Wavelengths for observed objects - emitted or observed?

Wavelengths for observed objects - emitted or observed?

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The WISE craft (Widefield Infrared Survey Explorer) surveys the sky in 4 wavelength bands; 3.4, 4.6, 12, and 22 $mu$m. When it's observing an object with an estimated redshift (given by

$z = frac{lambda_{obs} - lambda_{emit}}{lambda_{emit}}$

from Wikipedia) of, say, 0.373 (the lowest redshift for one of the objects I'm studying), would the wavelength it's seen in be its observed wavelength? I'm interested in calculating the wavelength actually emitted by the object.

If the redshift is $z$, then the wavelength of light you observe is at a wavelength that is $1+z$ times longer than the wavelength emitted in the frame of reference of the galaxy.

In this case $z=0.373$, so when you observe light centered on say the 3.4 micron band, then the light was emitted at a wavelength of 3.4/1.373 microns.

The WISE craft (Widefield Infrared Survey Explorer) surveys the sky in 4 wavelength bands; 3.4, 4.6, 12, and 22 $mu$m.

They are only using 3.4 and 4.6 since the coolant ran out. Source: FAQ "The Near-Earth Object Wide-field Infrared Survey Explorer at IPAC".

When it's observing an object with an estimated redshift of, say, 0.373 (the lowest redshift for one of the objects I'm studying), would the wavelength it sees be in its observed wavelength?

Yes, as long as it's within the very wide bandwidth of the imager filters it will be observable at such a small redshift (as long as you don't need the 12 and 22 $mu$m bands, see spectrum below).


  • "Optical identifications of high-redshift galaxy clusters from Planck Sunyaev-Zeldovich survey" (Jan 13 2018), by R. A. Burenin, I. F. Bikmaev, I. M. Khamitov, I. A. Zaznobin, G. A. Khorunzhev, M. V. Eselevich, V. L. Afanasyev, S. N. Dodonov, J. A. Rubiño-Martín, N. Aghanim, and R. A. Sunyaev, page 2:

    "Clusters located at redshifts below z ≈ 0.6 can be identified using the data of SDSS survey (Rykoff et al., 2014), using additional data from WISE all-sky survey it is possible to identify galaxy clusters at higher redshifts, up to z ≈ 0.7 (Burenin, 2017). To identify clusters at even higher redshifts, deeper direct imaging data in red and near IR bands are required.".

  • "An extension of the Planck galaxy cluster catalogue" (Mar 20 2017), by R. A. Burenin, page 3:

    "For galaxy cluster observations, the 3.4 µm photometric band is most useful. In this band, distant galaxy clusters are well detected at redshifts up to z ≈ 1-2 (e.g., Burenin, 2015).".

Source: "IV. WISE Data Processing - 4. Pipeline Science Modules":

Figure 5a - The QE-based (response per photon) relative system response (RSR) curves normalized to a peak value of unity, on a logarithmic scale.

Doppler Effect Equations for Light

The Doppler Effect equations for light show the change in the observed wavelength or color compared with that emitted from a moving source.

Note: Typically, the observed frequency is measured in the Doppler Effect. However in same cases, the change in wavelength is measured.

The source of light or electromagnetic radiation must travel at a high speed for the Doppler effect to cause an observable shift in the wavelength. Since the speed of light is much greater than the speed of the source, an approximate equation can be used to determine the shift of the radiation.

The shift in wavelength is used in astronomy to tell when a distant galaxy or star is moving toward the Earth (blue-shift) or away (red-shift). Equations are available for determining the new frequency and wavelength, as well as the velocity of the source.

Questions you may have include:

  • What are the equations for calculating frequency?
  • How do you calculate the wavelength shift?
  • What are the equations for velocity?

This lesson will answer those questions. Useful tool: Units Conversion

  • Periodic fluctuation in the intensity of coupled electric and magnetic fields.
  • Wave travels through a vacuum at the speed of light.
  • Doesn't need a medium to "wave" in.

This speed is independent of the wavelength or frequency of the light!

