Summary
Highlights
Light is a wave, a form of energy called electromagnetic radiation, comprised of intertwining electric and magnetic fields. Its most important feature is its wavelength, which determines its energy: shorter wavelengths mean higher energy, and longer wavelengths mean lower energy. Our eyes detect these different energies as colors, forming a spectrum from violet (short wavelength, high energy) to red (long wavelength, low energy).
Visible light is just a small part of the entire electromagnetic (EM) spectrum. Shorter wavelengths than violet include ultraviolet, X-rays, and gamma rays (highest energy). Longer wavelengths than red include infrared, microwaves, and radio waves (lowest energy). Most of the universe is invisible to our eyes, so different types of telescopes are built to detect these various forms of light.
Matter emits light when it's heated and gains energy. The type of light emitted depends on its temperature: hotter objects emit higher energy, shorter wavelength (bluer) light, while cooler objects emit lower energy, longer wavelength (redder) light. This principle applies to dense objects like stars and even humans, who emit light in the infrared spectrum.
For less dense objects like gas clouds, light emission is understood by looking at individual atoms. Atoms consist of protons, neutrons, and electrons. Electrons occupy specific energy levels or 'steps' around the nucleus. When electrons absorb the precise amount of energy (from light), they jump to a higher energy level. When they fall back down, they emit light with the exact amount of energy they absorbed.
Since energy corresponds to wavelength and color, electrons absorb or emit very specific colors of light. Different atoms have different energy level structures, meaning they emit distinct 'colors' or wavelengths. Spectrometers can precisely measure these wavelengths, allowing astronomers to determine the chemical composition of distant astronomical objects like gas clouds and stars.
Similar to the Doppler effect for sound, light also experiences wavelength shifts due to motion. If an object moves towards us, its light waves are compressed, resulting in a 'blue-shift' (shorter wavelength). If it moves away, the waves are stretched, causing a 'red-shift' (longer wavelength). By measuring this shift in a spectrum, astronomers can determine if objects are moving towards or away from Earth, a crucial concept in understanding the expanding universe.
Spectroscopic techniques allow astronomers to determine numerous fundamental properties of astronomical objects, including their spinning speed, magnetic field strength, mass, and density. Almost all our knowledge about the universe comes from analyzing the light emitted by objects within it, providing a 'blueprint' for understanding their nature and history.