Physics · Ch 5 — Electromagnetic Waves
Types of Spectrum -- Emission and Absorption Spectrum -- Fraunhofer Lines
Types of Spectrum -- Emission and Absorption Spectrum -- Fraunhofer Lines
When an object is heated, it emits electromagnetic radiation whose character depends on its temperature: a moderately hot object glows red, a hotter one glows reddish-orange, and the hottest objects glow white -- the same continuous, temperature-dependent curve (the black body spectrum) studied for Class XI Physics, whose peak shifts to shorter wavelengths as temperature rises. Passing a beam of white light through a prism disperses it into its seven constituent colours, a phenomenon called dispersion of light, and the resulting definite pattern of colours on a screen is called a spectrum; spectra are broadly classified into two categories, emission spectra and absorption spectra. An emission spectrum is obtained by directly examining the light coming from a self-luminous source, and every source has its own characteristic emission spectrum, of three possible types. A continuous emission spectrum contains every wavelength across the visible range with no gaps, as seen from an incandescent solid or a carbon arc; a line emission spectrum (also called a discontinuous spectrum) consists of a small number of sharp lines at definite wavelengths, arising from excited atoms of a gas (for example atomic hydrogen or helium), and because these lines are characteristic of -- and different for -- each element, they act as that element's unique fingerprint; a band emission spectrum consists of many very closely spaced spectral lines that overlap to form distinct bands separated by dark gaps, with a sharp edge at one end fading out at the other, arising when molecules (not just isolated atoms) are excited, as seen for example in the spectrum of ammonia gas in a discharge tube -- because it arises from molecular, not purely atomic, transitions, the band spectrum is characteristic of the molecule and can be used to study molecular structure. An absorption spectrum, by contrast, is obtained by passing light through a medium or an absorbing substance and is characteristic of that absorbing substance, and is likewise classified into three types. A continuous absorption spectrum results when, for instance, white light is passed through a blue glass plate, which absorbs every colour except blue. A line absorption spectrum results when light from an incandescent lamp is passed through a cooler gas (for example passing carbon-arc light through sodium vapour produces two dark lines in the yellow region); the cold gas absorbs exactly the same specific wavelengths its atoms would otherwise emit if excited, leaving dark lines at those positions against an otherwise continuous bright background. A band absorption spectrum results when white light is passed through iodine vapour, or through a dilute solution of blood, chlorophyll, or certain organic or inorganic compounds, producing dark bands against a continuous bright background, analogous to band emission but in absorption. Finally, when the Sun's own spectrum is examined closely, it shows a large number of dark lines -- a natural line absorption spectrum -- kn …
What this figure shows. A graph of radiated intensity versus wavelength, plotted for three different black-body temperatures -- 3000 K, 4000 K and 6000 K -- each drawn as its own smooth, hump-shaped curve, with the ultraviolet end of the spectrum marked on the left and the infrared end marked on the right. As the temperature rises from 3000 K (roughly a lamp filament) through 4000 K (roughly a carbon arc lamp) to 6000 K (roughly the Sun's surface), each successive curve's peak shifts towards shorter wavelengths and grows taller, visually illustrating why a hotter object glows progressively more blue-white while a cooler one glows a duller red, and grounding the chapter's later discussion of continuous emission …
What this figure shows. A beam of white light from a source is shown entering a triangular glass prism and emerging on the far side spread out into a fan of separate coloured beams landing on a screen, illustrating the phenomenon of dispersion: because the prism material's refractive index is slightly different for each wavelength, violet light bends the most and red light the least, so a single beam of white light splits into its seven constituent colours ranging continuously from violet to red. This dispersion by a prism is the basic tool used throughout the rest of the section to obtain and compare the …
What this figure shows. A rectangular strip of colour is shown running unbroken from violet through blue, green, yellow, orange to red with no gaps or dark lines anywhere across it, representing the spectrum obtained when light from an incandescent solid (such as a filament bulb) or a carbon arc is passed through a prism. Because every wavelength across the visible range is present with no missing bands, the spectrum forms one continuous smear of colour rather than a pattern of separate lines, which is exactly what distinguishes a continuous emissio …
What this figure shows. A dark background strip is shown crossed by only a handful of sharp, bright, narrow coloured lines at specific fixed positions, with a small label "Hot gas" indicating the source, in clear visual contrast to the unbroken smear of Figure 5.13. This is the spectrum obtained when light from a hot, excited gas -- such as atomic hydrogen or helium -- is dispersed by a prism: because excited atoms of a given element only emit at their own specific, characteristic wavelengths, the result is a small set of sharp bright lines rather than a continuous band, and the exact pattern of lines is …
What this figure shows. A continuous coloured background spectrum (as in Figure 5.13) is shown but now interrupted by a small number of sharp, narrow dark lines at fixed positions, with a small label "Cold gas" indicating the medium the light passed through before reaching the prism. This is the spectrum obtained when light from an incandescent lamp is passed through a cooler gas: the cold gas absorbs light only at the exact wavelengths its own atoms would otherwise emit, removing just those narrow slices from the otherwise continuous background and leaving behind dark lines at precisely the positions where that gas's characteristic bright emission lines …
What this figure shows. A long horizontal strip representing the Sun's spectrum spread out by wavelength, running from the ultraviolet and X-ray/gamma-ray end on the left, through the visible range with a wavelength scale marked in Angstrom units from about 4000 to 7500 A, and on to the infrared-and-radio end on the right. Superimposed on this otherwise continuous solar spectrum is a dense set of dark vertical lines at fixed positions, individually labelled with the conventional Fraunhofer letters (A, B, C, D, E, F, G, H and several lower-case letters), representing the actual dark absorption lines Joseph von Fraunhofer catalogued in sunlight -- each line corresponds to a wavelength absorbed by a specific element present in the cooler outer layers of the Sun's atmosphere as sunlight from the hotter interior passes through it, so matching each Fraunhofer line's exact w …