Quantum_I_Notes

 2014 Yeow Kok Han Page 1 QQuuaannttuumm PPhhyyssiiccss II 11 HHiissttoorriiccaall BBaacckkggrroouunndd Hundreds of years back, scientists were speculating about whether light was made up of particles or waves. By 1800s, it was generally accepted that light was a wave. The definitive feature of waves is their abil ity to diffract, suppose and produce interference patterns. Particles on the other hand, cannot superpose. Instead, they collide and bounce off each other. In 1887 Hertz discovered the photoelectric effect. It was found that electrons, called photoelectrons, were emitted from a clean metal surf ace when high frequency light such as ultraviolet light fell on it. The experimental data could not be explained by treating light as a wave but they could be explained by treating light as particles. Particles of lig ht or p ackets of electromagnetic radiation are called photons. Each packet of energy or radiation is called a quantum. Since then, many quantities other than energy have been found to be quantised under certain conditions. This quantisation of physical qua ntities and the probabilistic nature of their measurements are two of the features of Quantum Physics that we will study. In Quantum Physics, many phenomena seem very bizarre. This is because quantum behaviour is mostly encountered in the microscopic realm , of which we would normally have very little experience of. 22 PPhhoottooeelleeccttrriicc EEffffeecctt A positive p.d is applied when collector is at higher potential while negative p.d. is when collector is at lo wer potential than emitter . The data in Fig. 2.2 is obtained when p.d. V is varied while light intensity is kept fixed. A positive p.d creates an E-field that accelerates the electrons towards the collector. It also steers some electrons that would other wise have collided with the casing and be absorbed towards the collector. When all the electrons reach the collector, the current is called saturation current. Photon is a packet of electromagnetic radiation energy. Photoelectric effect is the emission of electrons from metals by electromagnetic radiation of sufficiently high frequency. Saturation current is the maximum photoelectric current which occurs when all the emitted electrons are able to reach the collector. Photoelectric effect is the emission of electrons from metals by electromagnetic radiation of sufficiently high frequency. Fig. 2.1 The incident light falls on the emitter metal, ejecting electrons (photoelectrons), some of which are collected by the collector, giving rise to a current (photoelectric current). The vacuum inside is needed to prevent formation of oxide layer on metal and to facilitate the movement of emitted electrons from emitter to collector. variable voltage source incident radiation V A quartz window emitter colle