(Chemistry Ch-2) 3. Wave Nature of EM Radiation, Particle Nature of EM Radiation, Dual Behaviour of EM Radiation

Wave Nature of Electromagnetic Radiations

• Radiations which are associated with electric and magnetic field are called electromagnetic radiations.

• Properties of Electromagnetic Radiation
  • The oscillating electric and magnetic field produced by the oscillating charged particles are perpendicular to each other and also perpendicular to the direction of propagation of wave.

• Electromagnetic waves do not require any medium. They can move in vacuum.
• There are many types of electromagnetic waves which differ from one another in wavelength (or frequency). These electromagnetic radiations constitute electromagnetic spectrum.

• Different kinds of units are used for the representation of electromagnetic waves.

• Electromagnetic radiations are characterized by the properties − frequency (v) and wave length (λ).

• Frequency − Number of waves that pass a given point in one second

The S.I. unit of frequency (ν) is hertz (Hz, s⁻¹).

• The S.I. unit of wavelength is metre (m).

• Electromagnetic waves travel at the speed of 3.0 × 10⁸ m/s, which is the speed of light (denoted by c).
• The relationship between frequency (ν), wavelength (λ), and velocity of light (c) is given by,

c = vλ

• Wave number − It is defined as the number of wavelengths per unit length or it is the reciprocal of wavelength. The SI unit of wavelength is cm⁻¹.

Examples (i) The Red FM radio station of India Today Network is broadcasted on a frequency range of 93.5 MHz. The wavelength (λ) of the electromagnetic radiation emitted by the transmitter can be calculated as The radiation of wavelength 3.2 m belongs to radio wave region of the spectrum. (ii) Wave numberof red light having wave length 750 mm can be calculated as λ = 750 mm = 750 × 10−9 m Wave number=  ∴

Particle Nature of Electromagnetic Radiation

Wave nature of electromagnetic radiation failed to explain many phenomena such as black body radiation and photoelectric effect.

Black Body Radiation

• Solids on heating emit radiations over a wide range of wavelengths.
• Radiations emitted shift from lower frequency to a higher frequency as the temperature increases.
• An ideal body which absorbs and emits all radiation is called a black body and the radiation emitted by it is called black body radiation.
• An ideal black body is defined as a perfect absorber and a perfect emitter of radiations.
• The distribution of frequency of the emitted radiation from a black body depends upon temperature.
• The intensity of emitted radiation at a given temperature increases with the decrease in wavelength. It attains a maximum value at a given wavelength and then starts decreasing with further decrease of wavelength.

• These experimental results cannot be explained on the basis of wave theory of light.

Photoelectric Effect

• When certain metals such as potassium, rubidium, caesium, etc. are exposed to a beam light, the electrons are ejected from the surface of the metals as shown in the figure below.

• The phenomenon is called Photoelectric effect.

• Results of the Experiment

• The electrons are ejected from the surface of the metal as soon as the beam strikes the metal surface.
• The number of electrons ejected from the metal surface is directly proportional to the intensity of light.

• For each metal, there is a certain minimum frequency of light below which photoelectric effect is not observed. This minimum frequency is called threshold frequency.

When, the ejected electrons come out with certain kinetic energy. The kinetic energy of the emitted electrons is directly proportional to the frequency of incident radiation and is independent of incident radiation.

• The results of photoelectric effect could not be explained by using law of classical physics.

Planck’s Quantum Theory of Radiation

Main features of Planck’s quantum theory of radiation are as follows:

• Radiant energy is not emitted or absorbed in continuous manner, but discontinuously in the form of small packets of energy called quanta.
• Each quantum of energy is associated with definite amount of energy.
• The amount of energy (E) associated with quantum of radiation is directly proportional to frequency of light (ν).

i.e., E ∝ ν

Or, E = hν

‘h’ is known as Planck’s constant and has the value 6.626 × 10⁻³⁴ Js.

Example Energy of one mole of photons of radiation whose frequency is 3.0 × 1015 s−1 can be calculated as: Energy (E) of one photon is given by, E = hν h = 6.6.26 × 10−34 Js ν = 3.0 × 1015 s−1 ∴ E = (6.626 × 10−34 Js) × (3.0 × 1015 s−1) E = 1.99 × 10−18 J Energy of one mole of photons

Explanation of Photoelectric Effect Using Quantum Theory

• According to Einstein,

Energy of the striking photon = Binding energy + Kinetic energy of ejected electron

Energy of the striking photon = hν

Binding energy = hν₀ (also called work function or threshold energy)

Kinetic energy of ejected electron =

• If ν < ν₀, then no electrons will be ejected, no matter how high the intensity is.

• If ν > ν₀, then the excess energy is imported to the ejected electron as kinetic energy. As the frequency of radiation increases, the kinetic energy of the electron will increase.

• As the intensity increases, more electrons will be ejected, but their kinetic energy does not change.

Dual Behaviour of Electromagnetic Radiations

• Some phenomena (reflection, refraction, diffraction) were explained using wave nature of electromagnetic radiation and some phenomena (photoelectric effect and black body radiation) were explained by using particle nature of radiation.
• This suggests that microscopic particles exhibit wave-particle duality.