METR 415/715

Wednesday February 6 2013

Imaginary part of index of refraction

• When ni is non-zero, absorption of an EM wave occurs as it passes through a medium.

• ni is sometimes referred to as the absorption index

• ni depends on the wavelength

• Figure 4.1 (page 77) shows values of ni for water and ice – small for visible and higher for UV and IR

Intensity as a function of travel through a medium

• Eq 4.2, p.77• Βa= absorption coefficient

Intensity as a function of travel through a medium

• Eq 4.3, p.77

Transmittance t• The fraction of radiation intensity that survives

the trip over the distance x.• The fraction that was NOT transmitted must

have been absorbed and thereofre converted to another form of energy such as heat

Beer’s Law

• The intensity of a beam of monochromatic radiation falls off exponentially as it traverses a uniform medium

• The rate at which it falls off is proportional to βa

Penetration Depth

• Defined as the value of x for which t(x) = e-1 or approximately 37%• Check Figure 4.2 on page 79 for water and ice• How does that influence color of pools water? Light penetration into the ocean?