Q.Explain the quantum concept of light.

Step 1. Max Planck, in 1900, proposed that matter is composed of a large number of oscillating atomic particles, each vibrating with its own characteristic frequency and each emitting or absorbing electromagnetic radiation only at that same frequency.

Step 2. Planck's key new idea was that such an oscillator's energy is not free to take any value; it is quantized, restricted to a discrete ladder $E_n=nh\nu$ for $n=1,2,3,\ldots$, where $h$ is Planck's constant.

Step 3. Equivalently, the oscillators emit or absorb energy only in small indivisible packets, or quanta, each of energy $E=h\nu$ -- a genuine break from the classical assumption that energy varies continuously.

Step 4. In 1905, Einstein extended this quantum idea from the atomic oscillators to light itself: rather than a continuous wave spreading energy over its wavefront, light of frequency $\nu$ is a stream of discrete energy quanta called photons, each carrying $E=h\nu$ and momentum $p=h\nu/c$. …

Explain Planck's oscillator-quantization hypothesis first, then Einstein's extension o …

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