Light particles and matter waves?

Light particles and matter waves? Physics 12

Clips of the day: • Minutephysics on wave particle duality…. • http://www.youtube.com/watch?v=Q_h4IoPJXZw • http://www.youtube.com/watch?v=_riIY-v2Ym8

And the story continues…. • After the verification of the photoelectric effect, Physicists started to ask more questions about the extent to which photons resemble particles of matter…. • Lead to questions about momentum and collisions of massless particles…would the law of conservation of momentum hold? How can you determine the momentum of a massless particle?

Compton 1923: • Used highly energetic photons from x-rays (high frequency waves) • He collided x-ray photons with and electrons • Discovered the Compton Effect • http://www.kcvs.ca/site/projects/physics.html

Compton Effect: • When a high energy x-ray photon collides with a “free” electron, it gives some of its energy to the electron and lower energy photon scatters off the electron • Electron starts at rest • Following the collision, the photon is scattered at an angle as is the electron • Since the photon has less energy than the incident photon, it must have a lower frequency

Compton Effect: • Much like the photoelectric effect, Compton also used the particle theory to explain his results • Since we know that in a collision momentum must be conserved and the scattered electron has a momentum, the incident photon must also have momentum • This leads to the equation: • How can a 'particle' with no mass have momentum?*Flashback* → E = mc2 • So X-rays with energy E have a mass-equivalence of E/c2

Momentum of a photon:

example • Calculate the momentum of a photon of light that has a frequency of 5.09x1014Hz.

Implications: • These collisions obeyed all the laws for collision between two masses • Compton's analysis supported the particle theory of light • Photons have a discrete energy and value for momentum • Compton won the Nobel Prize in 1927 for his work

What? • We’ve seen light acting as a particle….. • But matter acting like a wave???????? • De Broglie (pronounced 'de-Broy) proposed that electrons, too, have a wave nature and a wavelength and that all material objects have a wave nature.

Matter Waves: • de Broglie determined the wavelength of matter waves using the following equation:

Matter Waves: • Determine the wavelength of the following: • You travelling in a car (1400kg) at 100km/h • An electron (9.11X10-31kg) orbiting the nucleus at .9c • Are either of these wavelengths significant? • Planck's constant (h) has such an extremely small • value that the wavelength associated with any • ordinary object is far too small to be experimentally • detected.

Verifying Matter Waves: • In order to demonstrate that matter (like electrons) have wavelike behaviour, it must be possible to show that they will have interference patterns • von Laue and Bragg developed an X-ray diffraction technique that displays these patterns

Wave-Particle Duality: • Within 30 years of Planck’s presentation of quantization, the particle nature of light and wave-like behaviour of subatomic particles had been accepted • Matter has a dual nature but only subatomic particles have a small enough mass and thus a large enough wavelength to exhibit their wavelength properties

Try it : • Page 857 • 1-5 • Page 859 • 6-11 • Mass alpha particle= 6.64x10-27kg • Section Review (Page 861) • 2, 4, 6 • Chapter Review (Page 862) • 16, 21, 22, 24

Light particles and matter waves?