Hertz and Lenard's observation of photoelectric effect

First Observation of photoelectric effect

   In 1887 when Heinrich Hertz was conducting experiments to prove Maxwell’s electromagnetic theory of light, that he noticed a strange phenomenon. Hertz used a spark gap (two sharp electrodes placed at a small distance so that electric sparks can be generated) to detect the presence of electromagnetic waves. To get a closer look, he placed it in a dark box and found that the spark length was reduced. When he used a glass box, the spark length increased and when he replaced it with a quartz box, the spark length increased further. Later it was explained that this occured because quartz does not block the high frequency UV rays which are highly energetic.This was the first observation of the photo-electric effect.


    A year later, Willhelm Hallwachs confirmed these results and showed that UV light on a Zinc plate connected to a battery generated a current (because of electron emission). In 1898, J.J. Thompson found that the amount of current varied with the intensity and frequency of the radiation used.

    In 1902, Lenard observed that the kinetic energy of electrons emitted increased with the frequency of radiation used. This could not be explained as Maxwell’s electromagnetic theory (which Hertz proved correct) predicted that the kinetic energy should be only dependent on light intensity (not frequency).
The resolution would only come a few years later by Einstein when he would provide an explanation to the photoelectric effect.

Experimental Setup

J.J. Thompson’s set up (later improved by Lenard) to study photoelectric effect is of great importance. It consists of two zinc plate electrodes placed on the opposite ends of an evacuated glass tube. A small quartz window illuminates one of the electrodes that is made the cathode.and is used because it does not block

Ultra-Violet light (an ordinary glass blocks UV light). A variable voltage is exerted across the two electrodes using a Battery and Potentiometer. The current in the circuit can be recorded using an Ammeter as the potential and light intensity is changed. The set up is shown below:

Observations:

1. The photoelectric current (same as the rate of emission of electrons) is directly proportional to the intensity of light falling on the electrode. It can be noted from the figure below that with increasing intensity the current is increasing, and with the voltage decreasing the current decreases. But to obtain zero current, the voltage has to be reversed to a certain V­₀ known as the stopping potential. The voltage must be reversed to such an extent that the electrons cannot reach the anode. This is the maximum kinetic energy an emitted electron can achieve,
   

            Maximum Kinetic energy,  KE=eV₀eV0
            (e is the charge of the electron)

             Note that the stopping potential is independent to the intensity of light.

2. The Maximum kinetic energy increases with increase in frequency of light. With a higher frequency of light (ν), the stopping potential becomes more negative which implies that the kinetic energy of electrons also increases.
   

3. All frequencies of light however cannot cause a photoelectric current to develop. Only light above a certain frequency (ν₀) can produce a photoelectric current. This frequency is known as the threshold frequency. This varies with the electrode material. Also, the maximum kinetic energy of the electrons increases linearly with increasing light frequency. If we extend the graph below the x-axis, the intercept on the Kinetic energy axis represents the minimum energy required for emission of the electron; this is known as the work function of the material.
   
4. Lastly the electron emission occurs instantly without any time lag.

Planck's Quantum Theory changed the view of Physicists

Introduction

By the late 18th century there was a great progress in the field of Physics. However by the early 20th century many phenomena could not be predicted by Classical (Newtonian) Physics which was widely accepted at that time. Classical Mechanics failed especially at the atomic levels and completely contradicted with the modern experiments like photoelectric effect. As a result a new set of theories were articulated and these was collectively called Quantum Mechanics. Quantum mechanics changed the view of how Physicists viewed the Universe. It marked the end of a Clockwise Universe (Idea that a universe is predictable).

EM Waves

Electromagnetic (EM) radiation is a form of energy with both wave and particle nature; visible light being a well-known example. From the wave perspective, all forms of EM radiation may be described in terms of their wavelength and frequency. While the wavelength and frequency of EM radiation may vary, its speed in a vacuum remains constant at c=3.0 x 10⁸ m/sec, the speed of light. The wavelength or frequency of any specific occurrence of EM radiation determine its position on the electromagnetic spectrum and can be calculated from the following equation:

                                                          c=νλ
where λ = wavelength in meters, and ν=frequency in hertz (1/sec).

Discovery of Quantum (Photoelectric Effect)

Photoelectric effect is observed when light focused on certain metals emits electrons.This effect occurs when an EM radiation greater than a certain frequency falls on a metal. This minimum threshold frequency also called cutoff frequency is different for different metals. One important observation was that the emission of electrons did not depend on intensity of incident light, i.e. even light with twice intensity and a frequency less than the the threshold was unable to emit electrons. This was contrary to the effect that was expected if the light acted as a wave strictly; the effect of light would then be cumulative adding up the intensities little by little until electrons are emitted. But instead there is a clear cutoff frequency which triggers this phenomena known as photoelectric emission. This led to the conclusion that the energy of light is directly proportional to the frequency, higher light frequencies have greater energy. This led to the discovery that an atom could loss or gain a minimum amount of energy this minimum energy being called "quantum" plural "quanta". One photon of light carries one quantum of energy. According to Planck we have
                                                           E=hν

                         where h is Planck's constant and h=6.634x10^-34