Like other waves, light waves have a frequency, wavelength, and amplitude, usually symbolized with ν, λ, and A, respectively.

The wavelength and frequency of light waves are connected by

Wavelengths for observed objects - emitted or observed? - Astronomy

For most of history, visible light was the only known part of the electromagnetic spectrum. The ancient Greeks recognized that light traveled in straight lines and studied some of its properties, including reflection and refraction. In 1666, Sir Issac Newton observed that the spectrum of colors exiting a prism creates a rainbow. By 1670 he demonstrated that the multicolored spectrum produced by a prism could be recombined into white light by a lens and a second prism.

The dispersion of white light through a prism

Today we now know that visible light is only a small portion of what is called the electromagnetic spectrum. Continue to learn more below and also click on this companion learning source from NASA.


The first discovery of electromagnetic radiation other than visible light came in 1800, when William Herschel discovered infrared radiation. He was studying the temperature of different colors by moving a thermometer through light split by a prism. He noticed that the highest temperature was beyond red. He theorized that this temperature change was due to "calorific rays" which would be in fact a type of light ray that could not be seen. It wasn't until the latter 20th century when we could build imaging infrared detectors to view the universe. The infrared region of the spectrum is divided into near-, mid-, and far-infrared.

The constellation of Orion seen in visible and infrared light

The Orion Nebula in infrared and visible light

Molecules tend to absorb infrared energy causing them to vibrate based upon their atomic structure. In space, there are many regions which are hidden from optical telescopes because they are embedded in dense regions of gas and dust. However, infrared radiation, having wavelengths which are much longer than visible light, can pass through dusty regions of space without being scattered. This means that we can study objects hidden by gas and dust in the infrared, which we cannot see in visible light, such as the center of our galaxy and regions of newly forming stars. Many objects in the universe which are much too cool and faint to be detected in visible light, can be detected in the infrared. These include cool stars, infrared galaxies, clouds of particles around stars, nebulae, interstellar molecules, brown dwarfs and planets.

Infrared light is absorbed at many wavelengths by water vapor in the Earth's atmosphere, so most infrared telescopes are at high elevations in dry places, above as much of the atmosphere as possible. NASA operates infrared telescopes in orbit such as WISE and event has an aircraft equipped with infrared telescopes called SOFIA . Slightly more than half of the total energy from the Sun arrives on Earth in the form of infrared light. The balance between absorbed and emitted infrared radiation has a critical effect on Earth's climate.

Did You Know? We can feel infrared light as heat but cannot see it. Heat lamps at a fast food restaurant that keep your french fies warm give off most their light in the infrared.


In 1801, the German physicist Johann Wilhelm Ritter made the hallmark observation that invisible rays just beyond the violet end of the visible spectrum darkened silver chloride-soaked paper more quickly than violet light itself. He called them "oxidizing" or "chemical" rays to emphasize chemical reactivity and to distinguish them from "heat rays", discovered the previous year at the other end of the visible spectrum.

Messier 51 in visible and ultraviolet light

Very hot objects emit UV radiation. In M51, young hot stars show up well in ultraviolet yet the companion galaxy shows no new star formation. In our solar system, the Sun is a source of the full spectrum of ultraviolet radiation, which is commonly subdivided into UV-A, UV-B, and UV-C. These are the classifications most often used in Earth sciences. UV-C rays are the most harmful and are almost completely absorbed by our atmosphere. UV-B rays are the harmful rays that cause sunburn. Exposure to UV-B rays increases the risk of DNA and other cellular damage in living organisms. Fortunately, about 95 percent UV-B rays are absorbed by ozone in the Earth's atmosphere.

Did You Know? Although UV waves are invisible to the human eye, some insects, such as bumblebees, can see them.


In 1895 Wilhelm Röntgen noticed a new type of radiation emitted during an experiment with an evacuated tube subjected to a high voltage. He called these radiations x-rays and found that they were able to travel through parts of the human body but were reflected or stopped by denser matter such as bones. Before long, many uses were found for them in the field of medicine. The first attempt at X-ray astronomy was conducted at White Sands Missile Range in January 1949 when a detector was lofted by a V-2 suborbital rocket.

