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Visible light is not the only kind of electromagnetic radiation emitted. More energetic or less energetic transitions can produce ultraviolet or infrared radiation. However, because each atom has its own distinct set of energy levels its fingerprint! Atoms do not only emit photons; they also absorb photons. If a photon hits an atom and the energy of the photon is the same as the gap between two electron energy levels in the atom, then the electron in the lower energy level can absorb the photon and jump up to the higher energy level.

If the photon energy does not correspond to the difference between two energy levels then the photon will not be absorbed it can still be scattered. Using this effect, if we have a source of photons of various energies we can obtain the absorption spectra for different materials. To get an absorption spectrum, just shine white light on a sample of the material that you are interested in.

White light is made up of all the different wavelengths of visible light put together. In the absorption spectrum there will be gaps. The gaps correspond to energies wavelengths for which there is a corresponding difference in energy levels for the particular element. The absorbed photons show up as black lines because the photons of these wavelengths have been absorbed and do not show up.

Because of this, the absorption spectrum is the exact inverse of the emission spectrum. Look at the two figures below. In Figure It is the exact opposite of the emission spectrum!

Both emission and absorption techniques can be used to get the same information about the energy levels of an atom. The dark lines correspond to the frequencies of light that have been absorbed by the gas. As the photons of light are absorbed by electrons, the electrons move into higher energy levels. This is the opposite process of emission. The dark lines, absorption lines, correspond to the frequencies of the emission spectrum of the same element.

The amount of energy absorbed by the electron to move into a higher level is the same as the amount of energy released when returning to the original energy level. I have an unknown gas in a glass container. I shine a bright white light through one side of the container and measure the spectrum of transmitted light. I have a hunch that the gas might be hydrogen. If I am correct, between which 2 energy levels does this transition occur? Hint: look at Figure This means that the substance in the glass container absorbed photons with a wavelength of nm.

We need to calculate which 2 energy levels of hydrogen this transition would correspond to. Therefore we need to know what energy the absorbed photons had. The transitions related to visible wavelengths are marked as the transitions beginning or ending on Energy Level 2. Let us find the energy of those transitions and compare with the energy of the absorbed photons we have just calculated.

NOTE: The minus sign means that absorption is occurring. This is the same energy as the photons which were absorbed by the gas in the container! Therefore, since the transitions of all elements are unique, we can say that the gas in the container is hydrogen. The transition is absorption of a photon between Energy Level 2 and Energy Level 3. The spectral signature of an object is a function of the incidental EM wavelength and material interaction with that section of the electromagnetic spectrum.

Spectral Signature: Quantitative measurement of the properties of an object at one or several wavelength intervals For example, at some wavelengths, sand reflects more energy than green vegetation but at other wavelengths it absorbs more reflects less than does the vegetation. In principle, we can recognize various kinds of surface materials and distinguish them from each other by these differences in reflectance.

Of course, there must be some suitable method for measuring these differences as a function of wavelength and intensity as a fraction of the amount of irradiating radiation. Please note the positions of points for each When we use more than two wavelengths, the plots in multidimensional space tend to show more separation among the materials. This improved ability to distinguish materials due to extra wavelengths is the basis for multispectral remote sensing I Referring to the above spectral plots, which region of the spectrum stated in wavelength interval shows the greatest reflectance for a grasslands; b pinewoods; c red sand; d silty water.

Worked example 4: Absorption

By measuring the energy that is reflected or emitted by targets on the Earth's surface over a variety of different wavelengths, we can build up a spectral response for that object. It has a peak at the green region. However, the reflectance is maximum at the blue end of the spectrum and decreases as wavelength increases.

Hence, water appears dark bluish to the visible eye. In the example shown, the reflectance increases monotonically with increasing wavelength. Hence, it should appear yellowish-red to the eye. The shape of the reflectance spectrum can be used for identification of vegetation type. These properties enable vegetation condition to be monitored using remotely sensed images.

Vegetation generally has low reflectance and low transmittance in the visible part of the spectrum. This is mainly due to plant pigments absorbing visible light. Chlorophyll pigments absorb violet-blue and red light for photosynthetic energy. Green light is not absorbed for photosynthesis and therefore most plants appear green. In the autumn, some plant leaves turn from green to a brilliant yellow. This change in foliage color is caused by the normal autumn breakdown of chlorophyll which usually is the dominant pigment during the summer.

After the breakdown of chlorophyll, other pigments such as carotenes and xanthophylls become dominant and therefore the foliage color changes from green to yellow. Carotene and xanthophyll pigments absorb blue light and reflect green and red light. The vertical axis shows the percentage of incident sunlight that is reflected by the materials.

The horizontal axis shows wavelengths of energy for the visible spectral region 0.

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Reflected IR energy consists largely of solar energy reflected from the earth at wavelengths longer than the sensitivity range of the eye. The thermal portion of the IR region 3. Spectral bands recorded by remote sensing systems. Spectral reflectance curves are for vegetation and sedimentary rocks. These minerals have distinctive absorption features reflectance minima at wavelengths within the bandpass of TM band 7 which is shown with a stippled pattern in Fig.

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Recognition of hydrothermal clays and alunite from TM data, Goldfield mining district. Recognition of hydrothermal iron minerals from TM data, Goldfield mining district. Sometimes this warming of air above cities can influence weather, which is called the "urban heat island" effect. Diffraction is the bending and spreading of waves around an obstacle.


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It is most pronounced when a light wave strikes an object with a size comparable to its own wavelength. An instrument called a spectrometer uses diffraction to separate light into a range of wavelengths—a spectrum.

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In the case of visible light, the separation of wavelengths through diffraction results in a rainbow. A spectrometer uses diffraction and the subsequent interference of light from slits or gratings to separate wavelengths. Faint peaks of energy at specific wavelengths can then be detected and recorded.


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  • A graph of these data is called a spectral signature. Patterns in a spectral signature help scientists identify the physical condition and composition of stellar and interstellar matter. Scattering occurs when light bounces off an object in a variety of directions. The amount of scattering that takes place depends on the wavelength of the light and the size and structure of the object. The sky appears blue because of this scattering behavior. Light at shorter wavelengths—blue and violet—is scattered by nitrogen and oxygen as it passes through the atmosphere.

    Longer wavelengths of light—red and yellow—transmit through the atmosphere. This scattering of light at shorter wavelengths illuminates the skies with light from the blue and violet end of the visible spectrum. Even though violet is scattered more than blue, the sky looks blue to us because our eyes are more sensitive to blue light. Aerosols in the atmosphere can also scatter light.

    Atmospheric Transmission Emission And Scattering

    Refraction is when light waves change direction as they pass from one medium to another. Light travels slower in air than in a vacuum, and even slower in water. As light travels into a different medium, the change in speed bends the light. Different wavelengths of light are slowed at different rates, which causes them to bend at different angles.