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 Page 859 Section Review (Page 861) Chapter Review 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