Messier 51 in visible and X-ray light

The Sun emits X-rays giving clues to the temperature of the corona. A live image of the Sun in X-rays can be seen on the AAG 'Planets' page. Look for the live images of the Sun and the one on the far right (yellow) is how our Sun looks in X-rays.

Did You Know? X-ray light shows us some of the more energetic phenomenon in the universe like supernova remnants, binary star systems with white dwarfs, neutron stars and black holes.

Gamma rays

The last portion of the electromagnetic spectrum was filled in with the discovery of gamma rays. In 1900 Paul Villard was studying the radioactive emissions of radium when he identified a new type of radiation that he first thought consisted of particles similar to known alpha and beta particles, but with the power of being far more penetrating than either. A discovery in gamma-ray astronomy came in the late 1960s and early 1970s from a constellation of military defense satellites. Detectors on-board the Vela satellites, designed to detect flashes of gamma rays from nuclear bomb blasts, began to record bursts of gamma rays from deep space rather than the vicinity of the Earth.

Recent observations by the Fermi Space Telescope hint at large gamma-ray bubbles eminating from our galactic core

Gamma rays have the smallest wavelengths and the most energy of any wave in the electromagnetic spectrum. They are produced by the hottest and most energetic objects in the universe, such as neutron stars and pulsars, supernova explosions, and regions around black holes. On Earth, gamma waves are generated by nuclear explosions, lightning, and the less dramatic activity of radioactive decay.

Did You Know? The Vela satellites were attempting to see if the Soviet Union were testing nuclear weapons behind the Moon.

Radio and Microwaves

In the 1860's, Scottish scientist James Clerk Maxwell developed a scientific theory to explain electromagnetic waves. He noted that electrical fields and magnetic fields can couple together to form electromagnetic waves. The initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky observed radiation coming from the Milky Way. Subsequent observations have identified a number of different sources for radio emissions. These include stars and galaxies, as well as entirely new classes of objects, such as radio galaxies, quasars, pulsars, and masers. The discovery of the cosmic microwave background radiation, regarded as evidence for the Big Bang theory, was made through radio astronomy.

Messier 51 in visible and radio light

Radio astronomy primarly detects electrons caught in magnetic fields. Radio astronomy has led to substantial increases in astronomical knowledge, particularly with the discovery of several classes of new objects, including pulsars, quasars and radio galaxies. This is because radio astronomy allows us to see things that are not detectable in optical astronomy. Such objects represent some of the most extreme and energetic physical processes in the universe. The cosmic microwave background radiation was also first detected using radio telescopes. However, radio telescopes have also been used to investigate objects much closer to home, including observations of the Sun and solar activity, and radar mapping of the planets.

Did You Know? A microwave oven "cooks" food by radiating at a frequency of 2.45 GHz, which is the natural oscillation frequency of water molecules.

The Electromagnetic Spectrum

Did You Know? If you can imagine the electromagnetic spectrum spans the distance from Los Angeles to New York City, the part visible to the human eye spans the width of a dime.

The Effects of the Earth's Atmosphere

For centuries, astronomers have only examined the visible portion of the electromagnetic spectrum. The Earth's atmosphere effectively blocks low frequency radio waves, most infrared and ultraviolet light and all X-ray and gamma ray sources. Sorry Marconi, your radio signals are never going to be picked up by the aliens - the Earth's ionosphere reflected them back down to the surface.

Filtering of the EM spectrum by Earth's atmosphere

The Universe sends us light at all wavelengths of the electromagnetic spectrum. However, most of this light does not reach us at ground level here on Earth. Why? Because we have an atmosphere which blocks out many types of radiation while letting other types through. Fortunately for life on Earth, our atmosphere blocks out harmful, high energy radiation like X-rays, gamma rays and most of the ultraviolet rays. It also block out most infrared radiation, as well as very low energy radio waves. On the other hand, our atmosphere lets visible light, most radio waves, and small wavelength ranges of infrared light through, allowing astronomers to view the Universe at these wavelengths. Most of the infrared light coming to us from the Universe is absorbed by water vapor and carbon dioxide in the Earth's atmosphere. Only in a few narrow wavelength ranges, can infrared light make it through (at least partially) to a ground based infrared telescope.

