lecture 4 -light waves , particles, phtotns

What is black body radiation

An ideal perfect radiator that absorbs all incident radiation and emits only thermal radiation

Describe how black body emission changes with temperature

Higher temp → higher intensity at all wavelengths and peak intensity shifts to shorter wavelengths

State weins displacement law

λPeak = b/ T, with b = 2898 um

Example of black body temp measurement in astronomy

Using the color/peak wavelength of a star

What clsssical law described blackbody radiation and where did it fail

Fails at high frequencies / short wavelength

What did plank propose to fix the ultraviolet catastrophe

Oscillators can only exchange energy in discrete quanta . E=nhv

How does planks work imply photons

Radiation is admitted/ absorbed in packets of energy . E=hv

Relationship between photon energy and wavelength

E= hv = hc/λ

Momentum of a photon

P = h/ λ

Key observation of the photoelectric effect

Light of frequency below a threshold v0 ejects no electrons regardless of intensity

Equation for photo electron kinetic energy

Eknin- hv - work

How does light intensity affect electron emission

Higher intensity = more photons = more electrons emitted but not higher electron energy

What change will cause ejection if no electrons are emitted

Decrease the wavelength/ increase phtoton energy

Show the duality relationship for lights momentum and energy

E = hv = hc/ λ, p = h/ λ

What did einsteins special relativity show for a massless particle

E= pc when m=0

Shining light on a metal surface can

Cause electrons to be ejected

If the wavelength is too long,

No electrons are emitted no matter how intense (bright ) the light is

For more intense light

More electrons are ejected

For shorter wavelength

Electrons are ejected with higher kninetic energy

Only light of a sufficiently short wavelength ( sufficiently high energy )

Causes photo electron emission

Light intensity and particle nature

Photon : indivisible ‘ particle ‘ of light

Light intensity is the number of phtotons in the beam

E photon = h v