LIGHT

PART

ICLE

S

AND MAT

TER

WAV

ES? PH

YS

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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….o After the verification of the

photoelectric effect, Physicists started to ask more questions about the extent to which photons resemble particles of matter….

o 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:o Used highly energetic photons

from x-rays (high frequency waves)

o He collided x-ray photons with and electrons

o Discovered the Compton Effect

o http://www.kcvs.ca/site/projects/physics.html

COMPTON EFFECT:o 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

o Electron starts at resto Following the collision, the

photon is scattered at an angle as is the electron

o Since the photon has less energy than the incident photon, it must have a lower frequency

COMPTON EFFECT:o Much like the photoelectric effect, Compton also

used the particle theory to explain his resultso 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

o This leads to the equation:

o How can a 'particle' with no mass have momentum?*Flashback* → E = mc2

o So X-rays with energy E have a mass-equivalence of E/c2

c

hfhp

MOMENTUM OF A PHOTON:

EXAMPLE• Calculate the momentum of a photon of light

that has a frequency of 5.09x1014Hz.

IMPLICATIONS:o These collisions obeyed all the laws for

collision between two masseso Compton's analysis supported the particle

theory of lighto Photons have a discrete energy and value for

momentumo Compton won the Nobel Prize in 1927 for his

work

WHAT?o We’ve seen light acting as a

particle…..o But matter acting like a

wave????????

o 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:o de Broglie determined the wavelength of

matter waves using the following equation:

mv

h

MATTER WAVES:o Determine the wavelength of the following:

a.You travelling in a car (1400kg) at 100km/hb.An electron (9.11X10-31kg) orbiting the nucleus

at .9c

o 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:o In order to demonstrate that matter (like

electrons) have wavelike behaviour, it must be possible to show that they will have interference patterns

o von Laue and Bragg developed an X-ray diffraction technique that displays these patterns

WAVE-PARTICLE DUALITY:o Within 30 years of Planck’s presentation of

quantization, the particle nature of light and wave-like behaviour of subatomic particles had been accepted

o 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 :o Page 857

1-5o Page 859

6-11Mass alpha particle= 6.64x10-27kg

o Section Review (Page 861)

2, 4, 6o Chapter Review

(Page 862)16, 21, 22, 24