The Earth's atmosphere causes another problem for infrared astronomers. The atmosphere itself radiates strongly in the infrared, often putting out more infrared light than the object in space being observed. This atmospheric infrared emission peaks at a wavelength of about 10 microns (micron is short for a micrometer or one millionth of a meter). So the best view of the infrared universe, from ground based telescopes, are at infrared wavelengths which can pass through the Earth's atmosphere and at which the atmosphere is dim in the infrared. Ground based infrared observatories are usually placed near the summit of high, dry mountains to get above as much of the atmosphere as possible. Even so, most infrared wavelengths are completely absorbed by the atmosphere and never make it to the ground. From the table below, you can see that only a few of the infrared "windows" have both high sky transparency and low sky emission. These infrared windows are mainly at infrared wavelengths below 4 microns.

The Nature of Light

The earliest scientific theories of the nature of light were proposed around the end of the 17th century by Dutch astronomer Christian Huygens. He proposed a theory that explained light as a wave phenomenon. Sir Issac Newton, who had discovered the visible spectrum in 1666, held that light is composed of tiny particles, or corpuscles, emitted by luminous bodies. For more than 100 years, Newton's corpuscular theory of light was favored over the wave theory, partly because of Newton's great prestige and partly because not enough experimental evidence existed to provide an adequate basis of comparison between the two theories. However, the wave theory fell back into favor with from the electromagnetic theory of James Clerk Maxwell (1864), who showed that electric and magnetic fields were propagated together and that their speed was identical with the speed of light.

The Electromagnetic Wave

Mawell showed that the electric field is pushed along by the perpendicular magnetic field, which in turn pushes the electric field along and the propagation occurs at 300,000 meters per second. It thus became clear that visible light is a form of electromagnetic radiation, constituting only a small part of the electromagnetic spectrum. Maxwell's theory was confirmed experimentally with the discovery of radio waves by Heinrich Hertz in 1886.

Propagation of the EM Wave

In 1905, Einstein extended the quantum theory of thermal radiation proposed by Max Planck in 1900 to cover not only vibrations of the source of radiation but also vibrations of the radiation itself. He thus suggested that light, and other forms of electromagnetic radiation as well, travel as tiny bundles of energy called light quanta, or photons. According to our understanding today, the electromagnetic field itself is produced by photons, which in turn result from a local (gauge) symmetry and the laws of quantum field theory.

Decoding Spectra

As soon as astronomers understood the nature of light and could build sensors to detect various forms of the electromagntic spectrum, this opened up the universe's true nature. To learn about how astronomers can determine the nature of objects across the light years, click here: Decoding Cosmic Spectra

Did You Know? With the advent of satellites and new generations of detectors - We are living in the golden age of astronomy RIGHT NOW!

Laboratory experiments here on Earth have determined that each element in the periodic table emits photons only at certain wavelengths (determined by the excitation state of the atoms). These photons are manifest as either emission or absorption lines in the spectrum of an astronomical object, and by measuring the position of these spectral lines, we can determine which elements are present in the object itself or along the line of sight.

However, when astronomers perform this analysis, they note that for most astronomical objects, the observed spectral lines are all shifted to longer (redder) wavelengths. This is known as ‘cosmological redshift’ (or more commonly just ‘redshift’) and is given by:

for relatively nearby objects, where z is the cosmological redshift, λobs is the observed wavelength and λrest is the emitted/absorbed wavelength.

