Chapter 2 - Electromagnetic Radiation Principles Flashcards
Interactions that EMR from Sun has before becoming data
Radiated by atomic particles (the sun)
Travels through vacuum of space at speed of light
Interacts with Earth’s atmosphere
Interacts with Earth’s surface
Interacts with the atmosphere again
Reaches remote sensor, interacts with various optics, filters, film emulsions, or detectors
Energy
Ability to do work, often transferred from one body/ place to another 3 ways of transfer: - conduction - convection - radiation
Conduction
Transfer of kinetic E by collision (one hot object transfers heat to another hot object this way
Convection
Transfer of E by physically moving the bodies (hot air rising)
Radiation
Transfer of E by emission of electromagnetic waves
Electromagnetic Radiation (Structure, creation)
Perpendicular fluctuation fields - one electric and one magnetic, created when an electron jumps down an energy level after excitation
Frequency
Number of wavelengths that pass a point per unit time
wavelength-free equation
c = 𝛌v
Blackbody
Theoretical construct that absorbs & radiates energy at maximum possible rate per unit area at each wavelength 𝛌, for given T
Total emitted energy from a blackbody
M𝛌 = σT^4
where σ = Stephan Boltzmann constant
T = abs temp in K
Dominant wavelength emitted from blackbody
𝛌max = k/T where k = 2898 µm K T = abs temp in K
Excitation
Electrons can move “up” an energy level if an energy input threshold is reached (otherwise no work is accepted)
Potential energy is increased, after ~10^-8 seconds, electron falls back down & gives off radiation - possibly to an intermediate rung first
Quantum leap/jump
The movement of an electron to a different rung (electron changes states discretely without ever being in between). If an electron makes stops at intermediate rungs, the energy of small jumps sum to the energy of what the large jump would be
Energy of a specific wavelength
Q = hv = hc/𝛌
wavelength is inversely proportional to 𝛌
Photoelectric effect
Matter can be heated to such high T that electrons that normally move in non-radiating orbits break free & the atoms are ionized. When a free electron drops in to fill the vacant energy level, then radiation is given off in a continuous spectrum.
Radiant energy
Capacity of radiation within a spectral band to do work
Refraction
“Bending” of light as it passes through material of different optical density. Angle is predicted by snell’s law.
Index of refraction
Measure of optical density of a substance - ratio of the speed of light in a vacuum, c, to the speed in the substance, cn
n = c/cn
Snell’s law
n1sinθ1 = n2sinθ2
Relates the angle of traveling ray with indexes of refraction of multiple materials
Types of scattering
Rayleigh, Mie, Nonselective
Rayleigh scattering
(Molecular scattering) - diameter of matter are many times smaller than the wavelength of incident EMR - impossible to predict direction of photon reemission
Rayleigh scattering cross-section algorithm
Describes the amount of Rayleigh scattering
τm = 8π^3(n^2-1)^2/(3N^2λ^4)
n = index of refraction
N = # of air molecules per unit volume
Amt is inversely proportional to λ^4, so longer wavelengths are effected much less.
Explain why sunset is red, sky is blue
Sky is blue b/c Rayleigh scattering affects blue light more than red -> most red light makes it to the ground, while some blue light is “trapped” in atmosphere
Sunset is red/ orange because there’s more atmosphere to pass through and most blue light is scattered completely before it can reach your eye
Mie scattering
(Non-molecular/ aerosol scattering) - main scattering agent for visible light -> dust & smoke & other particles
amt of scattering > Rayleigh scattering
Pollution increases mie scattering (& makes sunsets beautiful)
