QUANTIZATION OF LIGHT

 Understanding Quantization

Until the late 19th century, light was considered to exhibit only wave-like properties as are evident in phenomena such as interference, diffraction, refraction etc. The wave theory of light was the basis of the entire classical electromagnetism. However, one peculiar experiment called the photoelectric effect was not understood by scientists using the wave theory of light.

There are a large number of free electrons that are present on the surface of a metal. When light of sufficiently low wavelength hits the surface of the metal, these free electrons are ejected from the metal. This is the photoelectric effect. It was discovered that if the wavelength is too high (or frequency too low) then these electrons are not ejected, even if the beam of light has very high intensity. This was very counter-intuitive because according to the wave theory of light (classical electromagnetism), the energy of emitted electrons should be proportional to the intensity of the incident light. To explain this, the photon theory or the particle theory of light was formulated by Einstein in 1905 for which he later won the Nobel prize. He theorized that light is made of packets of energy called photons. 

A photon can be thought of as a particle of light.

When light is considered to be a wave, it will have wave-like properties such as the wavelength, λ. Each particle of light, or a photon, will have the energy E=hc/λ, where c is the speed of light (≈3×108 m/s) and h is known as the Planck's constant (≈6.6×10−34 J.s).

For any given wavelength, λ, the energy of light is an integer times (hc\λ, ). Since the energy of light can be varied only in the quantum of hc/λ, the photon theory of light is called the quantum theory of light.

A quantum (smallest packet of energy) of light is called a photon.

When light is incident on a metal surface, the photons of light collide with the free electrons and the electrons are ejected from the metal surface. This phenomenon is the photoelectric effect. The electrons will only get emitted from the metallic surface if the wavelength is low enough. For low frequency or higher wavelength, the electrons may not be emitted, irrespective of the intensity of the beam of light.  Strong beams of light toward the red end of the spectrum might not be able to eject electrons at all, while weak beams of light toward the violet end of the spectrum would produce a higher and higher number of electrons.

QUANTIZATION OF ENERGY LEVELS IN HYDROGE

Consider an electric bulb which gives off white light when heated. If this light is made to pass through a prism, it will result in spreading of components of visible light (such as that in a rainbow)  to result in a continuous spectrum. 

If hydrogen gas is enclosed in a sealed tube and is heated to high temperatures, electromagnetic radiation is emitted. Electromagnetic radiation is basically a combination of visible as well as invisible light. When we pass this radiation through a prism we would see the different wavelengths emitted by hydrogen. Instead of a continuous spectrum, we would see only specific wavelengths, resulting in a discrete spectrum. No matter where this experiment is conducted, hydrogen spectrum always has only these specific wavelengths. 

n 1913, Niels Bohr provided an explanation for observing such a discrete spectrum of hydrogen. He proposed that electrons revolve around the nucleus of an atom in very specific fixed orbits.  An electron revolving in an orbit would have fixed energy. If the electron jumps from a higher energy to a lower energy, it would do so by releasing a photon of energy hc/λ. If the electron wants to jump from lower orbit to a higher orbit, it would absorb the energy hc/λ. This would result in the observation of discrete spectrum as electrons can only absorb or release specific quantum of energy.  

Conclusion

Furthermore, the study of quantized light has contributed to our comprehension of the universe at the most fundamental levels. It has become an integral part of the broader exploration into the nature of matter and energy, shedding light on the mysterious and counterintuitive aspects of quantum phenomena.

As our knowledge of quantized light continues to evolve, it is likely to lead to further breakthroughs in both theoretical and applied sciences. The quantization of light stands as a testament to the power of human curiosity and ingenuity in unraveling the mysteries of the natural world, providing a solid foundation for ongoing exploration and discovery.