Caused solely by the expansion of the Universe, the value of the cosmological redshift indicates the recession velocity of the object, or its distance. For small velocities (much less than the speed of light), cosmological redshift is related to recession velocity ( v ) through:

where c the speed of light. At larger distances (higher redshifts), using the theory of general relativity gives a more accurate relation for recession velocities, which can be greater than the speed of light. Note this doesn’t break the ultimate speed limit of c in Special Relativity as nothing is actually moving at that speed, rather the entire distance between the receding object and us is increasing. This is a complex formula requiring knowledge of the overall expansion history of the universe to calculate correctly but a simple recession velocity is given by multiplying the comoving distance (D) of the object by the Hubble parameter at that redshift (H) as:

Although cosmological redshift at first appears to be a similar effect to the more familiar Doppler shift, there is a distinction. In Doppler Shift, the wavelength of the emitted radiation depends on the motion of the object at the instant the photons are emitted. If the object is travelling towards us, the wavelength is shifted towards the blue end of the spectrum, if the object is travelling away from us, the wavelength is shifted towards the red end. In cosmological redshift, the wavelength at which the radiation is originally emitted is lengthened as it travels through (expanding) space. Cosmological redshift results from the expansion of space itself and not from the motion of an individual body.

For example, in a distant binary system it is theoretically possible to measure both a Doppler shift and a cosmological redshift. The Doppler shift would be determined by the motions of the individual stars in the binary – whether they were approaching or receding at the time the photons were emitted. The cosmological redshift would be determined by how far away the system was when the photons were emitted. The larger the distance to the system, the longer the emitted photons have travelled through expanding space and the higher the measured cosmological redshift.

The wavelengths of the Sun

The telescopes can collect light in ranges of frequencies inaccessible to us.
This lovely movie of the Sun, based on data from the Solar Dynamics Observatory NASA shows the wide range of visible wavelengths by the instruments of the telescope. SDO converts the wavelengths into an interpretable image by the human eye. Each of the light wave length (each color) shows the solar material at specific temperatures. So by looking at the sun in a variety of wavelengths, the images generated by SDO but also by imaging spectrographs NASA Earth Solar Observatory and NASA Solar Heliospheric Observatory and the European Space Agency, scientists can track and analyze the movement of particles and the temperature of the Sun's atmosphere.

Video: Sun characteristics appear radically different when viewed in different wavelengths.

Atomic Spectra

In the 1670s, Isaac Newton, during optical experiments observed that a beam of white light was decomposed in a continuum spectrum of all visible colors, like a rainbow, when it pass through a prism. Newton classified this spectrum in a range of seven different colors (Red, Orange, Yellow, Green, Bleu, Indigo and Violet). In sequence of this experiment, Newton combined those colored beams in another prism which resulted in another white light beam. This means that the white light is formed by the combination all visible colors. The decomposition of the white light in different colors results from different wavelengths, as a consequence, they move at different speeds in the prism, with red light moving faster than violet. The result is that red light bends less sharply than violet as it passes through the prism, creating a spectrum of colors. The table below shows the range of wavelength for each of seven colors of the rainbow.

Color Wavelength
Red 620 -750
Orange 590 – 620
Yellow 570 – 590
Green 495 – 570
Blue 450 – 500
Indigo 420 – 450
Violet 380 – 420

In the 19 th century, scientist discovered that they could use the light emitted by heated or electrical discharged materials to analyze their properties. This was the beginning of the Spectrum Analyze Technique. In 1850s, Anders Jonas Angstrom observed that the light emitted by a high heated gas did not have a continuum spectrum, like white light, when it passes through a prism. The spectrum of the heated gas was discrete lines or spectra lines. This means that a heated gas emits light in just some specific wavelength. Scientist found that each element of periodic table has a unique set of spectral lines, as a signature of the element. That way, it is possible to use the spectrum analyze to identify the composition of material. This is a technique very important in astronomy where scientists use to analyze the properties of distant objects, like their chemical elements.

The specific wavelengths emitted by the gas are related to their atomic structure. As well known, the atom is compounded by a nucleus with positive (protons) and neutral (neutrons) charges surrounded by a cloud of negative charges (electrons). The electrons of an atom are bounded to the nucleus by the electromagnetic force. By the quantum mechanical theory, a bounded electron can only occupy a set of states centered on the nucleus, and each state corresponds to a specific energy level. When an electron is excited, by heart or electrical discharge, it jumps to a higher energy level. However, the electron tends to be in the lowest unoccupied energy level available. That way, the electron returns to its original state. To the electron fall back, it emits an amount of energy in form of light (photon) with a specific wavelength. This energy is equal the difference between the excited and the original level. In conclusion, the spectra lines are result of the energy quantization of the atom.

