Particles of Light?

Mark LesmeisterPearland ISD Physics

Select graphics from Serway and Faughn, Holt Physics, Holt Rinehart and Winston, 2002.

PARTICLES OF LIGHT

Is Light a Particle or a Wave

Some light phenomena can be explained with either a particle model or a wave model.

Light does many things that only waves do. Most are the result of

interference

The Young Double Slit Experiment

In 1801, Thomas Young investigated the nature of light using a double slit experiment.

If light consisted of particles, there would just be two bright spots behind the slits.

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The Young Double Slit Experiment

However, Young found that the light formed a pattern of bright and dark lines, indicating that light is a wave.

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The Young Double Slit Experiment

Studying the pattern that light waves make allows us to determine the frequency of the waves. The frequency is the number of wave peaks

that move past a point in one second.

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: Wikip

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Infrared Video: Bats and Wind Turbines

Videos from Boston University Center for Ecology and Conservation Biology, http://www.bu.edu/cecb/wind/video/

Blackbody Radiation All objects emit

electromagnetic radiation. The frequencies of this

radiation and the amount emitted depend on the temperature of the object.

“Room temperature” objects emit infrared frequencies.

Very hot objects (like glowing coals) emit visible radiation.

The universe, which has a temperature of 3 Kelvin, is awash with microwave radiation.

© Holt. Rinehart and Winston

Quantization of Energy In the 19th

century, experiments were conducted to measure the blackbody radiation emitted at each wavelength of light.

Source: Wikipedia Commons

Quantization of Energy The wave-

theory of EM radiation did not agree with the experimental results for blackbody radiation.

Source: Wikipedia Commons

Quantization of Energy In 1900, Max Planck

developed a new theory that assumed energy isn’t emitted in any amount, but only in packets, called “quanta”.

This “quantum” theory agreed with the blackbody experiments.

© Holt. Rinehart and Winston

Planck’s Equation

Planck’s equation relates the energy of a quantum of light with the frequency of light.

Energy = Planck’s constant x frequency

In atoms, energy is measured in electron volts (eV) 1 eV = 1.60 x 10-19 J h=4.14 x 10-15 eV-s

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The Photoelectric Effect When light strikes a

metal surface, it will give off electrons.

Graphic from Serway and Faughn, “Holt Physics” © Holt, Rinehart and Winston

The Photoelectric Effect If light is a wave, then a

light beam that has greater intensity (greater energy) should cause the electrons given off to have more energy.

A more intense light beam causes more electrons to be emitted, but they all have the same energy. Graphic from Serway and Faughn,

“Holt Physics” © Holt, Rinehart and Winston

The Photoelectric Effect Einstein explained the

photoelectric effect by assuming that light was made of particles.

A more intense light beam has more photons, but each carries the same energy.

Thus, more intense light would produce more electrons, with the same energy.

Graphic from Serway and Faughn, “Holt Physics” © Holt, Rinehart and Winston

Quantum Theory of Light and the Photoelectric Effect When a photon strikes a metal plate, it

transfers energy to the electrons in the plate.

If an electron acquires enough energy, it can be ejected from the plate. The energy necessary to eject an electron is

called the “work function” of the metal. The kinetic energy of the electrons will be

the energy of the photon minus the work function. ionWork Funct hfKE

Photoelectric Effect: Practice

A certain beam of light has photons with 5 eV of energy. When this light strikes a metal plate, electrons with 3 eV of kinetic energy are released. What is the work function of the metal?

Answer: 2 eV

Wave Particle Duality In some experiments,

such as blackbody radiation and the photoelectric effect, light acts like a particle (called a photon).

In others, like double slit interference and thin film interference, light acts like a wave.

Light never appears as both in the same experiment.

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