Electromagnetic Spectrum
The electromagnetic spectrum is a family of energy that have both wave and particle properties, which are directly associated with electric and magnetic fields. Visible light is a member of the electromagnetic spectrum as is radio waves, TV signals, and microwaves. Members of this family have similar properties like light, but there are numerous differences. For example visible light will not pass through the wall of a building, but TV signals will. In physics at times the electromagnetic spectrum properties are viewed as a set of transverse waves[1] and at other times as a particle to explain the physical observations. A combinational particle/wave model[2] is not used; but instead for certain contexts only particle models can be used to explain the observations and for others it must be understood as a wave. One model or the other is used and not combined.
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When light is considered as a wave it is a three dimensional transverse wave, but in general is represented in only two dimension, for simplicity of understanding. A transverse wave has an amplitude and a frequency, They also have electrical fields that are perpendicular to the magnetic fields and moves perpendicular to the directions of vibration. The electric fields may be in the X, Z plane and magnetic fields in the Y, Z plane and the wave moving in the Z direction. The amplitude of the wave is the height above or below the central axis, in this discussion the Z direction. The frequency is the number of vibrations of the wave in a given unit of time and is usually measured in the Hertz, cycles per second. Since the frequency of waves is a very large number you see Megahertz and Kilohertz when discussing the frequency of radio waves and usually scientific notation for other members of the spectrum. The wavelength of the wave is related to the frequency of the wave since these waves travel at the speed of light when in a vacuum.
The higher the frequency the shorter the wavelength and the opposite is also true. The wavelength of waves might be from several meters to tiny fraction of a meter such as nanometers.
The frequency, wavelength and speed are interrelated by an equation; frequency times wavelength equals the speed of light. For example, red light and blue have a different wavelength, thus a different frequency, but the same velocity. The shorter the wavelength the more energetic the wave packet.
At other times in physics, we need to consider this family as a group of particles, called photons. A photon is a particle of light which is charge less, massless, with no physical dimensions that travels at the velocity of light. These particles behave much like elastic spheres. For example, when pool ball strikes the side of the table it reflects (bounces) off the table at the same angle in which it struck the bumper as measured to a normal (a perpendicular line to the side of the table); a photon does the same thing such as how light reflects off of a mirror. Different photons have different amounts of energy, thus a red photon has less energy than a blue one. But if we have 10 blue photons and 1000 red photons, the red light will be more intense and have more net energy, but each individual photon will have a discrete amount of energy.
For our discussions we will use either photon or wave. In the discussion of how a sensor works physically it is important that the discussion focuses on the photon. The basic principles of the detector will not be explained. The assumption is that the detector receives a photon of a certain color (energy) and counts it appropriately.
In remote sensing it is very important to understand the nature of the electromagnetic spectrum to understand the type of data that can be received from an aircraft or satellite. Without an understanding of this data type you cannot fully appreciate the nature of the data that you will be using. For example why is one object red and another object green? This has to do with what colors of light are reflected and what colors are absorbed. For something to look red then more red light must be reflected than any other color. Likewise certain wavelength might be absorbed easier by the earth’s atmosphere than other wavelengths and thus the atmosphere is transparent to certain wavelengths and opaque to other wavelengths. It is important to realize that the visible spectrum of light contains all colors, but since the sun is a yellow-orange color more wavelengths are emitted in this part of the spectrum.
Most members of the electromagnetic spectrum are invisible to the human eye. For completeness we will discuss all members of the family initially, but then concentrate on those members of the spectrum that is more commonly used in the study of remote sensing. We will begin with the most energetic members of the spectrum which will have the highest frequency and shortest wavelengths and finish with those on the lower end of the spectrum with long wavelengths and low frequency.
Gamma Rays – are the most energetic member of the spectrum and are produced in the nuclear decay of atoms and is always associated with other types of nuclear decay (alpha or beta). It is a way in which the nucleus of an atom loses energy. Gamma rays are extremely penetrable and requires a large thickness of lead to protect yourself from these waves.
X-rays – are members of the spectrum which we have some familiarity with, since most people have had an x-ray for a medical purpose. While X-rays can penetrate your body they can be blocked very easily by a thin layer of lead. X-rays are produced when an electrons interact with a metal target in an x-ray machine, they are also produced in stellar processes as well. Few x-rays penetrate the earth’s atmosphere.
