Short Answer

Classical physics failed to explain blackbody radiation because it assumed that energy is continuous and can take any value. Using this idea, classical theories predicted that the energy emitted at short wavelengths should become extremely large. This incorrect prediction was known as the “ultraviolet catastrophe.”

However, experiments showed that blackbodies emit limited energy at short wavelengths. Classical physics could not match the observed radiation curve. Only after Max Planck introduced the idea of quantised energy, called quanta, could the correct behaviour of blackbody radiation be explained.

Detailed Explanation :

Failure of classical physics to explain blackbody radiation

Classical physics was very successful in explaining everyday events such as motion, heat, sound, electricity, and magnetism. However, it failed completely in explaining the behaviour of blackbody radiation. A blackbody is an ideal object that absorbs all radiation that falls on it and emits radiation depending only on its temperature. When scientists studied the radiation spectrum of blackbodies, they found a specific pattern. At low frequencies, the intensity increases, reaches a maximum, and then decreases at higher frequencies.

Classical theories tried to explain this curve using the laws of thermodynamics and electromagnetism. One method used was based on the equipartition theorem, which said that energy is shared equally among all possible modes of vibration. According to this idea, every frequency of radiation inside a blackbody cavity should have the same average energy. The problem began when this idea was applied to high frequencies.

Using classical physics, scientists calculated that the energy emitted by a blackbody should increase without limit as the frequency becomes higher. This meant that at ultraviolet wavelengths, the radiation intensity should be extremely large. But experiments showed that blackbodies emit very little radiation at short wavelengths. The huge difference between theory and experiment proved that classical physics was not correct in this area.

This incorrect prediction was named the ultraviolet catastrophe, because it suggested infinite energy output in the ultraviolet region. No real object behaves like this, and such predictions were physically impossible. Scientists realised that classical laws were incomplete and could not describe the true nature of radiation.

Where classical physics failed (Subheading)

Classical physics failed mainly because it assumed that energy is continuous. According to classical ideas, energy can be divided into infinitely small parts. For radiation inside a blackbody, classical theory predicted that every frequency mode has the same amount of energy. Since there are an infinite number of high-frequency modes, the total energy should also be infinite. This was unreasonable and clearly did not match experiments.

The Rayleigh-Jeans law, which was based on classical physics, described the radiation intensity at long wavelengths correctly. But as the wavelength became shorter, the law predicted extremely high values for intensity, which were not seen in real measurements. This mismatch showed that classical mechanics and electromagnetism were not sufficient to describe radiation.

Another problem was that classical theories could not explain why the intensity of radiation peaks at a certain wavelength that depends on temperature. Experiments showed that hotter objects emit more radiation at shorter wavelengths. Classical physics did not predict this behaviour accurately.

Furthermore, classical physics did not consider the microscopic behaviour of atoms inside the blackbody. It treated radiation as waves only, without including any particle-like nature. This led to wrong assumptions and incorrect results.

How quantum theory solved the problem (Subheading)

The failure of classical physics encouraged scientists to look for a better explanation. Max Planck proposed a revolutionary idea in 1900. He suggested that energy is not continuous but is emitted in small packets called quanta. The energy of each quantum is directly proportional to the frequency of radiation. This means higher frequency radiation has higher-energy quanta.

Planck showed that if the energy of radiation is quantised, the predicted intensity curve matches the experimental results perfectly. His formula explained the behaviour at all wavelengths, including the ultraviolet region. At low frequencies, quantum theory matched classical predictions. At high frequencies, it prevented the intensity from becoming infinite.

This idea solved the ultraviolet catastrophe and provided the first correct explanation of blackbody radiation. Planck’s theory also marked the beginning of quantum physics, which later explained many other phenomena such as the photoelectric effect, atomic structure, and the behaviour of electrons.

Quantum theory showed that classical physics fails at microscopic levels because it cannot describe the discrete nature of energy. Classical physics assumed continuous behaviour, but nature actually works in tiny steps at small scales. This was the major reason behind the failure of classical physics in explaining blackbody radiation.

Planck’s idea also led to new concepts such as photons, wave particle duality, and quantised energy levels in atoms. These ideas became the foundation of modern physics and helped explain many natural phenomena that classical physics could not handle.

Conclusion

Classical physics failed to explain blackbody radiation because it assumed energy was continuous and predicted infinite radiation at high frequencies. This did not match experimental results. Max Planck solved the problem by introducing the idea of quantised energy, which accurately explained the blackbody radiation curve. The failure of classical physics led to the birth of quantum theory, which changed our understanding of light, radiation, and atomic behaviour.