Electromagnetic Radiation Principles

Wave Model of EMR

Relationship between wavelength and frequency is inverse

• i.e., gamma rays are the shortest and they have a high frequency

• Wavelength that are shorter have the most amount of energy while the longest have to lowest energy

EMR was conceptualized as an electromagnetic (EM) Wave by James Clerk Maxwell in 1860s

Wavelength

of EMR depends on length of time that a charged particle is accelerated

• Mean distance between maximums(or minimums) or a roughly periodic pattern, normally measured in micrometers(micrometers or nanometers)

Frequency

or EMR depends on number of accelerations per second(typically periodic)

• Number of wavelength that pass a point unit time.

• a wave that sends one crest every second(1 complete cycle) is said to have a frequency of one cycle per second or one hertz, abbreviated 1 Hz

What produces EM energy?

• all objects above absolute zero(-273°C or 0K)

• The sun produces a continuous spectrum of EMR ranging from a very short, extremely high frequency gamma waves to long, very low frequency radio waves

• We can think of the Sun as a 6000 K blackbody

Stefan-Boltzmann Law

• The total emitted radiation (M_wavelength) from a blackbody is proportional to the fourth power of its absolute temperature.

  • σ is the Stefan Boltzmann constant:

    • 5.6697 × 10^-8 W m^-1 K^-4

• it determines how much radiation is emitted based on its temperature

  • energy emmited is a function of temperatures

  • part of the wave model

Wien’s Displacement Law

• the peak wavelength of radiation emitted by a blackbody is inversely proportional to its absolute temperature

  •  meaning that as an object gets hotter, the wavelength of its peak emitted light becomes shorter.

• can determine the dominant wavelength(Where is the spectrum the radiation is most intense, what wavelength is the object emitting most?)

  • Wavelength_max

  • ***where “k” is a constant equaling 2898 μm K and T is the absolute temperature in Kelvin

 i.e., if the sun approximates a 6000K blackbody, its dominant wavelength is :

0.483 μm = 2898 μm K/ 6000K

What is the Suns Dominant Wavelength?

0.48μm(blue-green light)

What is the Earths Dominant Wavelength?

9.66μm

EMR and Spectral Resolution

• In remote sensing, spectral resolution is the ability of a sensor to detect distinct ranges of wavelengths within the EMR

• Sensors divide the electromagnetic spectrum into several "bands",  "channels" or “regions”. Each channel captures information within a specific wavelength range. 

• Spectral signatures: Different materials (like healthy vegetation, soil, or water) absorb and reflect EMR in unique ways at different wavelengths. 

Max Planck(1900)

• describe spectral distribution of radiation

• proposed that energy is emitted or absorbed in discrete amounts, or quanta

• Plancks Radiation Law: E = hv

  • E= energy of Photon

  • h = Planck constant

  • v = frequency

Albertas Input on Plancks Law

• proposed light consists of energy packets or particles, each carrying energy(E=hv)

  • the particles were named photons 

  • each reacts with matter like a particle, but also have wave like properties

Quantum Theory of EMR

• posits that EMR energy is not continuous but comes in discrete packets called photons

  • the relationship between the frequency of radiation is expressed by wave theory and the quantum theory is : E= hv

• where E is the energy of a quantum measured in Joules h is the Planck constant (6.626 × 10^-34 J·s), and v is the frequency of the radiation

• Energy of a quantum is inversely proportional to its wavelength(i.e., the longer the wavelength involved the lowers ints energy content

  • ** it is more difficult to detect longer wavelengths compared to shorter visible wavelengths

Creation of Light From Atomic Particles

• photon of light is emitted when an electron drops from a higher energy state to a lower energy state

  • Add energy = electrons will jump up and go up levels for 10 ^-8? Seconds then it will go back down to the next available level and release the enregy it had

Photo Electric Effect

• is physical mechanism insides optical detectors to convert photons to electric signals

  • photons hit a detector material

  • if their energy is above a threshold the free electrons

  • the freed electrons create an electrical signal that is crowded to digital

The Particle Model

• Behaviour of EM radiation as photon

• E=hv

• used to show hoe photons interact with matter and how sensors detect them

• energy quantization detection

EMR and the Atmosphere

• EMR goes through space in at the speed of light in a vacuum

• The atmosphere affects the speed, wavelength, intensity spectral distribution, and/or direction of EMR

  • refreation, scattering, and absorption happens before things hit the ground; when it does hit the ground more interactions happen again

Refraction

• occurs when EMR encounters substances of different density, i.e., air to water; space to atmosphere

  • Bending or light when it passes from one medium to another of different density

Index of Refraction, n

• measure of the optical density of a substance

  • speed of light in a  vacuum and how it changes in a substance

  • n is always >1

  • n of atmosphere = 1.0002926

  • n of water = 1.33

Snells Law

• for a given frequency of light, the product of the index of refraction and the sine of the angle between the ray and  a line normal to the interface is constant

• If we know the insect of refraction we can find out how much it is?

• in RS understanding light behaviour can help  explain distortion in imagery or discrepancy in measurement for distance and elevation(optical path calculations)

• most atmospheric correction involves snells law

When is Refraction Most Important in RS

• near the horizon at oblique angles there is noticeable bending

  • the more oblique the more the problem grows

• high res. or long range systems

• microwave RS affecting signal delays

Refraction Impact on RS

• affects RS the least compared to the other interactions