1. The nature of light

2. Velocity of light (ms-1) = wavelength(m) x frequency(Hz or s-1)
What is light
Around 1680 Isacc Newton had suggested that light was made up of a stream of particles.
Others suggested that light was made up of waves, without moving matter.
The dispute seemed ended when light was shown to have the exact properties of wave motion: Reflection, Refraction, Diffraction and Interference (Young 1801)
The formula which relates the wavelength and frequency of the light is
Velocity of light (ms-1) = wavelength(m) x frequency(Hz or s-1)
c=λf

3. The number of waves passing a point in 1 second is the frequency of the waveλ

4. Reflection

11. A little problem
Nobody could explain the problem of “black body radiation.
The electromagnetic radiation obeyed the known wave laws when the body was at low temperature.
At high temperature the equations went haywire and did not explain the shape of the curve

12. Changing the temperature
Think of a black cooker hot plate.
It does not remain black but glows when heated
Getting Hotter

13. This part of the curve is easy to explain

14. The problem with heated “black” bodies
in 1901 Max Planck explained the whole curve using the idea that light was emitted and absorbed as a steam of particles.
Each particle of light (or photon) has an energy which is related only to the frequency f of the light by
E =hf
Where E is the energy of a single photon f is its frequency and h is a constant known as the Planck constant