Particle Properties of Light

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 1014 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.