Electromagnetic Wave theory – given by Clark Maxwell. Main points are

1. energy is emitted from any source continuously in the form of radiations (or waves) called Radiant energy.
2. Radiations consist of electric and magnetic field Oscillating perpendicular to each other and both perpendicular to the direction of propagation of the radiation.
3. The radiations possess wave character + travel with the velocity of low. So these radiations are called electromagnetic radiation or electromagnetic waves.
4. These waves do not require any material medium for propagation (example- rays from the sun reaches us through space.)

Characteristics of waves (Type of disturbance wave length (λ ) distance between any two consecutive crusts or trough)

Crest is the highest point on a wave

Trough is the lowest point on a wave

Frequency (ν) is the number of waves passing through a point

 in are second.

Velocity (c) is the linear distance travelled by the wave in are second.

Amplitude (a) is the height of the crest or the depth of the trough .

Wave number () is the number of waves present in 1 cm length.

It is equal to the reciprocal of wavelength

Relation between velocity, wavelength and Frequency of a wave 

or    c = ν λ

                ν = frequency

                C = velocity of propagated wave

                λ = wavelength

 Electromagnetic spectrum – when electromagnetic radiations are arranged in order of their increasing wave length or decreasing frequencies, the complete spectrum is called electromagnetic spectrum.

Different types of electromagnetic radiations differ only in their wave length.

Limitations of Electromagnetic Wave Theory –

1. Black body radiation
2. Photo electric effect

These phenomena could be exploited if electromagnetic waves are supposed to have particle nature

1. Black body radiation – When a substance being heated as a black body , the radiation emitted is called black body radiation. Black body waves which is not only a perfect absorber but also a perfect radiator of energy i.e. which can emit and  absorb all frequencies of radiations.

When iron bar is heated first it becomes red then yellow and  finally glow with white and  then blue light.

According to electromagnetic wave theory energy is emitted or absorbed continuously. Thus

radiation is directly proportional to intensity

and independent to its frequency , so when body heated it gives same color , although intensity vary as heating  continued

 The change of color shows that on heating frequency is increasing , (read possess lower frequency while blue color shows higher frequency) 

At a given temp intensity increases to a max value+ then decrease with rise of wave length.

Explanation of black body radiation on particle nature

When solid substance is heated, atoms of a substance are oscillates and  emit radiations. As heating is continued, more and more energy is being absorbed by atoms and  emits radiations of higher and higher frequency.

Red light – has less ν, while yellow more ν. so on heating body first becomes red then yellow .