♦ Spectroscopy
• Spectroscopy is the study of the way in which atoms absorb and emit electromagnetic
radiation. Spectroscopy pertains to the dispersion of an object's light into its component colors (or energies). By performing the analysis of an object's light, scientists can infer the physical properties of that object (such as temperature, mass, luminosity, and chemical composition).
• We first realize that light acts like a wave. Light has particle-like properties too.
• The speed of a light wave is simply the speed of light, and different wavelengths of light manifest themselves as different colors. The energy of a light wave is inversely-proportional to its wavelength; in other words, low-energy light waves have long wavelengths, and high- energy light waves have short wavelengths.
♦ Electromagnetic spectrum
• Physicists classify light waves by their energies or wavelengths. Labeling in increasing energy or decreasing wavelength, we might draw the entire electromagnetic spectrum, as shown in Figure 43.
• Notice that radio, TV, and microwave signals are all ‘light’ waves; they simply lie at
wavelengths (energies) that our eyes do not respond to. On the other end of the scale, beware the high energy UV, x-ray, and gamma-ray photons. Each one carries a lot of energy compared to their visible-and radio-wave counterparts.
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Figure 44 In a prism, material dispersion (a wavelength-dependent refractive index) causes different colors to refract at different angles, splitting white light into a rainbow.
♦ Dispersion
• In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency. Media having such a property are termed dispersive media.
• The most familiar example of dispersion is probably a rainbow, in which dispersion causes the spatial separation of a white light into
components of different colors (different wavelengths), see Figure 44. Dispersion is most often described for light waves, but it may occur for any kind of wave that interacts with a medium or passes through an
inhomogeneous geometry. In optics, dispersion is sometimes called chromatic dispersion to emphasize its wavelength- dependent nature.
• The dispersion of light by glass prisms is used to construct spectrometers. Diffraction gratings are also used, as they allow more accurate discrimination of wavelengths.
• The most commonly seen consequence of
dispersion in optics is the separation of white light into a color spectrum by a prism. From Snell's law, it can be seen that the angle of refraction of light in a prism depends on the refractive index of the prism material. Since that refractive index varies with wavelength, it follows that the angle that the light is refracted by will also vary with wavelength, causing an angular separation of the colors known as angular dispersion.
Figure 43 The electromagnetic spectrum. Notice how small the visible region of the spectrum is, compared to the entire range of wavelengths.
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• A white light consists of a collection of component colors. These colors are often observed as white light passes through a triangular prism. Upon passing through the prism, the white light is separated into its component colors - red, orange, yellow, green, blue, and violet.
♦ Spectroscope
• A spectroscope is a device used for splitting a beam of radiation (light) into its component frequencies (or wavelengths) and delivering them onto a screen or detector for detailed study (see Figure 45). In other words, spectroscope is an optical system used to observe luminous spectra of light sources.
• In its most basic form, this device consists of an opaque barrier with a slit in it (to define a beam of light), a prism or a diffraction grating (to split the beam into its component colors), and an eyepiece or screen (to allow the user to view the resulting spectrum). Figure 45 shows such an arrangement.
• In many large instruments, the prism is replaced by a diffraction grating, consisting of a sheet of transparent material with many closely spaced parallel lines ruled on it. The spaces between the lines act as many tiny openings, and light is diffracted as it passes through these openings.
Because different wavelengths of electromagnetic radiation are diffracted by different amounts as they pass through a narrow gap, the effect of the grating is to split a beam of light into its component colors.
♦ Principle of operation of a spectroscope
• We use the source of interest to light a narrow slit. A collimating lens is placed on the path of light to send a parallel beam on a prism or a diffraction grating. After the dispersion of the light, a second lens projects on a screen the image of the slit, resulting many color lines. Each line corresponds to a wavelength. This series of lines constitutes the spectrum of the light source.
Examples are shown in Figure 46, including:
Figure 45 Diagram of a simple spectroscope. A small slit in the opaque barrier on the left allows a narrow beam of light to pass. The light passes through a prism and is split up into its component colors. The resulting spectrum can be viewed through an eyepiece or simply projected onto a screen.
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i. White light is broken up into a continuous spectrum, from red to blue (visible light).
ii. An incandescent gas gives bright lines of specific wavelengths; it is an emission spectrum and the positions of the lines are characteristic of the gas.
iii. The same cold gas is placed between the source of white light and the spectroscope. It absorbs some of the radiations emitted by this source. Dark lines are observed at the same positions as the bright lines of the previous spectrum. It is an absorption spectrum.
♦ SPECTRA
• The term ‘spectrum’ (plural form, spectra) is applied to any class of similar entities or
properties strictly arrayed in order of increasing or decreasing magnitude. In general, a spectrum is a display or plot of intensity of radiation (particles, photons, or acoustic radiation) as a
function of mass, momentum, wavelength, frequency, or some other related quantity.
• In the domain of electromagnetic radiation, a spectrum is a series of radiant energies arranged in order of wavelength or frequency. The entire range of frequencies is subdivided into wide intervals in which the waves have some common characteristic of generation or detection, such as the radio-frequency spectrum, infrared spectrum, visible spectrum, ultraviolet spectrum, and x-ray spectrum.
• Spectra are also classified according to their origin or mechanism of excitation, as emission, absorption, continuous, line, and band spectra.
- An emission spectrum is produced whenever the radiation from an excited light source is dispersed.
- A continuous spectrum contains an unbroken sequence of wavelengths or frequencies over a long range.
- Line spectra are discontinuous spectra characteristic of excited atoms and ions, whereas band spectra are characteristic of molecular gases or chemical compounds.
- An absorption spectrum is produced against a background of continuous radiation by interposing matter that reduces the intensity of radiation at certain wavelengths or spectral regions. The energies removed from the continuous spectrum by the interposed absorbing medium are precisely those that would be emitted by the medium if properly excited.
• Within the visible spectrum, various light wavelengths are perceived as colors ranging from red to blue, depending upon the wavelength of the wave. White light is a combination of all visible colors mixed in equal proportions. This characteristic of light, which enables it to be combined so that the resultant light is equal to the sum of its constituent wavelengths, is called additive color mixing.