As each element has different energy levels of their atoms, we can identify elements by their spectra lines. The table below shows the wavelength to the two strongest lines emitted on the visible spectrum by the gases of hydrogen (H), Argon (Ar), neon (Ne) and helium (He), the gases which fill the tubes on this experiment.

Wavelengths for observed objects - emitted or observed? - Astronomy

There are two basic ways to observe a galaxy with an optical telescope: by taking an image, or by taking a spectrum. A telescope takes an image of a galaxy in the same fashion that you might use a camera and black and white film to take a picture of a yourself.

In each case, all of the light from the object is collected areas which emit the most light (or, in the case of clouds and snow, reflect the most sunlight) appear as bright regions on the pictures, while fainter areas are reproduced in dimmer shades. Such an image conveys the overall brightness of the galaxy, but it cannot show us the colours of the stars which make up various parts of the galaxy, nor how these colours vary with location within the galaxy.

In order to produce a colour image, we need to take multiple exposures at different wavelengths along the optical portion of the electromagnetic spectrum. A different filter is placed before the camera for each exposure, one which transmits light within only a narrow range of wavelengths.

This filter lets only yellow light pass through. [NASA/HST]

By combining the exposures, we can determine which parts of the object are brightest at short wavelengths (ultraviolet light, and blue colours), at intermediate wavelengths (yellow colours), or at long wavelengths (red colours, and infrared light). For spiral galaxies like our own Milky Way, for example, we find that the outer regions of the galaxy disk tends to have blue colours (showing the presence of bright, young stars), while the central bulge is populated by redder, longer-lived stars.

The figure below shows seven images of the galaxy NGC 1512, which was observed with the Hubble Space Telescope. Each image was taken through a different filter, and so sampled a different portion of the optical spectrum, and contains light of different colours. Observe how different parts of the galaxy light up and assume prominence at different wavelengths. If you were describing the morphology (appearance) of this galaxy, how might your description change if you looked at only at a short, or long, wavelength image?

By combining the images, we can create a single colour image of the galaxy. Could you predict from the initial black and white images which components of the galaxy would be brightest in various colours?

A multiwavelength image of NGC 1512. [NASA/HST]

A spectrum takes the idea of breaking down the light according to colour, or wavelength, one step further than a series of images. Just as droplets of water in the atmosphere can separate out the colours of the sunlight into a rainbow, or a prism can split up white light into a range of colours, a spectrograph can disperse the light emitted from an object according to wavelength.

The figures below show spectra of optical light. The x-axis runs from short, blue wavelengths on the left to long, red wavelengths on the right. The y-axis indicates the amount of light emitted at each wavelength - the higher the level of the signal, the more light is present. In each case, the top plot is a line plot showing intensity versus wavelength, while the lower plot represents the spectrum as it would appear at the telescope.

By breaking the light down by wavelength, we are able to search for key features which indicate the presence of certain elements in the stars which form the galaxy. These features may not be strong enough for the eye to find them hidden in an image containing light from a range of wavelengths, because they are very narrow (covering only a few wavelengths out of thousands), but once the light is distributed by wavelength they are easy to identify.

    Continuum spectra show a relatively smooth shape, with no strong features as a function of wavelength. In the case shown below, the peak of the curve lies just below 6000 Angstroms (like the shape of the spectrum of light emitted from the Sun).


Radio emission reveals a few different things about the Milky Way depending on which part of the radio spectrum we observe. Parts of the radio continuum tell us about where electrons are being accelerated in the galaxy. Other parts tell us about where hydrogen lies in the Milky Way.

