In 1900, Max Planck introduced a revolutionary idea that marked the beginning of quantum physics. His theory explained a major scientific problem known as blackbody radiation.

???? The Problem: Blackbody Radiation

Classical physics predicted that as the frequency of radiation increased, the energy emitted by a hot object would increase infinitely — a contradiction known as the ultraviolet catastrophe.

But experiments showed:

• Energy increases to a peak,

• Then decreases at higher frequencies.

Classical physics failed to explain this.

???? Planck’s Bold Idea

Planck proposed that:

Energy is not emitted or absorbed continuously, but in small discrete packets called quanta.

???? Main Postulates of Planck’s Quantum Theory

1?? Energy is Quantized

Energy is emitted or absorbed in fixed packets called quanta.

2?? Energy of Each Quantum

The energy of a quantum is proportional to the frequency of radiation:

E=hfE = h fE=hf

Where:

• E = Energy of one quantum

• f = Frequency of radiation

• h = Planck’s constant

h=6.626×10−34 J\cdotpsh = 6.626 \times 10^{-34} \text{ J·s}h=6.626×10−34 J\cdotps

3?? Energy is Not Continuous

An oscillator (like atoms in a heated object) can only have energies:

E=nhfE = n h fE=nhf

Where:

• n = 1, 2, 3, ...

So energy exists in discrete levels — not infinitely divisible.

???? Why This Was Revolutionary

Before Planck:

• Energy was believed to be continuous (classical view).

After Planck:

• Energy behaves in discrete units at microscopic scales.

This idea later led to:

• Albert Einstein’s explanation of the photoelectric effect

• Niels Bohr’s atomic model

• The development of modern quantum mechanics

???? Simple Analogy

Think of energy like money:

• Classical physics: You can pay ?1.237489 continuously.

• Quantum theory: You must pay in fixed coins (?1, ?2, ?5).

Energy comes in “coins” called quanta.

???? Significance

Planck’s Quantum Theory:

• Solved the blackbody radiation problem

• Introduced the concept of quantization

• Laid the foundation for quantum mechanics

• Changed physics forever

It marked the transition from classical physics to modern physics.