we use a definition of light that involves photons.