Intensity of the radio continuum emission from the disk of the Milky Way at 408 MHz (top) and 2.4-2.7 GHz (bottom). These radio wavelengths show astronomers where electrons are being accelerated through a variety of processes. (Credit: Haslam, et. al (1982), A&AS, 47, 1 Duncan, et. al (1995) MNRAS, 277, 36 Fuerst, et. al (1990) A&AS, 85, 691 Reich, et al. (1990), A&AS, 85, 633)

Radio continuum emission comes from electrons accelerated through one of two different processes. The 408MHz continuum, shown above, primarily shows us places in the Milky Way where electrons are accelerated by the interstellar magnetic field at nearly the speed of light. As the electrons are accelerated, they spiral around the magnetic field lines and emit radiation at radio wavelengths. In the 2.4-2.7 GHz range, some of the bright spots also show where electrons are accelerated in magnetic fields. In that part of the continuum, though, we also see light emitted by electrons accelerated by protons in the hot, ionized gases of emission nebula.

These images show the amount of atomic (top, 1.4 GHz) and molecular (bottom, 115 GHz) hydrogen from radio observations. (Credit: Burton, (1985) A&AS, 62, 365 Hartmann, "Atlas of Galactic Neutral Hydrogen," Cambridge Univ. Press, (1997, book and CD-ROM) Kerr, (1986) A&AS, 66, 373 Dame, (2001) ApJ, 547, 792)

Looking at a couple of specific wavelengths, astronomers can see places where hydrogen resides in the Milky Way. Atomic hydrogen emits a rare spectral line at 1420 MHz (or 21-cm in wavelength). Even though the line is rare, we see this line fairly prominently in the Milky Way because there is so much hydrogen. Atomic hydrogen traces places where the interstellar medium is cold or warm, which is organized into diffuse clouds of gas and dust up to hundreds of light years across.

Molecular hydrogen is difficult to detect directly, so carbon monoxide is observed as a standard tracer of molecular hydrogen. Carbon monoxide has a spectral line in the radio at 115 GHz. We find that molecular hydrogen resides is the spiral arms of the Milky Way in "molecular clouds" that are often the site of star formation.

Common Applications of Emission and Absorption Spectroscopy

Spectroscopy is the study of the spectrum of a substance to investigate more about its properties. Both absorption and emission spectroscopy have a number of uses.

Emission Spectrum

To identify a substance: Every substance emits lights of different wavelengths. To identify the given substance, light is focused on it or the substance is heated. This causes the electrons to get excited and jump to a higher orbit. The energy emitted by these electrons while returning to their ground states is compared to the characteristic colors of the elements, and the chemical composition of the substance is determined.
To study the composition of stars: The emission spectra of stars can be recorded and then compared with standard emission spectra of known elements to determine their chemical composition.

Absorption Spectrum

To identify a substance and determine its concentration: An unknown substance can be identified by focusing light of a particular wavelength on it, and then studying the absorption spectrum of the substance. Since substances absorb light only from a particular wavelength or wavelength range, the wavelength of light focused on them is important. This spectrum can be compared with a set of reference values for identification. These reference values are known absorbance values of common elements and compounds. The concentration of the substance in the sample can also be determined.
To study the composition of stars: The light emitted by stars and planets passes through their atmosphere, where some of it is absorbed by the gases. When the absorption spectra of these gases is recorded and compared to the reference spectra values of gases, the composition of these planets or stars can be determined.
Remote sensing: Absorption spectroscopy can be used to collect details of the land, including attributes such as forest cover, health of forests or exposed rock surfaces, without any individual actually setting foot on it. When light is focused on the land terrain and its absorption spectra is recorded, it can be used to extract information about the terrain. This is done by comparing the recorded values with reference values of absorbance shown by land with forest cover or exposed rock. In fact, the absorbance values vary depending on the type of the forest, a healthy vegetation will show different values compared to an unhealthy forest cover. It can also provide details of atmospheric composition.

Both absorption and emission spectroscopy are exact opposites of each other. Since the electronic configurations of elements are different, the spectrum values of these elements will be their ‘atomic fingerprint’, i.e., it will be unique to each element. It is said that absorption spectrum is the ‘photographic negative’ of emission spectrum, because the wavelengths that are missing in absorption spectrum are seen in the emission spectrum.

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