attention: exam next friday (one week)!! * exam covers the reading chapters 1-6 * sample questions...

ATTENTION:

EXAM next FRIDAY (one week)!! * Exam covers the reading Chapters 1-6* Sample questions on the web.

HW – due Wednesday midnight

Atom-photon interactions

hc

fhE Photons:

Atom Light

Energy exchangedquantized

Most commonly by 1 photon

“photons”

Atom Light

Absorption

Atom Light

Emission

Absorption

Ground state

Atoms in ground state initially

Absorption

Ground state

hf /19.10

Have to have a photon present of the right frequency (n =1 → 2)

One photon absorbed

Absorption

Ground state

hf /74.12One photon absorbed

Have to have a photon present of the right frequency (n =1 → 4)

Another possibility …

White light H atoms

slit

Absorption spectrum

Dark lines appear in spectrum at frequencies (energies) where photons

can be absorbed by the atom

Emission

Ground state

Atom must be in excited state

Emission

Ground state

hf /55.2One photon emitted

Have to have an atom present in the n = 4 excited state (n = 4 → 2)

Emission

Ground state

hf /74.12One photon emitted

Have to have an atom present in the n = 4 excited state (n = 4 → 1)

Another possibility …

H atoms“Hot”

slit

emission spectrumHave to Jazz the atoms up somehow into their excited states

Bright spectral lines appear only at frequencies (energies) that the atoms can

emit

(Kirchhoff’s Law’s 2 and 3)

A diffuse gas emits or absorbs light only at discrete frequencies specific to the substance of the gas

Goofy-glasses demo: emission and absorption spectra

Matter in generalMore degrees of freedom more energy levels

Simple atomic Complicated molecule

lines ‘bands’

“white light”

A white object doesn’tabsorb anything

Reflective colors

WALL

“white light”

A red object ‘looks’Red because it absorbsThe blue and greens

Reflective colors

WALL

Why green leaves turn red

Chlorophyll dies/reabsorbed in autumn leaving longer-lived Carotenoids

If an object can emit light at a certain wavelength,then it can also absorb light at that same wavelength!

Thermal radiation from continuous sources

Recall …

Cavity radiation

T

Cavity radiation

T

Cavity radiation

T

Cavity radiation

T

The ‘color’ and brightness of the light you see in the cavity depends on the relative intensities of the different wavelengths of light present.

These intensities depend on the temperature of the walls!

For cavity radiation, we can work out the distribution of intensities of all wavelengths, given the temperature T

Colored objects ….

yellow body red body

Colored objects ….

yellow body red body

Absorbs: Red, Blue, GreenReflects: yellow

Absorbs: Blue & GreenReflects: Red

An ideal black body

Black body

Absorbs light at ALL wavelengths (not just visible!)

So, it must be able to emit light at any wavelength too!

It turns out that …..

T

It turns out that …..

T

Intensity of light of a given ,l emitted from a unit area of the blackbody

It turns out that …..

T

Intensity of light of a given ,l emitted from a unit area of the blackbody

IDEAL Thermal-radiation spectra:

Most large objects are very nearly likeideal black bodies, with an associated

thermal emission spectrum.

Cool object(most intensity in IR)

The higher an object’s temperature ….

The more intensely it emits electromagnetic radiation, and …

The shorter the wavelength of the light is at which it’s emitting most strongly!

Wien’s displacement law:

T

0029.0max

in °Kin m

Stefan-Boltzmann law

4TP

in °K

Total power radiated(at all wavelengths) per unit area of the emitting body.

in Watts/m2

5.67×10-8 W/(m2 °K4)

Doppler Effect

the water-wave analogy

drooling duck

drooling duck

drooling duck

drooling duck

drooling duck

drooling duck

drooling duck

drooling duck

lrest

drooling duck

drooling duck

drooling duck

drooling duck

drooling duck

drooling duck

drooling duck

lforward

drooling duck

lforward

lrest

source

l in front

l in back

Doppler effect with light: similar behaviorbut for very different reasons!

source movingaway

source movingtoward

RED BLUE

REDBLUE

Doppler shift measures Radial velocity

Radial velocity

Radial velocity

Transverse velocity

Radial velocity

Radial velocity

Transverse velocity

0

c

VR