Download The Compton Effect

Light has momentum, too!
The Compton Effect
 Discovered in 1923 by Arthur Compton
 Pointed x-rays at metal atoms
 X-rays are high frequency, high energy photons
 X-rays knocked electrons from the metal
 A low energy photon emitted after the collision
•Compton treated this like an elastic collision
•The incoming photon (x-ray) strikes the electron
•The electron emits a low energy photon
•Electron and photon scatter in opposite directions
•From the path and speed of the electron, Compton could
calculate the momentum of the scattered photon:
p = h/λ
Like the energy of a photon, the momentum depends only on its
frequency (or wavelength.
What is happening?
 The incoming x-ray is a very high energy photon
 The x-ray has more energy than is needed to eject the
electron from the metal.
 The ejected electron gets rid of the extra energy in the
form of a lower energy photon.
What is learned from this?
 We know that electrons have momentum because they
have mass
 Photons have momentum too, even though they have
no mass!
 Photons seem to behave much like particles when they
interact (collide?) with electrons.
The deBroglie Wavelength
 Louis deBroglie turned quantum physics upside down
when he asked: if light waves can behave like particles,
can particles behave like light waves?
 Taking Compton’s formula:
p = h/λ
The wavelength of a particle is given by:
λ = h/p
 Remember, momentum is given by p = mv
What does this mean?
 h = 6.6 x 10-34J s
 h is an extremely small number, so λ is very small
unless m or v is very small.
 For everyday objects like baseballs and ham
sandwiches, the wavelength is unnoticeable.
 For electrons and protons, the wavelength is
noticeable, especially if they have a low speed.
Ok, now what does that mean?
 If a small particle has a wavelength, then it can do
everything that waves do, such as:
 Diffract (bend around corners)
 Interfere with other small particles
Applications of Matter-Waves
 Optical microscopes are limited by the wavelength of
the light they use. The best microscopes possible can
only see down to the micrometer.
 An electron has a wavelength, but it is small, much
smaller than the wavelength of light.
 A microscope that uses electrons instead of light will
be able to see much better detail. This leads to the
invention of the electron microscope.
Electron Microscope
Ant’s head, magnified 2,000 times
Summary
 Light has momentum p = h/λ
 When a photon collides with an electron, the electron
scatters and emits a low energy photon. This is a
perfectly elastic collision, as if the photon was a
particle
 Matter can have wavelength λ = h/p
 The smaller and slower, the greater the wavelength
 Slow moving electrons can act like photons in this
sense, being able to diffract and interfere.