Lecture 6 Light and matter Harris Ch.18.

Light striking a sample can be 1. reflected 2. transmitted 3. absorbed 4. scattered

Reflected light Incident light Emergent light P0P0 P1P1 Absorbed light Scattered light

Refractive index

Air Glass Longer wavelength Shorter wavelength   Snell’s law

Reflected light P r Incident lightEmergent light P0P0

Incident lightEmergent light Transparent plate P0P0 P1P1

Air 1 Quarz 1.5 Water 1.33 T=0.85 R=0.04 Air-quarz Water-quarz R=0.003 T=0.92 emptyWith water

Absorbing plate

Incident light Emergent light Absorbing plate P0P0 P1P1 0 l dx dP P dP= N   P N=C  Volume Volume= 1  dx N = C  dx

Incident light Emergent light Absorbing plate P0P0 P1P1 0 l dx dP P

Incident light Emergent light Absorbing plate P0P0 P1P1 0 l dx dP P absorbance Sensitivity is the same for any power (P)

Beer’s law Bugert, Lambert and Beer A concentration Straight line!

When photon is absorbed, the system jumps to the next energy level

Luminescence A photon is absorbed and then re-emitted at different wavelength

Incident light P0P0 Emergent light P1P1 Re-emitted light Absorbed light

If absorbance is small:

Incident light P0P0 Emergent light P1P1 Re-emitted light Absorbed light sensetivity is proportional to the source power

Limits to Beer’s Law Polychromatic light (several wavelength)

Limits to Beer’s Law Incident light P0P0 Emergent light P1P1 Stray light