The atomic theory of night vision
Atoms are in perpetual motion. They are constantly vibrating, moving and spinning. Even the atoms that make up our chairs are in constant motion. Atoms have several different excited states. In other words, they have different energies. If we impart a large amount of energy to an atom, it moves out of the ground state energy level to an excited level. The level of excitation depends on the amount of energy applied to the atom in the form of heat, light or electricity.
An atom is made up of a nucleus (including protons and neutrons) and a cloud of electrons. We can think of the electrons in the electron cloud as moving around the nucleus in different orbits. It is not yet possible to observe the discrete orbitals of electrons, but it is easier to understand these orbitals by thinking of them as different energy levels of atoms. In other words, if we apply a certain amount of thermal energy to an atom, it is predictable that some of the electrons in lower energy orbitals will be transferred to higher energy orbitals, i.e. further away from the nucleus.
After electrons are transferred to high-energy orbits, they still have to return to the ground state eventually. In the process, the electrons release energy in the form of photons (particles of light). You see, atoms are constantly releasing energy in the form of photons. For example, when the heater in a toaster oven turns bright red, it is because atoms are excited by heat and emit red photons. An electron in an excited state has a higher energy than an unexcited electron, and because the electron has absorbed some energy to reach the excited level, it releases this energy to return to the ground state. This energy is released in the form of photons (light energy). The emitted photons have a specific wavelength (color), depending on the energy of the electron when the photon is released.
Any living thing consumes energy, and so do many inanimate objects, such as engines and rockets. Energy expenditure generates heat. In turn, the thermal energy causes the atoms in the object to emit photons in the thermal infrared spectrum. The hotter the object, the shorter the wavelength of the emitted infrared photons. If the temperature of the object is very high, the photons it emits can even enter the visible light spectrum, starting with red light, then orange light, yellow light, white light, and finally blue light.
