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?