Dual Natureof electromagnetic radiation

DAY 2
29.4.20

Good Morning Boys,
Today we will discuss about planks quantum theory , photoelectric effect, emmision and absorption spectra.
Learning Outcomes,
Students will be able to

Define photoelectric effect.
understand concept of emmission and absorption.

Particle nature of Electromagnetic radiations :There were two important phenomenon that couldn’t be explained by considering Light with wave character:

The phenomenon is:

Black body radiation
Photoelectric effect

Black body:

An ideal body, which emits and absorbs all frequencies, is called a black body.
The radiation emitted by such a body is called black body radiation.

Photoelectric effect:

The phenomenon of ejection of electrons from the surface of metal when light of suitable frequency strikes it is called photoelectric effect. The ejected electrons are called photoelectrons.
Number of electron ejected is directly proportional to intensity (or brightness) of light.
There is characteristic minimum frequency (ν₀ threshold frequency) below which photoelectric effect is not observed.
If ν > ν₀ then electrons comes out with kinetic energy which increases with increase in frequency of light.
Kinetic energy of ejected electrons is given by-

h ν = h ν₀+ ½(m_eV²)

Particle nature of electromagnetic radiation (Planck's Quantum Theory):

The radiant energy is emitted or absorbed not continuously but discontinuously in the form of small discrete packets of energy called ‘quantum’. In case of light, the quantum of energy is called a ‘photon’
The energy of each quantum is directly proportional to the frequency of the radiation, i.e. E α υ (nu)

Or E = hυ (nu)

Where h = Planck’s constant = 6.626 × 10^-27 Js

Energy is always emitted or absorbed as integral multiple of this quantum, i.e,

E = nhυ (nu)

Where n =1,2,3,4,.....

Dual behavior of electromagnetic radiation:

The light possesses both particle and wave like properties.
whenever radiation interacts with matter, it displays particle like properties (Black body radiation and photoelectric effect).
When it propagates, it shows wave like properties (interference and diffraction).

Spectrum:

When a white light is passed through a prism, it splits into a series of coloured bands known as spectrum.

Types of spectrum:

It is of two types:

(a) Continuous and line spectrum: The spectrum which consists of all the wavelengths is called continuous spectrum.

(b) Line spectrum: A spectrum in which only specific wavelengths are present is known as a line spectrum.

Electromagnetic spectrum:

It is a continuous spectrum.
It consists of a range of electromagnetic radiations arranged in the order of increasing wavelengths or decreasing frequencies.

Spectrum can also be classified as follows:

Emission spectrum: The spectrum of radiation emitted by a substance that has absorbed energy is called an emission spectrum.
Absorption spectrum: It is the spectrum of radiation transmitted through a substance, showing dark lines or bands due to absorption at specific wavelengths.