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##### The nature of light

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**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**The number of waves passing a point in 1 second is the**frequency of the wave λ**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**Getting Hotter**Changing the temperature • Think of a black cooker hot plate. • It does not remain black but glows when heated**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