Blackbody Radiation
‘All objects emit radiation whose total intensity is proportional to their temperature. This is called thermal radiation; a blackbody is one that emits thermal radiation ONLY.
The energy of atomic oscillation within atoms cannot have an arbitrary value; it is related to frequency:
The constant ‘h’ is now called Planck’s constant
Planck’s proposal was that the energy of an oscillation had to be an integral multiple of hf. This is called quantization of energy
Einstein took Planck’s ideas a step further and suggested that light must be emitted in small energy packets
These tiny packets, or particles, are called photons
Experimental results:
Max Planck (1858–1947) is credited with starting the quantum revolution
As a solid is heated to higher and higher temperatures, it begins to glow.
Initially, it appears red and then becomes white when the temperature increases.
The changes in the colours and the corresponding spectra do not depend on the composition of the solid.
In 1900 Planck developed a mathematical equation to explain the whole curve, by using a radical hypothesis. Planck saw that he could obtain agreement between theory and experiment by hypothesizing that the energies of the oscillating atoms in the heated solid were multiples of a small quantity of energy; in other words, energy is not continuous.
Albert Einstein later pointed out that the inevitable conclusion of Planck’s hypothesis is that the light emitted by a hot solid is also quantized — it comes in “bursts,” not a continuous stream of energy
One little burst or packet of energy is known as a quantum of energy
A logical interpretation is that as the temperature is increased, the proportion of each larger quantum becomes greater. The colour of a heated object is due to a complex combination of the number and kind of quanta
In the mid-19th century, James Maxwell produced a brilliant theory explaining the known properties of light, electricity, and magnetism. He proposed that light is an electromagnetic wave composed of electric and magnetic fields that can exert forces on charged particles. This electromagnetic-wave theory, known as the classical theory of light, eventually became widely accepted when new experiments supported this view. Most scientists thought this was the end of the debate about the nature of light — light is (definitely) an electromagnetic wave consisting of a continuous series of wavelengths
The photoelectric effect, discovered by Heinrich Hertz in 1887, involves the interaction of electromagnetic radiation with certain metals. Initially, it challenged classical theories, which suggested that the brightness of light determined the energy of liberated electrons. However, experiments showed that the frequency of light was the crucial factor. Albert Einstein later explained the photoelectric effect by proposing that light consists of discrete energy packets called photons. He suggested that when a photon collides with an electron in a metal, it transfers energy, enabling the electron to escape. This quantum explanation, based on the concept of minimum energy absorption per photon, resolved the inconsistencies in classical theory and led to Einstein being awarded the Nobel Prize in 1905. The analogy of an electron in an atom being akin to a marble trapped in a bowl helps illustrate why the energy of liberated electrons is independent of light intensity.
Quantum Theory received a huge boost in popularity for explaining this and other laboratory effects at the atomic and subatomic levels. Quantum theory is heralded as one of the major scientific achievements of the 20th century. There were results from many scientific experiments that could not be explained by classical chemistry and physics, but these experimental results could be explained by quantum theory. Two of the experiments leading to quantum theory are summarized below, but there were many more that could only be explained using quantum theory.