JNTU B.Tech I APPLIED PHYSICS Previous Question Paper (November/December - 2020 - R18)

A black body is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. When in thermal equilibrium, it emits radiation known as black body radiation, with a spectrum that depends solely on its temperature.

Planck resolved the ultraviolet catastrophe by proposing that energy is emitted or absorbed in discrete packets called quanta, where the energy of a quantum is given by E = hν. Using statistical mechanics, Planck derived the spectral energy density distribution:

In this expression, h is Planck's constant, c is the speed of light, k is the Boltzmann constant, T is the absolute temperature, and λ is the wavelength. This law successfully describes the intensity of radiation emitted by a black body across all wavelengths, matching experimental observations perfectly.

The Compton Effect is the scattering of a photon by a charged particle, usually an electron, resulting in a decrease in the photon's energy and an increase in its wavelength. This phenomenon provides direct evidence for the particle nature of electromagnetic radiation.

When an incident X-ray photon of wavelength λ strikes a stationary electron, it is scattered at an angle θ. The scattered photon has a longer wavelength λ'. The shift in wavelength, Δλ = λ' - λ, is given by the formula:

Here, h is Planck's constant, m₀ is the rest mass of the electron, c is the speed of light, and θ is the scattering angle. The term (h / m₀c) is known as the Compton wavelength of the electron. The shift is independent of the incident wavelength and depends only on the scattering angle, confirming that photons behave like particles colliding with electrons during the interaction.