Ultraviolet (UV) – waves are very similar to visible light, but are invisible to the human eye. It is used in some settings to sterilize equipment. UV will cause certain inks to glow in the visible wavelengths when exposed. This is the way in which mail is determined to have postage on an envelope, stamps have a layer of ink that is invisible in normal light. UV from the sun can cause skin cancers and the atmosphere blocks much of this radiation from reaching the surface of the earth in a layer known as the Ozone layer. There are concerns environmentally that the Ozone layer is gradually being destroyed. To protect ourselves when outside exposed to the sun, a sun screen with a UV blocker can be used.
Visible Light – Blue light is the most energetic member of this family and red light the least. We are fairly familiar with visible light since that is the way we determine visible factors in our daily lives. It is a set of wavelengths that are used extensively in remote sensing, usually as a passive technology (looking at the reflectance).
Infrared (IR) – radiation is divided into two groups, one known as near infrared and the other thermal infrared or heat.
Microwaves – are a set of wavelengths between heat and radio waves, we have a familiarity with this family of waves since they are used for cooking. Since the discussion is from more to less energetic members of the spectrum this raises questions knowing that cooking is quicker with microwaves than most stovetops (thermal IR). Microwaves are less energetic than thermal IR when we look at individual photons, but in a microwave oven we concentrate the number photons used and thus allows most items to cook in a shorter amount of time. Radar is a set of waves that is at the interface between radio and microwaves. Radar is used as an active sensing area in remote sensing also in weather forecasting.
Radio Waves – are a broad spectrum of waves which includes multiple wavelengths such as TV, traditional radio AM and FM, as well as cell phone communications.
In remote sensing when an airborne detector is used (airplane or satellite) information is sensed in multiple wavelengths, one of the simplest systems is to receive reflectance in red, green and blue. Some sensors will use multiple sensors to expand beyond these three colors and also some sensors will sense in the near infrared and the ultraviolet. By knowing the spectral reflectance of an object in different wavelengths the identification of an object can occur. In addition, a diseased plant looks the same in visible wavelengths and very different when observed at other wavelengths such as near IR. For example, a measurement of the amount of salt in coastal area waters can be monitored over time to determine if fresh water is being depleted.
[1] A transverse wave oscillates perpendicular to the direction propagation. Sound is a longitudinal wave composed of physical particles and not a member of the electromagnetic spectrum.
[2] A model is a representation of what is observed to occur and allows for mathematical calculation of events. A model is not always an actual physical representation.
At other times in physics, we need to consider this family as a group of particles, called photons. A photon is a particle of light which is charge less, massless, with no physical dimensions that travels at the velocity of light. These particles behave much like elastic spheres. For example, when pool ball strikes the side of the table it reflects (bounces) off the table at the same angle in which it struck the bumper as measured to a normal (a perpendicular line to the side of the table); a photon does the same thing such as how light reflects off of a mirror. Different photons have different amounts of energy, thus a red photon has less energy than a blue one. But if we have 10 blue photons and 1000 red photons, the red light will be more intense and have more net energy, but each individual photon will have a discrete amount of energy.
For our discussions we will use either photon or wave. In the discussion of how a sensor works physically it is important that the discussion focuses on the photon. The basic principles of the detector will not be explained. The assumption is that the detector receives a photon of a certain color (energy) and counts it appropriately.
In remote sensing it is very important to understand the nature of the electromagnetic spectrum to understand the type of data that can be received from an aircraft or satellite. Without an understanding of this data type you cannot fully appreciate the nature of the data that you will be using. For example why is one object red and another object green? This has to do with what colors of light are reflected and what colors are absorbed. For something to look red then more red light must be reflected than any other color. Likewise certain wavelength might be absorbed easier by the earth’s atmosphere than other wavelengths and thus the atmosphere is transparent to certain wavelengths and opaque to other wavelengths. It is important to realize that the visible spectrum of light contains all colors, but since the sun is a yellow-orange color more wavelengths are emitted in this part of the spectrum.
Most members of the electromagnetic spectrum are invisible to the human eye. For completeness we will discuss all members of the family initially, but then concentrate on those members of the spectrum that is more commonly used in the study of remote sensing. We will begin with the most energetic members of the spectrum which will have the highest frequency and shortest wavelengths and finish with those on the lower end of the spectrum with long wavelengths and low frequency.
