Evidence For The Wave Nature Of Light

When a partition containing two thin parallel slits is placed in front of a light source, the light emerging from the slits strikes a screen and forms a series of bands of light. This phenomena led to the postulation that light has wave properties. When the waves emerge from the slits, some waves from one slit merge with those from the other slit in phase; that is, they reinforce one another. This merger of waves produces the bands that appear on the screen. The dark areas between the bands result when waves from one slit merge with waves that are "out of phase"; that is, the waves cancel each other and no light is observed.

The Electromagnetic Spectrum

It is important to have the entire electromagnetic spectrum firmly fixed in your mind. At one end of the spectrum, the radio, TV region, the waves are long and their frequency is low. These waves have very low energy. At the other end of the spectrum are the very short waves with high frequencies. These regions, the X-ray and gamma-ray regions, have very high energies. In the middle of the spectrum is the visible region to which our eyes respond. You should also familiarize yourself with the colors of the visible region, ranging from red at the longer wavelengths to blue at the shorter wavelengths.

The Line Spectrum Of Hydrogen

A spark is passed through a bulb containing hydrogen. The light emitted is directed through a prism, which dissects the light into its consituent waves. Some of these waves and their colors are shown in the foreground at the end of the animation. This line spectrum was one of the phenomena that led to the postulation of discrete, quantized energy levels for the hydrogen atom.

Low-Energy Bohr Orbits

According to the Bohr model, an electron remains in an orbit about the nucleus until it either accepts the right amount of energy to "jump" to an orbit farther from the nucleus, or it losses energy by "jumping" to an orbit closer to the nucleus. As the animation begins the electron is in the lowest energy level; that is, in the orbit closest to the nucleus. A photon of just the right energy for the transition to the second orbit is then absorbed and its energy is converted into a higher energy for the hydrogen atom. [Notice that the electron moves faster in the second orbit.] Finally, another photon of greater wavelength and lower energy is absorbed and a transition to the third orbit is accomplished. The energy required for this transition is lower than that required for the first transition because the energy levels get closer together as the value of n increases.

Relaxation Of Electron According To The Bohr Model

This animation is essentially the reverse of the previous animation. It portrays the relaxation of the hydrogen atom according to the Bohr model. As the electron "relaxes" to a lower orbit, the atom loses energy. This energy is emitted in the form of a photon.

Relaxation Of Electron According To The Wave Model

In the wave model the electron is assumed to have some characteristics of waves. The electron is characterized by a set of four quantum numbers that determine the relative location of the electron and its energy. This animation shows the eight electrons of a neutral oxygen atom in an excited state electron configuration. In other words, the atom is not in its lowest possible energy. When it relaxes to the ground state it emits a photon. Hence, the wave model also explains the phenomena of line spectra.

A Schematic Representation Of The "Aufbau" Prinicple

This schematic representation of the aufbau principle shows electrons entering the 1s, 2s, and 2p orbitals of an oxygen atom. Notice that both the Pauli exclusion principle and Hund's first rule are obeyed. After the ground state electron configuration is obtained, the atom is irradiated with a photon of precisely the correct energy to cause a transition from a 2p to a 3s orbital. At the end of the animation the atom is in an excited state.