Ex

  Physics (NTSE/Olympiad)  

3. Light Reflection

The Nature of Light

The Nature of Light
Light is a form of energy (optical energy) which helps us in seeing objects by its presence.
(A) THEORIES ABOUT NATURE OF LIGHT :
(i) Particle nature of light (Newton's corpuscular theory) :
According to Newton light travels in space with a great speed as a stream of very small particles called corpuscles.
This theory was failed to explain interference of light and diffraction of light. So wave theory of light was discovered.
(ii) Wave nature of light :
Light waves are electromagnetic waves so there is no need of medium for the propagation of these waves. They can travel in vacuum also. The speed of these waves in air or in vacuum is maximum i.e., 3 × 108 m/s हैं।
Photoelectric effect was not explained with the help of wave theory, so Plank gave a new theory which was known as quantum theory of light.
(iii) Quantum theory of light :
When light falls on the surface of metals like caesium, potassium etc., electrons are given out. These electrons are called 'photo-electrons' and phenomenon is called 'photo-electric effect'.
This was explained by Einstein. According to plank light consisted of packets or quanta's of energy called photons. The rest mass of photon is zero. Each quanta carries energy E = hv
h → Planck's constant = 6.6 × 10–34 J-s.
v → Frequency of light
Some phenomenons like interference of light, diffraction of light are explained with the help of wave theory but wave theory was failed to explain the photo electric effect of light. It was explained with the help of quantum theory. So, light has dual nature.
(i) Wave Nature (ii) Particle nature

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