Gamma Rays – are the most energetic member of the spectrum and are produced in the nuclear decay of atoms and is always associated with other types of nuclear decay (alpha or beta). It is a way in which the nucleus of an atom loses energy. Gamma rays are extremely penetrable and requires a large thickness of lead to protect yourself from these waves.
X-rays – are members of the spectrum which we have some familiarity with, since most people have had an x-ray for a medical purpose. While X-rays can penetrate your body they can be blocked very easily by a thin layer of lead. X-rays are produced when an electrons interact with a metal target in an x-ray machine, they are also produced in stellar processes as well. Few x-rays penetrate the earth’s atmosphere.
Ultraviolet (UV) – waves are very similar to visible light, but are invisible to the human eye. It is used in some settings to sterilize equipment. UV will cause certain inks to glow in the visible wavelengths when exposed. This is the way in which mail is determined to have postage on an envelope, stamps have a layer of ink that is invisible in normal light. UV from the sun can cause skin cancers and the atmosphere blocks much of this radiation from reaching the surface of the earth in a layer known as the Ozone layer. There are concerns environmentally that the Ozone layer is gradually being destroyed. To protect ourselves when outside exposed to the sun, a sun screen with a UV blocker can be used.
Visible Light – Blue light is the most energetic member of this family and red light the least. We are fairly familiar with visible light since that is the way we determine visible factors in our daily lives. It is a set of wavelengths that are used extensively in remote sensing, usually as a passive technology (looking at the reflectance).
Infrared (IR) – radiation is divided into two groups, one known as near infrared and the other thermal infrared or heat.
- Near infrared has a longer wavelength than red light and is less energetic, it is used extensively in remote sensing since, green plants emit large amounts of near infrared energy. Thus two objects that visibly appear the same color to our eye, such as a green tree and a green roof will look very different in the near infrared. Near infrared is not the wavelengths used in infrared glasses, these night vision glasses use thermal infrared.
- Thermal infrared is heat, thus different temperatures have different wavelengths. Thermal IR is used to determine the heat lost by objects. Different substances emit heat differently, so how a metal roof of a building emits heat and how the pavement of a road emits heat must be accounted for when determining the temperature of the object. Thus looking at just at the temperature of the object will not directly tell us the heat loss or gain. Temperature and heat from a physics stand point are not the same, temperature is the measurement of how hot or cold something is and heat is the measurement of the amount of energy. On a cloudy night the outside temperature usually stays relatively constant but on a clear night the temperature will drop rapidly, this is due to the amount of water vapor within the atmosphere, water vapor absorbs thermal IR and does not allow it to escape the earth.
Microwaves – are a set of wavelengths between heat and radio waves, we have a familiarity with this family of waves since they are used for cooking. Since the discussion is from more to less energetic members of the spectrum this raises questions knowing that cooking is quicker with microwaves than most stovetops (thermal IR). Microwaves are less energetic than thermal IR when we look at individual photons, but in a microwave oven we concentrate the number photons used and thus allows most items to cook in a shorter amount of time. Radar is a set of waves that is at the interface between radio and microwaves. Radar is used as an active sensing area in remote sensing also in weather forecasting.
Radio Waves – are a broad spectrum of waves which includes multiple wavelengths such as TV, traditional radio AM and FM, as well as cell phone communications.
In remote sensing when an airborne detector is used (airplane or satellite) information is sensed in multiple wavelengths, one of the simplest systems is to receive reflectance in red, green and blue. Some sensors will use multiple sensors to expand beyond these three colors and also some sensors will sense in the near infrared and the ultraviolet. By knowing the spectral reflectance of an object in different wavelengths the identification of an object can occur. In addition, a diseased plant looks the same in visible wavelengths and very different when observed at other wavelengths such as near IR. For example, a measurement of the amount of salt in coastal area waters can be monitored over time to determine if fresh water is being depleted.
[1] A transverse wave oscillates perpendicular to the direction propagation. Sound is a longitudinal wave composed of physical particles and not a member of the electromagnetic spectrum.
[2] A model is a representation of what is observed to occur and allows for mathematical calculation of events. A model is not always an actual physical representation.