Atoms emit light when their electrons transition from higher energy levels to lower energy levels. This emission of light occurs because electrons in atoms can only occupy certain discrete energy levels, and when an electron moves from a higher energy level to a lower one, it releases energy in the form of electromagnetic radiation, which we perceive as light.
The specific wavelengths or colors of light emitted depend on the energy difference between the initial and final energy levels. This energy difference corresponds to a specific frequency or color of light according to the relationship E=hf, where E is the energy, h is Planck's constant, and f is the frequency of the light.
There are a few different ways in which electrons can transition between energy levels and emit light:
Absorption and emission spectra: When atoms absorb energy, typically through the absorption of photons, electrons can be excited to higher energy levels. These excited electrons are unstable in these higher energy states and eventually return to their original energy levels by emitting photons of specific wavelengths. This emission produces a characteristic emission spectrum, which is unique to each element.
Fluorescence and phosphorescence: Some substances can absorb light energy and then re-emit it almost instantaneously. This phenomenon is known as fluorescence. When the re-emission of light continues even after the excitation source is removed, it is called phosphorescence. These processes involve electrons transitioning between energy levels in the atoms or molecules of the substance.
Gas discharges: When a gas is subjected to high voltage or heat, its atoms can become excited, and electrons move to higher energy levels. As these excited electrons return to lower energy levels, they emit light. This is the principle behind various light sources such as neon lights, fluorescent lamps, and plasma televisions.
Thermal radiation: Atoms and molecules at high temperatures, such as in a flame or an incandescent light bulb, have electrons in excited states due to thermal energy. When these excited electrons return to lower energy levels, they emit light as thermal radiation.
In summary, atoms emit light when their electrons transition from higher energy levels to lower energy levels. The specific colors and patterns of the emitted light depend on the energy differences between the levels, and these emissions can occur through absorption and emission spectra, fluorescence and phosphorescence, gas discharges, and thermal radiation.