ELECTROMAGNETIC RADIATION AND THE ELECTROMAGNETIC SPECTRUM Electromagnetic Radiation (EMR) THE ELECTROMAGNETIC SPECTRUM Electromagnetic Waves A wave is characterized by: Wavelength (λ - lambda) is the distance in meters between successive crests of the waves Frequency (nu - ν) is the number of oscillations completed per second - measured in hertz (Hz) Usually measured in meters (m), nanometers (nm, 10-9 meters), or micrometers (μm, 10-6 meters) or centimeters (cm, 10-2 meters). Gamma rays Wavelengths are less than ten trillionths of a meter Extreme penetrating capabilities Generated by radioactive atoms and in nuclear explosions, and are used in many medical applications Images of our universe taken in gamma rays have yielded important information on the life and death of stars, and other violent processes in the universe X-rays Wavelength range is from about ten billionths of a meter to about 10 trillionths of a meter High energy waves which have great penetrating power and are used extensively in medical applications X-ray images of our Sun can yield important clues to solar flares and other changes on our Sun that can affect space weather 1
Ultraviolet rays Wavelengths range from 400 billionths of a meter to about 10 billionths of a meter UV waves can burn skin however, most are blocked by ozone in the Earth's upper atmosphere. UV wavelengths are used extensively in astronomical observatories. Some remote sensing observations of the Earth are also concerned with the measurement of ozone. Visible rays Wavelengths between 400 and 700 billionths of a meter (400 to 700 nanometers or 0.4 0.7 microns). This is the spectrum that we see Best used for the identification of different objects by their visible colors VISIBLE LIGHT Infrared rays Violet: 0.4-0.446 μm Blue: 0.446-0.500 μm Green: 0.500-0.578 μm Yellow: 0.578-0.592 μm Orange: 0.592-0.620 μm Wavelengths extend from the visible region to about one millimeter Includes, but is not limited to thermal radiation Measured using electronic detectors and has applications in medicine and finding thermal pollution Infrared images yield important information on crop health and can help us see forest fires even when enveloped in smoke Red: 0.620-0.7 μm Microwave Radiation Radio Waves Wavelengths range from approximately one millimeter to thirty centimeters In a microwave oven, the radio waves generated are tuned to frequencies that can be absorbed by the food. The food absorbs the energy and gets warmer. The dish holding the food doesn't absorb a significant amount of energy and stays much cooler Wavelengths range from less than a centimeter to tens or even hundreds of meters Used to transmit radio and television signals Radio waves can also be used to create images. The reflected waves can be used to form an image of the ground in complete darkness or through clouds 2
Blue wavelengths In remote sensing we isolate and measure certain wavelengths of energy Green wavelengths Red wavelengths Landsat ETM+ data of Kent, Ohio, acquired 5 th September, 2000 Landsat ETM+ visible bands combined Visible Spectrum Landsat Band 1 Landsat Band 2 Landsat Band 3 Blue 0.45-0.52 μm Green 0.52-0.6 μm Red 0.63-0.69 μm Landsat ETM+ data of Kent, Ohio, acquired 5 th September, 2000 Landsat ETM+ infrared bands combined Infrared Spectrum Band 4 Band 5 Band 7 Near infrared 0.76 0.9 μm Mid infrared 1.55 1.75 μm Mid infrared 2.08 2.35 μm 3
SUMMARY ENERGY INTERACTIONS BETWEEN THE EARTH AND ATMOSPHERE Electromagnetic Interactions Electromagnetic Interactions Transmission Reflection Absorption Scattering TRANSMISSION Atmospheric Transmission The movement of light through a material Transmission is wavelength dependent The proportional amount of incident radiation passing through a surface 4
REFLECTANCE Specular reflectance THE PROCESS OF RADIATION BOUNCING OFF AN OBJECT The angle at which radiation hits an object will be the same angle at which it is reflected Smooth surfaces act as specular reflectors, rough surfaces act as diffuse reflectors Diffuse reflectance Near-perfect diffuse REFRACTION THE BENDING OF LIGHT WHEN IT PASSES FROM ONE MEDIUM TO ANOTHER Refraction is dependent on: angle with vertical distance through the medium that radiation is traveling air density Near-perfect specular 5
Light refraction through Earth s atmosphere SCATTERING THE UNPREDICTABLE REFLECTION OF RADIATION BY ATMOSPHERIC PARTICLES Most scattering occurs high in the atmosphere and gives makes the sky seem blue During sunsets, or in the evenings the sun is low on the horizon - the sun s rays have to travel through a longer slice of the atmosphere This efficiently and almost totally scatters blue light, letting the longer wave red light through, hence red skies. Effects of atmospheric scattering Types of scattering Rayleigh Occurs when the diameter of an atmospheric particle is significantly smaller than the EMR wavelength Mie Particles approximate the size of the EMR wavelength Non-selective Occurs when particles (e.g. water vapor) are significantly larger that the EMR wavelength being scattered Rayleigh and Mie Non-selective ABSORPTION Atmospheric Transmission and absorption THE ABSORPTION AND CONVERSION OF ELECTROMAGNETIC RADIATION TO OTHER FORMS OF ENERGY Occurs in the atmosphere and on the ground Tends to occur in absorption bands (range of wavelengths in electromagnetic spectrum where radiation is absorbed by substances) e.g. electromagnetic energy is efficiently absorbed by water in all wavelengths of the EMS except within the blue wavelength region between 0.4 and 0.5 um 6
Spectral curves of common Earth materials Everything under the lines represents reflection Everything above the lines represents absorption 7