Objectives To discuss the particle nature of light
Particle Properties There are certain properties of light that cannot be explained using the wave model. Blackbody radiation – emission of light from hot objects example: electric stove burner – wavelength of radiation depends on the temperature – Max Planck (1900) – energy can either be released or absorbed by atoms in discrete “chunks.”
Since energy can only be released in specific amounts, we say that the allowed energies are quantized. Quantum = smallest amount of energy that can be emitted or absorbed as electromagnetic radiation Planck developed an equation to determine the energy of a single quantum. E = hν – Planck’s constant (h) = 6.63 x 10 -34 J s.
Photoelectric effect studied by Albert Einstein the emission of electrons from a metal when light shines on the metal Only certain frequencies of light caused the effect.
Another way to think of a beam of light through space is as a stream of tiny packets of energy. He called these packets of light photons. That is, electromagnetic radiation can sometimes have properties that are characteristic of particles. Different wavelengths of electromagnetic radiation produce photons of different amounts of energy. The energy of the photons of electromagnetic radiation is directly related to its frequency and inversely related to its wavelength.
Example: – Red light has a wavelength of 700 nm and blue light has a wavelength 480 nm. Photons of red light have a smaller frequency than blue light. Therefore, photons of red light have less energy than photons of blue light.
Sample Problem Determine the energy of a photon whose frequency is 5.09 x 10 14 Hz.
Emission Spectra Another phenomenon that the wave model of light could not explain was the light released from electrically excited gas atoms. When an atom has its lowest possible energy, the atom is in its ground state. When an electric current is passed through a gas, the atoms of the gas absorb energy. This causes the atom to enter an excited state.
In order for the atoms to return to their ground state, they release the absorbed energy in the form of electromagnetic radiation. When light from a hydrogen discharge tube passes through a prism, discrete lines are produced. The wavelengths of light emitted by an element separate into discrete lines to give the emission spectrum (bright-line spectrum) of the element.
Summary Is light a wave or does it consist of particles? Light possess both wavelike and particle-like characteristics, and depending on the situation, will behave more like a wave or more like a particle. Since light can consist as a wave or a stream of particles, we say that light has wave-particle duality.