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To understand the nature of Radiation and Matter, you will first have to understand the duality of wave and particle nature. With the discovery of cathode rays, the rays were considered to be fast-moving negatively charged particles. With further advancements, Einstein photoelectric equation, where he explained the photoelectric effect with the energy quantum and charge being moving in discrete units called quanta of energy which represented the particle nature.
You know that metals contain free electrons which are required for conductivity. Still, these free electrons do not leave the metal surface as free electrons being negatively charged particles when leaving the surface, the metal surface acquires a positive charge and attracts back the electrons. To overcome this attractive pull minimum energy is required by an electron to leave the metal surface called the work function of the metal. It is denoted by 0 and measured in eV( electron-volt). For electron emission, this minimum energy required can be supplied to the free electrons by these methods – Thermionic emission, Field emission, Photoelectric emission.
The phenomena of electron emission from the metal surface, when the light of sufficient frequency (threshold frequency) falls on the emitter surface is called the photoelectric effect. Found in 1887, by Heinrich Hertz during his experimental study of the photoelectric effect, he observed that when the emitter plate was illuminated by ultraviolet light from an arc lamp, high voltage sparks were produced across the detector. Further, after the discovery of electrons, it was evident that incident light falling on the surface causes electrons to be emitted from the emitter surface, the intensity of light required to each surface is different. The metals which emit electrons when illuminated by light were called photosensitive substances, and the electrons were called photoelectrons.
The following experiment was conducted to study the photoelectric effect. The arrangement consists of an evacuated tube, a photosensitive (emitter) plate and another metal plate. Monochromatic light of short wavelength is made to fall on the emitter plate through a window. As light falls, the emitter plate emits electrons which are collected by the metal plate (collector), and an electric field is created through the battery. As the battery maintains the potential difference between the plates, the collector plate can be held at a desired positive or negative potential with respect to the emitter plate. When the collector plate is positive compared to the emitter plate, the electrons get attracted towards it; this causes electron emission. This arrangement is used to study the variation of photocurrent with
Keeping the frequency of incident radiation and accelerating potential is fixed. By varying the intensity of light, the change in the resulting photoelectric current is observed. Electron emission increases linearly with the intensity of the incident light, which implies that the number of photoelectrons emitted per second is directly proportional to the intensity of incident radiation.
While keeping the emitter plate at positive accelerating potential with respect to a collector plate, and illuminating the collector plate with the light of fixed frequency and fixed intensity we observe that on increasing the accelerating potential (positive) the photoelectric current increases. If we increase the positive potential of the emitter plate further, photocurrent will not increase. This maximum limit is known as saturation current.
Now, applying negative (retarding) potential to the emitter plate, the electrons are repelled, and most energetic are only able to reach the collector plate. The photocurrent decreases rapidly as until it drops to zero at a critical value of negative potential V0 on the emitter plate. This minimum negative potential V0 is called cut-off or stopping potential.
Now, on varying the intensity of Radiation, it is found that the saturation current increases with increased intensity. Thus, more numbers of electrons are emitted per second proportional to intensity.
To study the variation between the incident radiation frequency and the stopping potential V0, we fix the intensity of light at various frequencies and study the variation of photocurrent with collector plate potential. On observation, we get the energy of the emitted electrons depending on the frequency of the incident radiation. For higher frequencies of incident radiation, the stopping potential is more harmful.
Albert Einstein, in 1905, presented a new picture of Electromagnetic Radiation of the photoelectric effect. Instead of continuous absorption of energy, radiation energy is built up discrete units called quanta of the energy of Radiation. Each quantum of energy has the energy of hv, where h is Planck’s constant and v is the frequency of light.
When the quantum of energy exceeds the minimum energy, the minimum energy needed for the electrons to escape from the metal surface ( work function0), the electron is emitted with maximum kinetic energy. Eqn. 1 is known as Einstein’s Photoelectric Equation.
Wave nature is shown by many phenomena in physics, i.e. interference, diffraction and polarisation. Whereas phenomenons like the photoelectric effect, Compton effect which involve Radiation, energy transfer is explained based on the transfer of photons.
De-Broglie presented a hypothesis that moving particles display wave-like properties under suitable conditions. The waves associated with moving particles were named as de Broglie waves. The wavelength associated with the momentum is given as
The dual nature of Radiation and matter is inherently present in the de Broglie equation as the wave concept is represented by and particle nature is represented by momentum p. m is the mass of the matter. De Broglie wavelength is not evident for the heavier particles as they become very small.
This matter-wave scenario gives rise to the Heisenberg uncertainty principle according to which position and momentum of an electron can never be measured simultaneously. There’s always an uncertainty.
The wave nature of electrons was experimentally verified by Davisson and Germer experiment, which presented an agreement between the theoretical and the experimental value obtained by de Broglie.
Dual Nature of Radiation and matter concept was widely accepted, and the behaviour of light and matter was now explained based on particle or wave nature which led to more development to modern quantum mechanics—fields like electron optics where development of electron microscopes utilized the concept of wave property of matter.
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