Light as Particle (part 1)Light as Particle (part 1)

Newton separated white light into Newton separated white light into ROYGBV using prismROYGBV using prism

He believed that light was particle He believed that light was particle and that refraction was somehow and that refraction was somehow due to the gravitational force due to the gravitational force between light and the prismbetween light and the prism

Light as waveLight as wave

Young’s double-slit interference Young’s double-slit interference experimentexperiment

Explanation of thin-film Explanation of thin-film interference interference

Description of light as Description of light as electromagnetic wave (J. C. electromagnetic wave (J. C. Maxwell)Maxwell)

Diffraction Diffraction

The longer the wavelength (The longer the wavelength () ) compared to the gap or obstacle, the compared to the gap or obstacle, the greater the diffractiongreater the diffraction

The less diffraction, the more “particle-The less diffraction, the more “particle-like” the wave; the more diffraction, like” the wave; the more diffraction, the more “wave-like”the more “wave-like”

X-rays and gamma-rays are relatively X-rays and gamma-rays are relatively particle-like compared to longer particle-like compared to longer em em waveswaves

What happens to the color of What happens to the color of a bulb filament as it gets a bulb filament as it gets

hotter?hotter?

Max Planck Max Planck realized that realized that

not all not all frequencies frequencies

are are represented represented equally… equally…

Cooler objects Cooler objects have more have more ROY… as ROY… as

objects get objects get hotter, add hotter, add

more BVmore BV

Planck’s “quantum” Planck’s “quantum” – Einstein’s “photon”– Einstein’s “photon”

Planck proposed that light is composed Planck proposed that light is composed of particles of em energy – the light is of particles of em energy – the light is quantized into bundles called “quanta”quantized into bundles called “quanta”

One bundle would be called a quantum One bundle would be called a quantum and its energy would be given byand its energy would be given by

hfE h is now called Planck’s constant and is equal to

6.63 10-34 Js

(4.14 10-15 eVs)Einstein called the quantum Einstein called the quantum a a photonphoton

Planck’s explanation of Planck’s explanation of blackbody spectrumblackbody spectrum

Red photons are relatively small “buckets” of Red photons are relatively small “buckets” of energy compared to violet photons (use energy compared to violet photons (use E=hfE=hf))

As objects get hot, infrared photons are As objects get hot, infrared photons are produced first, then red, then orange… An produced first, then red, then orange… An object is white hot when it is producing object is white hot when it is producing photons of all ROYGBV (peaked in the YG)photons of all ROYGBV (peaked in the YG)

Violet and UV photons cannot be produced Violet and UV photons cannot be produced until the object is hot enough to fill those until the object is hot enough to fill those bucketsbuckets

Photoelectric EffectPhotoelectric Effect When light is projected on a metal, certain When light is projected on a metal, certain

wavelengths allowed electrons to escape, wavelengths allowed electrons to escape, while others did not. The brightness while others did not. The brightness (amplitude of E-field) did not matter, even (amplitude of E-field) did not matter, even though classical wave theory said it though classical wave theory said it should.should.

If energy delivered in packets, then only If energy delivered in packets, then only those packets with enough energy could those packets with enough energy could free electrons – increasing brightness of free electrons – increasing brightness of the wrong colors wouldn’t work the wrong colors wouldn’t work

See Photoelectric Effect slidesSee Photoelectric Effect slides

Energy and momentum of Energy and momentum of photonphoton

Even though a photon has no mass, it Even though a photon has no mass, it carries momentumcarries momentum

h

p

hchfpcE

When short light (why not long ?) interacts with free charges (not bound to atoms), momentum and energy must be conserved. By giving some of its energy to the charge, the photon’s frequency ________, while its ________.

Momentum is conserved separately in x- and -y dimensions.