Atoms & LightEmission &

absorption of radiant energy depends on electrons in atoms

Recall: Ground and excited states – moving e between energy levels (valence e)These transitions

are quantizedExcitations are

short lived ~ ns

Loss of energy either light of thermal agitation

For light E = h fThere are specific

transitions, and therefore frequencies

These are resonant frequencies – an atom efficiently absorbs and emits energy

Scattering and AbsorptionThe process of

absorbing a photon and emitting another photon

Incident photons on a substance can either: be absorbed (and “lost”)or be scattered

If the photons energy (frequency) matches one of the excited states of the atom, the atom absorbs it

The energy is transferred via collisions to random K (rather than reemitted)

The photon vanishes, its energy converted to heat

Most things have the colors they do because of absorption (dissipative)

Nonresonant ScatteringSuppose photons of

light have frequencies too small to cause e to move to a higher energy level

The EM field can cause e cloud into oscillation at the same frequency

The atom remains in ground state

This causes e to accelerate, creating photons

This scattered photon moves in same direction as original photon

Causing material to be transparent

Light passing through space has nothing to interact with and moves in a straight line at cThe beam cannot be

seen from the side

When atom and

molecules are very far apart, the scattered photons do not interfere in a substantial wayCommonly referred to

as Rayleigh scatteringSince air molecules

have electron resonance in UV, the closer the visible light is to UV, the greater the scattering (violet)

Why sky is blueOther colors interact

less often and pass through

RefractionA change in wave

direction when passing to different medium

Incident wave: some energy reflected & some transmittedTransmitted light

direction is different, it is “bent”

The wave speed is material dependent

As wave fronts pass

into material, the speed changes, therefore the λ changes

v1 = f λ1 & v2 = f λ2

Index of refraction, nRatio of speed of light in

vacuum per speed of light in medium

n = c / vLeads to Snell’s Law

n1 sin θ1 = n2 sin θ2

Where n1 = leaving

medium & n2 = entering medium

When n2 > n1 refracted ray is bent towards normal line

Each ray is reversibleThe > the change in

index, the > change in direction

Light along normal is not deflected

ApplicationsPencils appear to be

“bent” in waterObjects in water

appear closer to surface than they actually are

Dispersion & the RainbowIndex of refraction is frequency dependent

Glass is opaque to ultraviolet – the atoms resonate in those frequencies

The closer EM radiation is to resonance, the slower it travels

Therefore, the higher f, higher nViolet “bends” more than redWhite light spreads out by refraction - dispersion

Dispersion of white light

due to drops of rain causes rainbows

As sunlight enters drop:it refracted (& dispersed)internally reflected off

the back of dropRefracted again(&

dispersed) as the light leaves the drop

Light must come from behind viewerWhy most often viewed

at sunset

Rainbow is top half

of a cone between 40 & 42 degrees

Double rainbows

have two internal reflectionsThe order of colors

is reversedRange of 50 to 53

degreesFainter than primary

due to larger area encompassed and wider band of bow