When an atom absorbs energy—whether it be electrical energy, thermal energy, or electromagnetic radiation—it tends to re-emit that energy as a photon. This is the principle behind neon signs. If you pass electricity through a gas, the gaseous atoms give off light. Different atoms emit different wavelengths of light. For example, if you pass the light from a hydrogen lamp through a slit and a prism, the light will be divided into its component wavelengths. This result is called an atomic spectrum. Hydrogen's spectrum consists of four distinct wavelengths in the visible spectrum. On the other hand, if you perform this experiment with helium, you would get a different spectrum with six distinct wavelengths in the visible spectrum. Each atom has its own unique spectrum. If you conduct this experiment with white light, you would observe a continuous spectrum. The Bohr model was developed to explain these results. In the Bohr model, the energy of an electron is quantized, which means an electron in an atom can have only particular discrete values of energy. We label these values of energy as energy levels: n = 1, n = 2, n = 3, and so forth. Normally, an electron is in its ground state, which means it is in the lowest energy level available. However, when an electron absorbs energy, it is said to be excited and it jumps to a higher energy level. This higher energy level is less stable because the opposite charges have been separated. The nucleus is positive and the electron is negative, so energy is required to separate them. The electron will spontaneously drop back down to a lower energy level as it is attracted to the positive nucleus. When it drops from a higher energy level to a lower energy level, it emits a photon with an energy equal to the difference in energy between the two levels. Each of the distinct wavelengths present in the atomic spectrum for an atom corresponds to transitions between two energy levels in the atom. For instance, in hydrogen, when an electron drops from the n = 3 to the n = 2 energy level, it emits a photon with a wavelength of 657 nm, which corresponds to red light. Since each element has a unique atomic spectrum, we can use a flame to excite electrons in a metal and observe the color of light that is emitted, in order to identify the metal. This is called a flame test for a metal. The creators of fireworks applied the concept of atomic spectra when they used shavings of different metals to achieve different colored lights.