Wave-Particle Duality

Theories of light at the end of the 17th century

-Isaac Newton

-Huygens

Isaac Newton theory of light

Made up of tiny particles called corpuscles

Huygens theory of light

Light as a wave

Why did Isaac Newton’s theory fail

Corpuscles couldn’t explain diffraction and interference

Could corpuscles reflect and refract

Yes

Where did Huygens’ theory fail

Light as purely a wave wouldn’t explain the photoelectric effect

What was Einstein’s theory of light

A beam of light is a series of particle-like photons

How do photons interact with electrons

They transfer energy on a one-to-one basis

What was De Broglie’s thesis

If ‘wave-like’ light showed particle properties, ‘particles’ like electrons should be expected to show wave-like qualities

de Broglie equation

λ = h/p

What is p in the de Broglie equation

Momentum

How did Broglie link waves and particles

The de Broglie equation links wavelength (a wave property) with momentum (a particle property)

How did de Broglie describe particles

A particle is a probability wave

What does it mean that a particle is a ‘probability wave’

The likelihood of finding a particle at a point is directly proportional to the square of the amplitude of the wave at that point

Who diffracted electrons

Clinton Davisson and Lester Germer

What confirmed that electrons have wave-like properties

Electron diffraction

How were electrons diffracted

Accelerated electrons in a vacuum tube interact with the spaces between carbon atoms in polycrystalline graphite

How does the wavelength of a wave relate to its diffraction pattern

The greater the wavelength, the more spread the diffraction pattern

Explain how a smaller accelerating voltage in electron diffraction effect the diffraction pattern

Less acceleration gives slower electrons, giving widely spaced rings

Explain how electron diffraction proves the de Broglie equation

Electrons with higher speed create a more squashed diffraction pattern, whilst electrons with a slower speed create a more spread out pattern, so the higher the momentum, the shorter the wavelength

How does the wavelength of electrons relate to EM waves

The wavelength for electrons accelerated in a vacuum tube is about the same size as X-rays

What determines if an object can diffract a particle

If the particle is the same size as the object’s de Broglie wavelength

What structures are electrons with a wavelength of 10^-10 likely to be diffracted by

Polycrystalline structures

What wavelength of electrons are likely to be diffracted by polycrystalline structures

10^-10

Is a longer or shorter wavelength needed for less diffraction effects?

Shorter wavelength

Explain the wavelength of electron microscopes

A shorter wavelength is needed to resolve tiny detail

Why do electron microscopes have a higher resolution than light microscopes

Light blurs out more detail than electron waves