From Classical Physics to Quantum Theory

Classical physics that we have been studied in the first lecture have limited success to understand atoms and molecules – could not explain the forces that hold atoms together. So the properties of atoms and molecules are not governed by the same laws that work so well for larger objects.

To understand quantum theory, we must know something the nature of waves. A wave can be thought of as a vibrating disturbance by which energy is transmitted. The speed of the wave depends on the type of wave and the nature of the medium through which the wave is traveling. The distance between identical points on successive waves is called the wavelength (l), it is usually expressed in unit of meters, centimeters, or nanometers. The frequency (v) of the wave is the number of waves that pass through a particular pint in one second, it is measured in hertz (Hz = cycle/s). The amplitude is the vertical distance from the midline of a wave to the peak or trough (see figure 2.9). So the speed of the wave is given by the product of its wavelength and its frequency.

Figure 2.9 (a) wavelength and amplitude. (b) two waves having different wavelength and frequencies.