METR 415/715

Monday 2/4/2013

EM Radiation can be considered as particles as well as waves

• Small “packets” ot EM radiation that behave like particles

• Called “photons”

• E= Energy carried by photon• h= Planck’s constant = 6.626 X 10-34 J s• ν = Frequency

Photon’s are “indivisible”

• As text states on page 31 “very low intensity light deposits discrete packets of energy on a surface in a manner analogous to the occasional random splashes of fat raindrops on your windshield”

• If monochromatic radiation of wavelegth λ deposits F Watts per unit area on a surface, then this corresponds to F/hν photons per unit area per unit time

Flux (F)

• Flux is the rate at which radiation is incident on, or passes through a flat surface

• Expressed in units of Wm-2 for “broadband” radiation (radiation of many wavelengths)

• The flat surface may be oriented in any direction

• Monochromatic flux (Fλ) has units of Wm-2 per unit wavelength, Wm-2μm-1

Broadband Flux

• Encompasses a range of wavelengths. Defined as

Intensity

• Tells you in detail both the strength and the direction of various sources contributing to the incident flux on a surface

• Spherical coordinates are useful in describing intensity

• Coordinates are Φ (azimuth) and θ (elevation)

Solid angle

• Measured in steradians (sr)• Steradian – (ratio of area in a solid angle

compared to the area of a sphere of unit radius) 4π r2, where r=1

• In differential form, a steradian would be:

Reflection refraction scattering

Refraction of light in a raindrop

Scattering

Reflectivity of water

Absorption

• “Imaginary” part of the index of refraction• Gives you information on how much

absorption will take place