PARTICLE NATURE OF ELECTROMAGNETIC RADIATION AND PLANCK'S QUANTUM THEORY

PARTICLE NATURE OF ELECTROMAGNETIC RADIATION AND PLANCK'S QUANTUM THEORY

The electromagnetic wave theory of radiation believed in the continuous generation of energy. This theory explained the phenomenon of propagation of light such as diffraction and interference" quote successfully but it failed to explain many phenomena such as black body radiation and photoelectric effect.

(a) Black Body Radiation:- When solids are heated they emit radiation over a wide range of wavelengths. For example, when an iron basis heated in a furnace, it emits radiation and becomes then progressively becomes more and more dull the temperature increases. When heating is continued, it becomes brighter orange, then yellow, then white and finally it becomes blue at very high temperatures. As we know red light has higher wavelength and blue light has lower wavelength so in terms of wavelength, it means that the radiation emitted varies from a higher wavelength to a lower wavelength. Since the longer wavelengths (red) have a lower intensity and shorter wavelengths have a higher intensity, this means that if this trend were to continue, the intensity would keep on increasing indefinitely as the wavelength becomes shorter and shorter and may enter ultra violet region. However, the intensity of black body radiation does not show this expected behaviour. An ideal body which emits and absorbs radiation of all wavelengths or frequencies is called black body and the radiation emitted by this body is called black body radiation. It has characteristic distribution at a given temperature. At a given temperature, intensity of radiation emitted from a black body increases with decrease of wavelength. It reaches a maximum value at a given wavelength and then starts decreasing with further decrease of wavelength. The variation of intensity with wavelength at two temperatures T, and T, (T, >T,). These results could not be explained by the classical wave theory of light. According to this theory, energy is emitted or absorbed continuously. Therefore, the energy of any electromagnetic radiation in proportional to its intensity and independent of its frequency or wavelength Thun the radiation emitted by the body being heated shell have the same colour wavelength or frequency throughout, although the intensity of the colour might change with variation in temperature. 

It is clear that at each temperature, there is a wavelength at which the intensity of radiation is maximum (A). This maximum shifts to a lower wavelength as the temperature is increased, A black body is not only a perfect absorber of radiant energy but it is also a perfect radiator.

This means that a black body absorbs all the radiation falling on it. When such a body is heated, it emits radiation of all wavelengths or frequencies and it has been shown that no other body can emit radiant energy more than a black body. Hence, a black body which is the most efficient absorber of radiant energy is also the most perfect emitter.

(b) Photoelectric Effect:- In 1887, H. Hertz performed very interesting experiment. He observed that when light of certain frequency strikes the surface of some metals, electrons (or electric current) are ejected from the metals. The phenomenon of ejection of electrons from the surface of a metal when light of suitable frequency strikes on it, is called photoelectric effect. The emitted electrons are called photoelectrons. The apparatus shown photoelectric effect. The cell consists of an evacuated chamber which contains two electrodes connected to an external circuit. The metal that exhibits the photoelectric effect is made negative electrode. When light of sufficiently high energy strikes the metal, the electrons are ejected from its surface and move toward the positive electrode and form the current flowing through the circuit. It may be noted that only a few metals such as cesium, rubidium or potassium in which the electrons are loosely held by the nucleus show this effect when visible light falls upon them. Experimental studies of photoelectric effect under different conditions led to the following important observations:

(i) The electrons are ejected from the metal surface as soon as the beam of light strikes the surface i.e., there is no time lag between the striking of light beam and the ejection of electrons from the metal surface.

(ii) For each metal, certain minimum frequency of light is needed to eject the electrons. This is known as threshold frequency (v) and it is different for different metals. Light of frequency less than v, cannot eject electrons no matter how long it falls on the surface or how high is its intensity.

(iii) The kinetic energy of the ejected electrons is directly proportional to the frequency of the incident radiation and is independent of its intensity of electrons ejected per second from the metal surface depends upon the intensity or brightness of incident radiation but does not depend upon its frequency. The variation of kinetic energy of photoelectric with frequency of absorbed photons. It is clear from the figure that for ejection electrons, the frequency (V) of light used must greater than threshold frequency V: However, kinetic energy remains constant with chang intensity.

All these observations could not be explained the basis of classical laws of physics. According to classical laws of physics, the energy content of beam light depends upon the brightness of the light. In other words, number of electrons ejected and the kinetic energy associated with them should depend on brightness of light. However, as has been discussed above though the number of electrons ejected depends upon the brightness of light, the kinetic energy of the electrons does not. For example, red light (v = 4.3-48 x1014 s-1) of any brightness may shine on a potassium surface for hours but it does not eject photoelectrons But yellow light (v = 5.1 – 5.2 x 1014 s-1) of even a very weak brightness ejects photoelectrons. This is because the threshold frequency (v) for potassium metal is 5.0 x 1014 s- and light of frequency more than v; (i.e., yellow light and not red light) can cause photoelectric effect Planck's Quantum Theory of Radiation.  

All electromagnetic radiation are forms of energy Our body can feel the heat of sunlight and that is why we avoid the sunlight during the summer and welcome it during the winter. The electromagnetic wave theory believed in continuous generation of radiant energy, i.e., the energy may be emitted or absorbed in any value from infinitely small to infinitely large. However, this theory could not explain the experimental results of black body radiation and photoelectric effect. At the beginning of the twentieth century Max Planck in 1901, gave a new revolutionary theory known as quantum theory of radiation. The main features of Planck's quantum theory are:

(i)Radiant energy is not emitted or absorbed continuously but discontinuously in the form of small packets of energy called quanta. Each such quantum in associated with a definite amount of energy. 

(i i) The amount of energy associated with a quantum of radiation is proportional to the frequency of light, 

E= n h v or E = h v        ... (i) 

 where the proportionality constant, h, is a universal constant known as Planck's constant. the value of 6.626 x 10 J S or 3.99 x 10 K J sec mo l". This relation was found to be valid for all types of electromagnetic radiation.

(i i) The total amount of energy emitted or absorbed by a body will be some whole number multiple of quantum, i.e.,

E = n h v = E n h c / lambda .. (i i)

where n is an integer such as 1, 2, 3, .. This means that a body can emit or absorb energy equal to h v, 2 h v, 3 h v... or any other integral multiple of h v but cannot emit or absorb energy equal to 1.6 h v, 3.2 h v or any other fractional value of h v.

The relation [equations (i) and (i i)] give the relation between energy of the radiation and its wavelength or frequency. It shows that the higher the frequency (or the lower the wavelength), the more energetic are the corresponding photons. For example, a photon of violet light will be of more energy than that of red light because former is of larger frequency. The concept of energy packets of light supports the corpuscular character. It also explained the distribution of intense in radiation from a black body as a function of frequency at different temperatures.